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PERLFAQ4(1)                      Perl Programmers Reference Guide                     PERLFAQ4(1)

NAME
       perlfaq4 - Data Manipulation

VERSION
       version 5.20210411

DESCRIPTION
       This section of the FAQ answers questions related to manipulating numbers, dates, strings,
       arrays, hashes, and miscellaneous data issues.

Data: Numbers
   Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be
       getting (eg, 19.95)?
       For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know
       About Floating-Point Arithmetic"
       (<http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf>).

       Internally, your computer represents floating-point numbers in binary.  Digital (as in
       powers of two) computers cannot store all numbers exactly. Some real numbers lose
       precision in the process. This is a problem with how computers store numbers and affects
       all computer languages, not just Perl.

       perlnumber shows the gory details of number representations and conversions.

       To limit the number of decimal places in your numbers, you can use the "printf" or
       "sprintf" function. See "Floating-point Arithmetic" in perlop for more details.

           printf "%.2f", 10/3;

           my $number = sprintf "%.2f", 10/3;

   Why is int() broken?
       Your "int()" is most probably working just fine. It's the numbers that aren't quite what
       you think.

       First, see the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of
       the numbers I should be getting (eg, 19.95)?".

       For example, this

           print int(0.6/0.2-2), "\n";

       will in most computers print 0, not 1, because even such simple numbers as 0.6 and 0.2
       cannot be presented exactly by floating-point numbers. What you think in the above as
       'three' is really more like 2.9999999999999995559.

   Why isn't my octal data interpreted correctly?
       (contributed by brian d foy)

       You're probably trying to convert a string to a number, which Perl only converts as a
       decimal number. When Perl converts a string to a number, it ignores leading spaces and
       zeroes, then assumes the rest of the digits are in base 10:

           my $string = '0644';

           print $string + 0;  # prints 644

           print $string + 44; # prints 688, certainly not octal!

       This problem usually involves one of the Perl built-ins that has the same name a Unix
       command that uses octal numbers as arguments on the command line. In this example, "chmod"
       on the command line knows that its first argument is octal because that's what it does:

           %prompt> chmod 644 file

       If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat
       them as octal numbers either by prefixing the digits with a 0 or using "oct":

           chmod(     0644, $filename );  # right, has leading zero
           chmod( oct(644), $filename );  # also correct

       The problem comes in when you take your numbers from something that Perl thinks is a
       string, such as a command line argument in @ARGV:

           chmod( $ARGV[0],      $filename );  # wrong, even if "0644"

           chmod( oct($ARGV[0]), $filename );  # correct, treat string as octal

       You can always check the value you're using by printing it in octal notation to ensure it
       matches what you think it should be. Print it in octal  and decimal format:

           printf "0%o %d", $number, $number;

   Does Perl have a round() function? What about ceil() and floor()? Trig functions?
       Remember that "int()" merely truncates toward 0. For rounding to a certain number of
       digits, "sprintf()" or "printf()" is usually the easiest route.

           printf("%.3f", 3.1415926535);   # prints 3.142

       The POSIX module (part of the standard Perl distribution) implements "ceil()", "floor()",
       and a number of other mathematical and trigonometric functions.

           use POSIX;
           my $ceil   = ceil(3.5);   # 4
           my $floor  = floor(3.5);  # 3

       In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004,
       the Math::Trig module (part of the standard Perl distribution) implements the
       trigonometric functions. Internally it uses the Math::Complex module and some functions
       can break out from the real axis into the complex plane, for example the inverse sine of
       2.

       Rounding in financial applications can have serious implications, and the rounding method
       used should be specified precisely. In these cases, it probably pays not to trust
       whichever system of rounding is being used by Perl, but instead to implement the rounding
       function you need yourself.

       To see why, notice how you'll still have an issue on half-way-point alternation:

           for (my $i = -5; $i <= 5; $i += 0.5) { printf "%.0f ",$i }

           -5 -4 -4 -4 -3 -2 -2 -2 -1 -0 0 0 1 2 2 2 3 4 4 4 5

       Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose
       absolute values are integers under 2**31 (on 32-bit machines) will work pretty much like
       mathematical integers.  Other numbers are not guaranteed.

   How do I convert between numeric representations/bases/radixes?
       As always with Perl there is more than one way to do it. Below are a few examples of
       approaches to making common conversions between number representations. This is intended
       to be representational rather than exhaustive.

       Some of the examples later in perlfaq4 use the Bit::Vector module from CPAN. The reason
       you might choose Bit::Vector over the perl built-in functions is that it works with
       numbers of ANY size, that it is optimized for speed on some operations, and for at least
       some programmers the notation might be familiar.

       How do I convert hexadecimal into decimal
           Using perl's built in conversion of "0x" notation:

               my $dec = 0xDEADBEEF;

           Using the "hex" function:

               my $dec = hex("DEADBEEF");

           Using "pack":

               my $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));

           Using the CPAN module "Bit::Vector":

               use Bit::Vector;
               my $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
               my $dec = $vec->to_Dec();

       How do I convert from decimal to hexadecimal
           Using "sprintf":

               my $hex = sprintf("%X", 3735928559); # upper case A-F
               my $hex = sprintf("%x", 3735928559); # lower case a-f

           Using "unpack":

               my $hex = unpack("H*", pack("N", 3735928559));

           Using Bit::Vector:

               use Bit::Vector;
               my $vec = Bit::Vector->new_Dec(32, -559038737);
               my $hex = $vec->to_Hex();

           And Bit::Vector supports odd bit counts:

               use Bit::Vector;
               my $vec = Bit::Vector->new_Dec(33, 3735928559);
               $vec->Resize(32); # suppress leading 0 if unwanted
               my $hex = $vec->to_Hex();

       How do I convert from octal to decimal
           Using Perl's built in conversion of numbers with leading zeros:

               my $dec = 033653337357; # note the leading 0!

           Using the "oct" function:

               my $dec = oct("33653337357");

           Using Bit::Vector:

               use Bit::Vector;
               my $vec = Bit::Vector->new(32);
               $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
               my $dec = $vec->to_Dec();

       How do I convert from decimal to octal
           Using "sprintf":

               my $oct = sprintf("%o", 3735928559);

           Using Bit::Vector:

               use Bit::Vector;
               my $vec = Bit::Vector->new_Dec(32, -559038737);
               my $oct = reverse join('', $vec->Chunk_List_Read(3));

       How do I convert from binary to decimal
           Perl 5.6 lets you write binary numbers directly with the "0b" notation:

               my $number = 0b10110110;

           Using "oct":

               my $input = "10110110";
               my $decimal = oct( "0b$input" );

           Using "pack" and "ord":

               my $decimal = ord(pack('B8', '10110110'));

           Using "pack" and "unpack" for larger strings:

               my $int = unpack("N", pack("B32",
               substr("0" x 32 . "11110101011011011111011101111", -32)));
               my $dec = sprintf("%d", $int);

               # substr() is used to left-pad a 32-character string with zeros.

           Using Bit::Vector:

               my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
               my $dec = $vec->to_Dec();

       How do I convert from decimal to binary
           Using "sprintf" (perl 5.6+):

               my $bin = sprintf("%b", 3735928559);

           Using "unpack":

               my $bin = unpack("B*", pack("N", 3735928559));

           Using Bit::Vector:

               use Bit::Vector;
               my $vec = Bit::Vector->new_Dec(32, -559038737);
               my $bin = $vec->to_Bin();

           The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)  are left as an
           exercise to the inclined reader.

   Why doesn't & work the way I want it to?
       The behavior of binary arithmetic operators depends on whether they're used on numbers or
       strings. The operators treat a string as a series of bits and work with that (the string
       "3" is the bit pattern 00110011). The operators work with the binary form of a number (the
       number 3 is treated as the bit pattern 00000011).

       So, saying "11 & 3" performs the "and" operation on numbers (yielding 3). Saying "11" &
       "3" performs the "and" operation on strings (yielding "1").

       Most problems with "&" and "|" arise because the programmer thinks they have a number but
       really it's a string or vice versa. To avoid this, stringify the arguments explicitly
       (using "" or "qq()") or convert them to numbers explicitly (using "0+$arg"). The rest
       arise because the programmer says:

           if ("\020\020" & "\101\101") {
               # ...
           }

       but a string consisting of two null bytes (the result of "\020\020" & "\101\101") is not a
       false value in Perl. You need:

           if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
               # ...
           }

   How do I multiply matrices?
       Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) or the PDL
       extension (also available from CPAN).

   How do I perform an operation on a series of integers?
       To call a function on each element in an array, and collect the results, use:

           my @results = map { my_func($_) } @array;

       For example:

           my @triple = map { 3 * $_ } @single;

       To call a function on each element of an array, but ignore the results:

           foreach my $iterator (@array) {
               some_func($iterator);
           }

       To call a function on each integer in a (small) range, you can use:

           my @results = map { some_func($_) } (5 .. 25);

       but you should be aware that in this form, the ".." operator creates a list of all
       integers in the range, which can take a lot of memory for large ranges. However, the
       problem does not occur when using ".." within a "for" loop, because in that case the range
       operator is optimized to iterate over the range, without creating the entire list. So

           my @results = ();
           for my $i (5 .. 500_005) {
               push(@results, some_func($i));
           }

       or even

          push(@results, some_func($_)) for 5 .. 500_005;

       will not create an intermediate list of 500,000 integers.

   How can I output Roman numerals?
       Get the <http://www.cpan.org/modules/by-module/Roman> module.

   Why aren't my random numbers random?
       If you're using a version of Perl before 5.004, you must call "srand" once at the start of
       your program to seed the random number generator.

            BEGIN { srand() if $] < 5.004 }

       5.004 and later automatically call "srand" at the beginning. Don't call "srand" more than
       once--you make your numbers less random, rather than more.

       Computers are good at being predictable and bad at being random (despite appearances
       caused by bugs in your programs :-). The random article in the "Far More Than You Ever
       Wanted To Know" collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of
       Tom Phoenix, talks more about this. John von Neumann said, "Anyone who attempts to
       generate random numbers by deterministic means is, of course, living in a state of sin."

       Perl relies on the underlying system for the implementation of "rand" and "srand"; on some
       systems, the generated numbers are not random enough (especially on Windows : see
       <http://www.perlmonks.org/?node_id=803632>).  Several CPAN modules in the "Math" namespace
       implement better pseudorandom generators; see for example Math::Random::MT ("Mersenne
       Twister", fast), or Math::TrulyRandom (uses the imperfections in the system's timer to
       generate random numbers, which is rather slow).  More algorithms for random numbers are
       described in "Numerical Recipes in C" at <http://www.nr.com/>

   How do I get a random number between X and Y?
       To get a random number between two values, you can use the "rand()" built-in to get a
       random number between 0 and 1. From there, you shift that into the range that you want.

       "rand($x)" returns a number such that "0 <= rand($x) < $x". Thus what you want to have
       perl figure out is a random number in the range from 0 to the difference between your X
       and Y.

       That is, to get a number between 10 and 15, inclusive, you want a random number between 0
       and 5 that you can then add to 10.

           my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )

       Hence you derive the following simple function to abstract that. It selects a random
       integer between the two given integers (inclusive). For example:
       "random_int_between(50,120)".

           sub random_int_between {
               my($min, $max) = @_;
               # Assumes that the two arguments are integers themselves!
               return $min if $min == $max;
               ($min, $max) = ($max, $min)  if  $min > $max;
               return $min + int rand(1 + $max - $min);
           }

Data: Dates
   How do I find the day or week of the year?
       The day of the year is in the list returned by the "localtime" function. Without an
       argument "localtime" uses the current time.

           my $day_of_year = (localtime)[7];

       The POSIX module can also format a date as the day of the year or week of the year.

           use POSIX qw/strftime/;
           my $day_of_year  = strftime "%j", localtime;
           my $week_of_year = strftime "%W", localtime;

       To get the day of year for any date, use POSIX's "mktime" to get a time in epoch seconds
       for the argument to "localtime".

           use POSIX qw/mktime strftime/;
           my $week_of_year = strftime "%W",
               localtime( mktime( 0, 0, 0, 18, 11, 87 ) );

       You can also use Time::Piece, which comes with Perl and provides a "localtime" that
       returns an object:

           use Time::Piece;
           my $day_of_year  = localtime->yday;
           my $week_of_year = localtime->week;

       The Date::Calc module provides two functions to calculate these, too:

           use Date::Calc;
           my $day_of_year  = Day_of_Year(  1987, 12, 18 );
           my $week_of_year = Week_of_Year( 1987, 12, 18 );

   How do I find the current century or millennium?
       Use the following simple functions:

           sub get_century    {
               return int((((localtime(shift || time))[5] + 1999))/100);
           }

           sub get_millennium {
               return 1+int((((localtime(shift || time))[5] + 1899))/1000);
           }

       On some systems, the POSIX module's "strftime()" function has been extended in a non-
       standard way to use a %C format, which they sometimes claim is the "century". It isn't,
       because on most such systems, this is only the first two digits of the four-digit year,
       and thus cannot be used to determine reliably the current century or millennium.

   How can I compare two dates and find the difference?
       (contributed by brian d foy)

       You could just store all your dates as a number and then subtract.  Life isn't always that
       simple though.

       The Time::Piece module, which comes with Perl, replaces localtime with a version that
       returns an object. It also overloads the comparison operators so you can compare them
       directly:

           use Time::Piece;
           my $date1 = localtime( $some_time );
           my $date2 = localtime( $some_other_time );

           if( $date1 < $date2 ) {
               print "The date was in the past\n";
           }

       You can also get differences with a subtraction, which returns a Time::Seconds object:

           my $date_diff = $date1 - $date2;
           print "The difference is ", $date_diff->days, " days\n";

       If you want to work with formatted dates, the Date::Manip, Date::Calc, or DateTime modules
       can help you.

   How can I take a string and turn it into epoch seconds?
       If it's a regular enough string that it always has the same format, you can split it up
       and pass the parts to "timelocal" in the standard Time::Local module. Otherwise, you
       should look into the Date::Calc, Date::Parse, and Date::Manip modules from CPAN.

   How can I find the Julian Day?
       (contributed by brian d foy and Dave Cross)

       You can use the Time::Piece module, part of the Standard Library, which can convert a
       date/time to a Julian Day:

           $ perl -MTime::Piece -le 'print localtime->julian_day'
           2455607.7959375

       Or the modified Julian Day:

           $ perl -MTime::Piece -le 'print localtime->mjd'
           55607.2961226851

       Or even the day of the year (which is what some people think of as a Julian day):

           $ perl -MTime::Piece -le 'print localtime->yday'
           45

       You can also do the same things with the DateTime module:

           $ perl -MDateTime -le'print DateTime->today->jd'
           2453401.5
           $ perl -MDateTime -le'print DateTime->today->mjd'
           53401
           $ perl -MDateTime -le'print DateTime->today->doy'
           31

       You can use the Time::JulianDay module available on CPAN. Ensure that you really want to
       find a Julian day, though, as many people have different ideas about Julian days (see
       <http://www.hermetic.ch/cal_stud/jdn.htm> for instance):

           $  perl -MTime::JulianDay -le 'print local_julian_day( time )'
           55608

   How do I find yesterday's date?
       (contributed by brian d foy)

       To do it correctly, you can use one of the "Date" modules since they work with calendars
       instead of times. The DateTime module makes it simple, and give you the same time of day,
       only the day before, despite daylight saving time changes:

           use DateTime;

           my $yesterday = DateTime->now->subtract( days => 1 );

           print "Yesterday was $yesterday\n";

       You can also use the Date::Calc module using its "Today_and_Now" function.

           use Date::Calc qw( Today_and_Now Add_Delta_DHMS );

           my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );

           print "@date_time\n";

       Most people try to use the time rather than the calendar to figure out dates, but that
       assumes that days are twenty-four hours each. For most people, there are two days a year
       when they aren't: the switch to and from summer time throws this off. For example, the
       rest of the suggestions will be wrong sometimes:

       Starting with Perl 5.10, Time::Piece and Time::Seconds are part of the standard
       distribution, so you might think that you could do something like this:

           use Time::Piece;
           use Time::Seconds;

           my $yesterday = localtime() - ONE_DAY; # WRONG
           print "Yesterday was $yesterday\n";

       The Time::Piece module exports a new "localtime" that returns an object, and Time::Seconds
       exports the "ONE_DAY" constant that is a set number of seconds. This means that it always
       gives the time 24 hours ago, which is not always yesterday. This can cause problems around
       the end of daylight saving time when there's one day that is 25 hours long.

       You have the same problem with Time::Local, which will give the wrong answer for those
       same special cases:

           # contributed by Gunnar Hjalmarsson
            use Time::Local;
            my $today = timelocal 0, 0, 12, ( localtime )[3..5];
            my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; # WRONG
            printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;

   Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
       (contributed by brian d foy)

       Perl itself never had a Y2K problem, although that never stopped people from creating Y2K
       problems on their own. See the documentation for "localtime" for its proper use.

       Starting with Perl 5.12, "localtime" and "gmtime" can handle dates past 03:14:08 January
       19, 2038, when a 32-bit based time would overflow. You still might get a warning on a
       32-bit "perl":

           % perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
           Integer overflow in hexadecimal number at -e line 1.
           Wed Nov  1 19:42:39 5576711

       On a 64-bit "perl", you can get even larger dates for those really long running projects:

           % perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
           Thu Nov  2 00:42:39 5576711

       You're still out of luck if you need to keep track of decaying protons though.

Data: Strings
   How do I validate input?
       (contributed by brian d foy)

       There are many ways to ensure that values are what you expect or want to accept. Besides
       the specific examples that we cover in the perlfaq, you can also look at the modules with
       "Assert" and "Validate" in their names, along with other modules such as Regexp::Common.

       Some modules have validation for particular types of input, such as Business::ISBN,
       Business::CreditCard, Email::Valid, and Data::Validate::IP.

   How do I unescape a string?
       It depends just what you mean by "escape". URL escapes are dealt with in perlfaq9. Shell
       escapes with the backslash ("\") character are removed with

           s/\\(.)/$1/g;

       This won't expand "\n" or "\t" or any other special escapes.

   How do I remove consecutive pairs of characters?
       (contributed by brian d foy)

       You can use the substitution operator to find pairs of characters (or runs of characters)
       and replace them with a single instance. In this substitution, we find a character in
       "(.)". The memory parentheses store the matched character in the back-reference "\g1" and
       we use that to require that the same thing immediately follow it. We replace that part of
       the string with the character in $1.

           s/(.)\g1/$1/g;

       We can also use the transliteration operator, "tr///". In this example, the search list
       side of our "tr///" contains nothing, but the "c" option complements that so it contains
       everything. The replacement list also contains nothing, so the transliteration is almost a
       no-op since it won't do any replacements (or more exactly, replace the character with
       itself). However, the "s" option squashes duplicated and consecutive characters in the
       string so a character does not show up next to itself

           my $str = 'Haarlem';   # in the Netherlands
           $str =~ tr///cs;       # Now Harlem, like in New York

   How do I expand function calls in a string?
       (contributed by brian d foy)

       This is documented in perlref, and although it's not the easiest thing to read, it does
       work. In each of these examples, we call the function inside the braces used to
       dereference a reference. If we have more than one return value, we can construct and
       dereference an anonymous array. In this case, we call the function in list context.

           print "The time values are @{ [localtime] }.\n";

       If we want to call the function in scalar context, we have to do a bit more work. We can
       really have any code we like inside the braces, so we simply have to end with the scalar
       reference, although how you do that is up to you, and you can use code inside the braces.
       Note that the use of parens creates a list context, so we need "scalar" to force the
       scalar context on the function:

           print "The time is ${\(scalar localtime)}.\n"

           print "The time is ${ my $x = localtime; \$x }.\n";

       If your function already returns a reference, you don't need to create the reference
       yourself.

           sub timestamp { my $t = localtime; \$t }

           print "The time is ${ timestamp() }.\n";

       The "Interpolation" module can also do a lot of magic for you. You can specify a variable
       name, in this case "E", to set up a tied hash that does the interpolation for you. It has
       several other methods to do this as well.

           use Interpolation E => 'eval';
           print "The time values are $E{localtime()}.\n";

       In most cases, it is probably easier to simply use string concatenation, which also forces
       scalar context.

           print "The time is " . localtime() . ".\n";

   How do I find matching/nesting anything?
       To find something between two single characters, a pattern like "/x([^x]*)x/" will get the
       intervening bits in $1. For multiple ones, then something more like "/alpha(.*?)omega/"
       would be needed. For nested patterns and/or balanced expressions, see the so-called
       (?PARNO) construct (available since perl 5.10).  The CPAN module Regexp::Common can help
       to build such regular expressions (see in particular Regexp::Common::balanced and
       Regexp::Common::delimited).

       More complex cases will require to write a parser, probably using a parsing module from
       CPAN, like Regexp::Grammars, Parse::RecDescent, Parse::Yapp, Text::Balanced, or Marpa::R2.

   How do I reverse a string?
       Use "reverse()" in scalar context, as documented in "reverse" in perlfunc.

           my $reversed = reverse $string;

   How do I expand tabs in a string?
       You can do it yourself:

           1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;

       Or you can just use the Text::Tabs module (part of the standard Perl distribution).

           use Text::Tabs;
           my @expanded_lines = expand(@lines_with_tabs);

   How do I reformat a paragraph?
       Use Text::Wrap (part of the standard Perl distribution):

           use Text::Wrap;
           print wrap("\t", '  ', @paragraphs);

       The paragraphs you give to Text::Wrap should not contain embedded newlines. Text::Wrap
       doesn't justify the lines (flush-right).

       Or use the CPAN module Text::Autoformat. Formatting files can be easily done by making a
       shell alias, like so:

           alias fmt="perl -i -MText::Autoformat -n0777 \
               -e 'print autoformat $_, {all=>1}' $*"

       See the documentation for Text::Autoformat to appreciate its many capabilities.

   How can I access or change N characters of a string?
       You can access the first characters of a string with substr().  To get the first
       character, for example, start at position 0 and grab the string of length 1.

           my $string = "Just another Perl Hacker";
           my $first_char = substr( $string, 0, 1 );  #  'J'

       To change part of a string, you can use the optional fourth argument which is the
       replacement string.

           substr( $string, 13, 4, "Perl 5.8.0" );

       You can also use substr() as an lvalue.

           substr( $string, 13, 4 ) =  "Perl 5.8.0";

   How do I change the Nth occurrence of something?
       You have to keep track of N yourself. For example, let's say you want to change the fifth
       occurrence of "whoever" or "whomever" into "whosoever" or "whomsoever", case
       insensitively. These all assume that $_ contains the string to be altered.

           $count = 0;
           s{((whom?)ever)}{
           ++$count == 5       # is it the 5th?
               ? "${2}soever"  # yes, swap
               : $1            # renege and leave it there
               }ige;

       In the more general case, you can use the "/g" modifier in a "while" loop, keeping count
       of matches.

           $WANT = 3;
           $count = 0;
           $_ = "One fish two fish red fish blue fish";
           while (/(\w+)\s+fish\b/gi) {
               if (++$count == $WANT) {
                   print "The third fish is a $1 one.\n";
               }
           }

       That prints out: "The third fish is a red one."  You can also use a repetition count and
       repeated pattern like this:

           /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;

   How can I count the number of occurrences of a substring within a string?
       There are a number of ways, with varying efficiency. If you want a count of a certain
       single character (X) within a string, you can use the "tr///" function like so:

           my $string = "ThisXlineXhasXsomeXx'sXinXit";
           my $count = ($string =~ tr/X//);
           print "There are $count X characters in the string";

       This is fine if you are just looking for a single character. However, if you are trying to
       count multiple character substrings within a larger string, "tr///" won't work. What you
       can do is wrap a while() loop around a global pattern match. For example, let's count
       negative integers:

           my $string = "-9 55 48 -2 23 -76 4 14 -44";
           my $count = 0;
           while ($string =~ /-\d+/g) { $count++ }
           print "There are $count negative numbers in the string";

       Another version uses a global match in list context, then assigns the result to a scalar,
       producing a count of the number of matches.

           my $count = () = $string =~ /-\d+/g;

   How do I capitalize all the words on one line?
       (contributed by brian d foy)

       Damian Conway's Text::Autoformat handles all of the thinking for you.

           use Text::Autoformat;
           my $x = "Dr. Strangelove or: How I Learned to Stop ".
             "Worrying and Love the Bomb";

           print $x, "\n";
           for my $style (qw( sentence title highlight )) {
               print autoformat($x, { case => $style }), "\n";
           }

       How do you want to capitalize those words?

           FRED AND BARNEY'S LODGE        # all uppercase
           Fred And Barney's Lodge        # title case
           Fred and Barney's Lodge        # highlight case

       It's not as easy a problem as it looks. How many words do you think are in there? Wait for
       it... wait for it.... If you answered 5 you're right. Perl words are groups of "\w+", but
       that's not what you want to capitalize. How is Perl supposed to know not to capitalize
       that "s" after the apostrophe? You could try a regular expression:

           $string =~ s/ (
                        (^\w)    #at the beginning of the line
                          |      # or
                        (\s\w)   #preceded by whitespace
                          )
                       /\U$1/xg;

           $string =~ s/([\w']+)/\u\L$1/g;

       Now, what if you don't want to capitalize that "and"? Just use Text::Autoformat and get on
       with the next problem. :)

   How can I split a [character]-delimited string except when inside [character]?
       Several modules can handle this sort of parsing--Text::Balanced, Text::CSV, Text::CSV_XS,
       and Text::ParseWords, among others.

       Take the example case of trying to split a string that is comma-separated into its
       different fields. You can't use "split(/,/)" because you shouldn't split if the comma is
       inside quotes. For example, take a data line like this:

           SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

       Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we
       have Jeffrey Friedl, author of Mastering Regular Expressions, to handle these for us. He
       suggests (assuming your string is contained in $text):

            my @new = ();
            push(@new, $+) while $text =~ m{
                "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
               | ([^,]+),?
               | ,
            }gx;
            push(@new, undef) if substr($text,-1,1) eq ',';

       If you want to represent quotation marks inside a quotation-mark-delimited field, escape
       them with backslashes (eg, "like \"this\"".

       Alternatively, the Text::ParseWords module (part of the standard Perl distribution) lets
       you say:

           use Text::ParseWords;
           @new = quotewords(",", 0, $text);

       For parsing or generating CSV, though, using Text::CSV rather than implementing it
       yourself is highly recommended; you'll save yourself odd bugs popping up later by just
       using code which has already been tried and tested in production for years.

   How do I strip blank space from the beginning/end of a string?
       (contributed by brian d foy)

       A substitution can do this for you. For a single line, you want to replace all the leading
       or trailing whitespace with nothing. You can do that with a pair of substitutions:

           s/^\s+//;
           s/\s+$//;

       You can also write that as a single substitution, although it turns out the combined
       statement is slower than the separate ones. That might not matter to you, though:

           s/^\s+|\s+$//g;

       In this regular expression, the alternation matches either at the beginning or the end of
       the string since the anchors have a lower precedence than the alternation. With the "/g"
       flag, the substitution makes all possible matches, so it gets both. Remember, the trailing
       newline matches the "\s+", and  the "$" anchor can match to the absolute end of the
       string, so the newline disappears too. Just add the newline to the output, which has the
       added benefit of preserving "blank" (consisting entirely of whitespace) lines which the
       "^\s+" would remove all by itself:

           while( <> ) {
               s/^\s+|\s+$//g;
               print "$_\n";
           }

       For a multi-line string, you can apply the regular expression to each logical line in the
       string by adding the "/m" flag (for "multi-line"). With the "/m" flag, the "$" matches
       before an embedded newline, so it doesn't remove it. This pattern still removes the
       newline at the end of the string:

           $string =~ s/^\s+|\s+$//gm;

       Remember that lines consisting entirely of whitespace will disappear, since the first part
       of the alternation can match the entire string and replace it with nothing. If you need to
       keep embedded blank lines, you have to do a little more work. Instead of matching any
       whitespace (since that includes a newline), just match the other whitespace:

           $string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

   How do I pad a string with blanks or pad a number with zeroes?
       In the following examples, $pad_len is the length to which you wish to pad the string,
       $text or $num contains the string to be padded, and $pad_char contains the padding
       character. You can use a single character string constant instead of the $pad_char
       variable if you know what it is in advance. And in the same way you can use an integer in
       place of $pad_len if you know the pad length in advance.

       The simplest method uses the "sprintf" function. It can pad on the left or right with
       blanks and on the left with zeroes and it will not truncate the result. The "pack"
       function can only pad strings on the right with blanks and it will truncate the result to
       a maximum length of $pad_len.

           # Left padding a string with blanks (no truncation):
           my $padded = sprintf("%${pad_len}s", $text);
           my $padded = sprintf("%*s", $pad_len, $text);  # same thing

           # Right padding a string with blanks (no truncation):
           my $padded = sprintf("%-${pad_len}s", $text);
           my $padded = sprintf("%-*s", $pad_len, $text); # same thing

           # Left padding a number with 0 (no truncation):
           my $padded = sprintf("%0${pad_len}d", $num);
           my $padded = sprintf("%0*d", $pad_len, $num); # same thing

           # Right padding a string with blanks using pack (will truncate):
           my $padded = pack("A$pad_len",$text);

       If you need to pad with a character other than blank or zero you can use one of the
       following methods. They all generate a pad string with the "x" operator and combine that
       with $text. These methods do not truncate $text.

       Left and right padding with any character, creating a new string:

           my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
           my $padded = $text . $pad_char x ( $pad_len - length( $text ) );

       Left and right padding with any character, modifying $text directly:

           substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
           $text .= $pad_char x ( $pad_len - length( $text ) );

   How do I extract selected columns from a string?
       (contributed by brian d foy)

       If you know the columns that contain the data, you can use "substr" to extract a single
       column.

           my $column = substr( $line, $start_column, $length );

       You can use "split" if the columns are separated by whitespace or some other delimiter, as
       long as whitespace or the delimiter cannot appear as part of the data.

           my $line    = ' fred barney   betty   ';
           my @columns = split /\s+/, $line;
               # ( '', 'fred', 'barney', 'betty' );

           my $line    = 'fred||barney||betty';
           my @columns = split /\|/, $line;
               # ( 'fred', '', 'barney', '', 'betty' );

       If you want to work with comma-separated values, don't do this since that format is a bit
       more complicated. Use one of the modules that handle that format, such as Text::CSV,
       Text::CSV_XS, or Text::CSV_PP.

       If you want to break apart an entire line of fixed columns, you can use "unpack" with the
       A (ASCII) format. By using a number after the format specifier, you can denote the column
       width. See the "pack" and "unpack" entries in perlfunc for more details.

           my @fields = unpack( $line, "A8 A8 A8 A16 A4" );

       Note that spaces in the format argument to "unpack" do not denote literal spaces. If you
       have space separated data, you may want "split" instead.

   How do I find the soundex value of a string?
       (contributed by brian d foy)

       You can use the "Text::Soundex" module. If you want to do fuzzy or close matching, you
       might also try the String::Approx, and Text::Metaphone, and Text::DoubleMetaphone modules.

   How can I expand variables in text strings?
       (contributed by brian d foy)

       If you can avoid it, don't, or if you can use a templating system, such as Text::Template
       or Template Toolkit, do that instead. You might even be able to get the job done with
       "sprintf" or "printf":

           my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

       However, for the one-off simple case where I don't want to pull out a full templating
       system, I'll use a string that has two Perl scalar variables in it. In this example, I
       want to expand $foo and $bar to their variable's values:

           my $foo = 'Fred';
           my $bar = 'Barney';
           $string = 'Say hello to $foo and $bar';

       One way I can do this involves the substitution operator and a double "/e" flag. The first
       "/e" evaluates $1 on the replacement side and turns it into $foo. The second /e starts
       with $foo and replaces it with its value. $foo, then, turns into 'Fred', and that's
       finally what's left in the string:

           $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

       The "/e" will also silently ignore violations of strict, replacing undefined variable
       names with the empty string. Since I'm using the "/e" flag (twice even!), I have all of
       the same security problems I have with "eval" in its string form. If there's something odd
       in $foo, perhaps something like "@{[ system "rm -rf /" ]}", then I could get myself in
       trouble.

       To get around the security problem, I could also pull the values from a hash instead of
       evaluating variable names. Using a single "/e", I can check the hash to ensure the value
       exists, and if it doesn't, I can replace the missing value with a marker, in this case
       "???" to signal that I missed something:

           my $string = 'This has $foo and $bar';

           my %Replacements = (
               foo  => 'Fred',
               );

           # $string =~ s/\$(\w+)/$Replacements{$1}/g;
           $string =~ s/\$(\w+)/
               exists $Replacements{$1} ? $Replacements{$1} : '???'
               /eg;

           print $string;

   Does Perl have anything like Ruby's #{} or Python's f string?
       Unlike the others, Perl allows you to embed a variable naked in a double quoted string,
       e.g. "variable $variable". When there isn't whitespace or other non-word characters
       following the variable name, you can add braces (e.g. "foo ${foo}bar") to ensure correct
       parsing.

       An array can also be embedded directly in a string, and will be expanded by default with
       spaces between the elements. The default LIST_SEPARATOR can be changed by assigning a
       different string to the special variable $", such as "local $" = ', ';".

       Perl also supports references within a string providing the equivalent of the features in
       the other two languages.

       "${\ ... }" embedded within a string will work for most simple statements such as an
       object->method call. More complex code can be wrapped in a do block "${\ do{...} }".

       When you want a list to be expanded per $", use "@{[ ... ]}".

           use Time::Piece;
           use Time::Seconds;
           my $scalar = 'STRING';
           my @array = ( 'zorro', 'a', 1, 'B', 3 );

           # Print the current date and time and then Tommorrow
           my $t = Time::Piece->new;
           say "Now is: ${\ $t->cdate() }";
           say "Tomorrow: ${\ do{ my $T=Time::Piece->new + ONE_DAY ; $T->fullday }}";

           # some variables in strings
           say "This is some scalar I have $scalar, this is an array @array.";
           say "You can also write it like this ${scalar} @{array}.";

           # Change the $LIST_SEPARATOR
           local $" = ':';
           say "Set \$\" to delimit with ':' and sort the Array @{[ sort @array ]}";

       You may also want to look at the module Quote::Code, and templating tools such as
       Template::Toolkit and Mojo::Template.

       See also: "How can I expand variables in text strings?" and "How do I expand function
       calls in a string?" in this FAQ.

   What's wrong with always quoting "$vars"?
       The problem is that those double-quotes force stringification--coercing numbers and
       references into strings--even when you don't want them to be strings. Think of it this
       way: double-quote expansion is used to produce new strings. If you already have a string,
       why do you need more?

       If you get used to writing odd things like these:

           print "$var";       # BAD
           my $new = "$old";       # BAD
           somefunc("$var");    # BAD

       You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct:

           print $var;
           my $new = $old;
           somefunc($var);

       Otherwise, besides slowing you down, you're going to break code when the thing in the
       scalar is actually neither a string nor a number, but a reference:

           func(\@array);
           sub func {
               my $aref = shift;
               my $oref = "$aref";  # WRONG
           }

       You can also get into subtle problems on those few operations in Perl that actually do
       care about the difference between a string and a number, such as the magical "++"
       autoincrement operator or the syscall() function.

       Stringification also destroys arrays.

           my @lines = `command`;
           print "@lines";     # WRONG - extra blanks
           print @lines;       # right

   Why don't my <<HERE documents work?
       Here documents are found in perlop. Check for these three things:

       There must be no space after the << part.
       There (probably) should be a semicolon at the end of the opening token
       You can't (easily) have any space in front of the tag.
       There needs to be at least a line separator after the end token.

       If you want to indent the text in the here document, you can do this:

           # all in one
           (my $VAR = <<HERE_TARGET) =~ s/^\s+//gm;
               your text
               goes here
           HERE_TARGET

       But the HERE_TARGET must still be flush against the margin.  If you want that indented
       also, you'll have to quote in the indentation.

           (my $quote = <<'    FINIS') =~ s/^\s+//gm;
                   ...we will have peace, when you and all your works have
                   perished--and the works of your dark master to whom you
                   would deliver us. You are a liar, Saruman, and a corrupter
                   of men's hearts. --Theoden in /usr/src/perl/taint.c
               FINIS
           $quote =~ s/\s+--/\n--/;

       A nice general-purpose fixer-upper function for indented here documents follows. It
       expects to be called with a here document as its argument.  It looks to see whether each
       line begins with a common substring, and if so, strips that substring off. Otherwise, it
       takes the amount of leading whitespace found on the first line and removes that much off
       each subsequent line.

           sub fix {
               local $_ = shift;
               my ($white, $leader);  # common whitespace and common leading string
               if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) {
                   ($white, $leader) = ($2, quotemeta($1));
               } else {
                   ($white, $leader) = (/^(\s+)/, '');
               }
               s/^\s*?$leader(?:$white)?//gm;
               return $_;
           }

       This works with leading special strings, dynamically determined:

           my $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
           @@@ int
           @@@ runops() {
           @@@     SAVEI32(runlevel);
           @@@     runlevel++;
           @@@     while ( op = (*op->op_ppaddr)() );
           @@@     TAINT_NOT;
           @@@     return 0;
           @@@ }
           MAIN_INTERPRETER_LOOP

       Or with a fixed amount of leading whitespace, with remaining indentation correctly
       preserved:

           my $poem = fix<<EVER_ON_AND_ON;
              Now far ahead the Road has gone,
             And I must follow, if I can,
              Pursuing it with eager feet,
             Until it joins some larger way
              Where many paths and errands meet.
             And whither then? I cannot say.
               --Bilbo in /usr/src/perl/pp_ctl.c
           EVER_ON_AND_ON

       Beginning with Perl version 5.26, a much simpler and cleaner way to write indented here
       documents has been added to the language: the tilde (~) modifier. See "Indented Here-docs"
       in perlop for details.

Data: Arrays
   What is the difference between a list and an array?
       (contributed by brian d foy)

       A list is a fixed collection of scalars. An array is a variable that holds a variable
       collection of scalars. An array can supply its collection for list operations, so list
       operations also work on arrays:

           # slices
           ( 'dog', 'cat', 'bird' )[2,3];
           @animals[2,3];

           # iteration
           foreach ( qw( dog cat bird ) ) { ... }
           foreach ( @animals ) { ... }

           my @three = grep { length == 3 } qw( dog cat bird );
           my @three = grep { length == 3 } @animals;

           # supply an argument list
           wash_animals( qw( dog cat bird ) );
           wash_animals( @animals );

       Array operations, which change the scalars, rearrange them, or add or subtract some
       scalars, only work on arrays. These can't work on a list, which is fixed. Array operations
       include "shift", "unshift", "push", "pop", and "splice".

       An array can also change its length:

           $#animals = 1;  # truncate to two elements
           $#animals = 10000; # pre-extend to 10,001 elements

       You can change an array element, but you can't change a list element:

           $animals[0] = 'Rottweiler';
           qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!

           foreach ( @animals ) {
               s/^d/fr/;  # works fine
           }

           foreach ( qw( dog cat bird ) ) {
               s/^d/fr/;  # Error! Modification of read only value!
           }

       However, if the list element is itself a variable, it appears that you can change a list
       element. However, the list element is the variable, not the data. You're not changing the
       list element, but something the list element refers to. The list element itself doesn't
       change: it's still the same variable.

       You also have to be careful about context. You can assign an array to a scalar to get the
       number of elements in the array. This only works for arrays, though:

           my $count = @animals;  # only works with arrays

       If you try to do the same thing with what you think is a list, you get a quite different
       result. Although it looks like you have a list on the righthand side, Perl actually sees a
       bunch of scalars separated by a comma:

           my $scalar = ( 'dog', 'cat', 'bird' );  # $scalar gets bird

       Since you're assigning to a scalar, the righthand side is in scalar context. The comma
       operator (yes, it's an operator!) in scalar context evaluates its lefthand side, throws
       away the result, and evaluates it's righthand side and returns the result. In effect, that
       list-lookalike assigns to $scalar it's rightmost value. Many people mess this up because
       they choose a list-lookalike whose last element is also the count they expect:

           my $scalar = ( 1, 2, 3 );  # $scalar gets 3, accidentally

   What is the difference between $array[1] and @array[1]?
       (contributed by brian d foy)

       The difference is the sigil, that special character in front of the array name. The "$"
       sigil means "exactly one item", while the "@" sigil means "zero or more items". The "$"
       gets you a single scalar, while the "@" gets you a list.

       The confusion arises because people incorrectly assume that the sigil denotes the variable
       type.

       The $array[1] is a single-element access to the array. It's going to return the item in
       index 1 (or undef if there is no item there).  If you intend to get exactly one element
       from the array, this is the form you should use.

       The @array[1] is an array slice, although it has only one index.  You can pull out
       multiple elements simultaneously by specifying additional indices as a list, like
       @array[1,4,3,0].

       Using a slice on the lefthand side of the assignment supplies list context to the
       righthand side. This can lead to unexpected results.  For instance, if you want to read a
       single line from a filehandle, assigning to a scalar value is fine:

           $array[1] = <STDIN>;

       However, in list context, the line input operator returns all of the lines as a list. The
       first line goes into @array[1] and the rest of the lines mysteriously disappear:

           @array[1] = <STDIN>;  # most likely not what you want

       Either the "use warnings" pragma or the -w flag will warn you when you use an array slice
       with a single index.

   How can I remove duplicate elements from a list or array?
       (contributed by brian d foy)

       Use a hash. When you think the words "unique" or "duplicated", think "hash keys".

       If you don't care about the order of the elements, you could just create the hash then
       extract the keys. It's not important how you create that hash: just that you use "keys" to
       get the unique elements.

           my %hash   = map { $_, 1 } @array;
           # or a hash slice: @hash{ @array } = ();
           # or a foreach: $hash{$_} = 1 foreach ( @array );

           my @unique = keys %hash;

       If you want to use a module, try the "uniq" function from List::MoreUtils. In list context
       it returns the unique elements, preserving their order in the list. In scalar context, it
       returns the number of unique elements.

           use List::MoreUtils qw(uniq);

           my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7
           my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7

       You can also go through each element and skip the ones you've seen before. Use a hash to
       keep track. The first time the loop sees an element, that element has no key in %Seen. The
       "next" statement creates the key and immediately uses its value, which is "undef", so the
       loop continues to the "push" and increments the value for that key. The next time the loop
       sees that same element, its key exists in the hash and the value for that key is true
       (since it's not 0 or "undef"), so the next skips that iteration and the loop goes to the
       next element.

           my @unique = ();
           my %seen   = ();

           foreach my $elem ( @array ) {
               next if $seen{ $elem }++;
               push @unique, $elem;
           }

       You can write this more briefly using a grep, which does the same thing.

           my %seen = ();
           my @unique = grep { ! $seen{ $_ }++ } @array;

   How can I tell whether a certain element is contained in a list or array?
       (portions of this answer contributed by Anno Siegel and brian d foy)

       Hearing the word "in" is an indication that you probably should have used a hash, not a
       list or array, to store your data. Hashes are designed to answer this question quickly and
       efficiently. Arrays aren't.

       That being said, there are several ways to approach this. If you are going to make this
       query many times over arbitrary string values, the fastest way is probably to invert the
       original array and maintain a hash whose keys are the first array's values:

           my @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
           my %is_blue = ();
           for (@blues) { $is_blue{$_} = 1 }

       Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep
       the blues all in a hash in the first place.

       If the values are all small integers, you could use a simple indexed array. This kind of
       an array will take up less space:

           my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
           my @is_tiny_prime = ();
           for (@primes) { $is_tiny_prime[$_] = 1 }
           # or simply  @istiny_prime[@primes] = (1) x @primes;

       Now you check whether $is_tiny_prime[$some_number].

       If the values in question are integers instead of strings, you can save quite a lot of
       space by using bit strings instead:

           my @articles = ( 1..10, 150..2000, 2017 );
           undef $read;
           for (@articles) { vec($read,$_,1) = 1 }

       Now check whether "vec($read,$n,1)" is true for some $n.

       These methods guarantee fast individual tests but require a re-organization of the
       original list or array. They only pay off if you have to test multiple values against the
       same array.

       If you are testing only once, the standard module List::Util exports the function "any"
       for this purpose. It works by stopping once it finds the element. It's written in C for
       speed, and its Perl equivalent looks like this subroutine:

           sub any (&@) {
               my $code = shift;
               foreach (@_) {
                   return 1 if $code->();
               }
               return 0;
           }

       If speed is of little concern, the common idiom uses grep in scalar context (which returns
       the number of items that passed its condition) to traverse the entire list. This does have
       the benefit of telling you how many matches it found, though.

           my $is_there = grep $_ eq $whatever, @array;

       If you want to actually extract the matching elements, simply use grep in list context.

           my @matches = grep $_ eq $whatever, @array;

   How do I compute the difference of two arrays? How do I compute the intersection of two
       arrays?
       Use a hash. Here's code to do both and more. It assumes that each element is unique in a
       given array:

           my (@union, @intersection, @difference);
           my %count = ();
           foreach my $element (@array1, @array2) { $count{$element}++ }
           foreach my $element (keys %count) {
               push @union, $element;
               push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
           }

       Note that this is the symmetric difference, that is, all elements in either A or in B but
       not in both. Think of it as an xor operation.

   How do I test whether two arrays or hashes are equal?
       The following code works for single-level arrays. It uses a stringwise comparison, and
       does not distinguish defined versus undefined empty strings. Modify if you have other
       needs.

           $are_equal = compare_arrays(\@frogs, \@toads);

           sub compare_arrays {
               my ($first, $second) = @_;
               no warnings;  # silence spurious -w undef complaints
               return 0 unless @$first == @$second;
               for (my $i = 0; $i < @$first; $i++) {
                   return 0 if $first->[$i] ne $second->[$i];
               }
               return 1;
           }

       For multilevel structures, you may wish to use an approach more like this one. It uses the
       CPAN module FreezeThaw:

           use FreezeThaw qw(cmpStr);
           my @a = my @b = ( "this", "that", [ "more", "stuff" ] );

           printf "a and b contain %s arrays\n",
               cmpStr(\@a, \@b) == 0
               ? "the same"
               : "different";

       This approach also works for comparing hashes. Here we'll demonstrate two different
       answers:

           use FreezeThaw qw(cmpStr cmpStrHard);

           my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
           $a{EXTRA} = \%b;
           $b{EXTRA} = \%a;

           printf "a and b contain %s hashes\n",
           cmpStr(\%a, \%b) == 0 ? "the same" : "different";

           printf "a and b contain %s hashes\n",
           cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

       The first reports that both those the hashes contain the same data, while the second
       reports that they do not. Which you prefer is left as an exercise to the reader.

   How do I find the first array element for which a condition is true?
       To find the first array element which satisfies a condition, you can use the "first()"
       function in the List::Util module, which comes with Perl 5.8. This example finds the first
       element that contains "Perl".

           use List::Util qw(first);

           my $element = first { /Perl/ } @array;

       If you cannot use List::Util, you can make your own loop to do the same thing. Once you
       find the element, you stop the loop with last.

           my $found;
           foreach ( @array ) {
               if( /Perl/ ) { $found = $_; last }
           }

       If you want the array index, use the "firstidx()" function from "List::MoreUtils":

           use List::MoreUtils qw(firstidx);
           my $index = firstidx { /Perl/ } @array;

       Or write it yourself, iterating through the indices and checking the array element at each
       index until you find one that satisfies the condition:

           my( $found, $index ) = ( undef, -1 );
           for( $i = 0; $i < @array; $i++ ) {
               if( $array[$i] =~ /Perl/ ) {
                   $found = $array[$i];
                   $index = $i;
                   last;
               }
           }

   How do I handle linked lists?
       (contributed by brian d foy)

       Perl's arrays do not have a fixed size, so you don't need linked lists if you just want to
       add or remove items. You can use array operations such as "push", "pop", "shift",
       "unshift", or "splice" to do that.

       Sometimes, however, linked lists can be useful in situations where you want to "shard" an
       array so you have many small arrays instead of a single big array. You can keep arrays
       longer than Perl's largest array index, lock smaller arrays separately in threaded
       programs, reallocate less memory, or quickly insert elements in the middle of the chain.

       Steve Lembark goes through the details in his YAPC::NA 2009 talk "Perly Linked Lists" (
       <http://www.slideshare.net/lembark/perly-linked-lists> ), although you can just use his
       LinkedList::Single module.

   How do I handle circular lists?
       (contributed by brian d foy)

       If you want to cycle through an array endlessly, you can increment the index modulo the
       number of elements in the array:

           my @array = qw( a b c );
           my $i = 0;

           while( 1 ) {
               print $array[ $i++ % @array ], "\n";
               last if $i > 20;
           }

       You can also use Tie::Cycle to use a scalar that always has the next element of the
       circular array:

           use Tie::Cycle;

           tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];

           print $cycle; # FFFFFF
           print $cycle; # 000000
           print $cycle; # FFFF00

       The Array::Iterator::Circular creates an iterator object for circular arrays:

           use Array::Iterator::Circular;

           my $color_iterator = Array::Iterator::Circular->new(
               qw(red green blue orange)
               );

           foreach ( 1 .. 20 ) {
               print $color_iterator->next, "\n";
           }

   How do I shuffle an array randomly?
       If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or
       later installed, you can say:

           use List::Util 'shuffle';

           @shuffled = shuffle(@list);

       If not, you can use a Fisher-Yates shuffle.

           sub fisher_yates_shuffle {
               my $deck = shift;  # $deck is a reference to an array
               return unless @$deck; # must not be empty!

               my $i = @$deck;
               while (--$i) {
                   my $j = int rand ($i+1);
                   @$deck[$i,$j] = @$deck[$j,$i];
               }
           }

           # shuffle my mpeg collection
           #
           my @mpeg = <audio/*/*.mp3>;
           fisher_yates_shuffle( \@mpeg );    # randomize @mpeg in place
           print @mpeg;

       Note that the above implementation shuffles an array in place, unlike the
       "List::Util::shuffle()" which takes a list and returns a new shuffled list.

       You've probably seen shuffling algorithms that work using splice, randomly picking another
       element to swap the current element with

           srand;
           @new = ();
           @old = 1 .. 10;  # just a demo
           while (@old) {
               push(@new, splice(@old, rand @old, 1));
           }

       This is bad because splice is already O(N), and since you do it N times, you just invented
       a quadratic algorithm; that is, O(N**2).  This does not scale, although Perl is so
       efficient that you probably won't notice this until you have rather largish arrays.

   How do I process/modify each element of an array?
       Use "for"/"foreach":

           for (@lines) {
               s/foo/bar/;    # change that word
               tr/XZ/ZX/;    # swap those letters
           }

       Here's another; let's compute spherical volumes:

           my @volumes = @radii;
           for (@volumes) {   # @volumes has changed parts
               $_ **= 3;
               $_ *= (4/3) * 3.14159;  # this will be constant folded
           }

       which can also be done with "map()" which is made to transform one list into another:

           my @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;

       If you want to do the same thing to modify the values of the hash, you can use the
       "values" function. As of Perl 5.6 the values are not copied, so if you modify $orbit (in
       this case), you modify the value.

           for my $orbit ( values %orbits ) {
               ($orbit **= 3) *= (4/3) * 3.14159;
           }

       Prior to perl 5.6 "values" returned copies of the values, so older perl code often
       contains constructions such as @orbits{keys %orbits} instead of "values %orbits" where the
       hash is to be modified.

   How do I select a random element from an array?
       Use the "rand()" function (see "rand" in perlfunc):

           my $index   = rand @array;
           my $element = $array[$index];

       Or, simply:

           my $element = $array[ rand @array ];

   How do I permute N elements of a list?
       Use the List::Permutor module on CPAN. If the list is actually an array, try the
       Algorithm::Permute module (also on CPAN). It's written in XS code and is very efficient:

           use Algorithm::Permute;

           my @array = 'a'..'d';
           my $p_iterator = Algorithm::Permute->new ( \@array );

           while (my @perm = $p_iterator->next) {
              print "next permutation: (@perm)\n";
           }

       For even faster execution, you could do:

           use Algorithm::Permute;

           my @array = 'a'..'d';

           Algorithm::Permute::permute {
               print "next permutation: (@array)\n";
           } @array;

       Here's a little program that generates all permutations of all the words on each line of
       input. The algorithm embodied in the "permute()" function is discussed in Volume 4 (still
       unpublished) of Knuth's The Art of Computer Programming and will work on any list:

           #!/usr/bin/perl -n
           # Fischer-Krause ordered permutation generator

           sub permute (&@) {
               my $code = shift;
               my @idx = 0..$#_;
               while ( $code->(@_[@idx]) ) {
                   my $p = $#idx;
                   --$p while $idx[$p-1] > $idx[$p];
                   my $q = $p or return;
                   push @idx, reverse splice @idx, $p;
                   ++$q while $idx[$p-1] > $idx[$q];
                   @idx[$p-1,$q]=@idx[$q,$p-1];
               }
           }

           permute { print "@_\n" } split;

       The Algorithm::Loops module also provides the "NextPermute" and "NextPermuteNum" functions
       which efficiently find all unique permutations of an array, even if it contains duplicate
       values, modifying it in-place: if its elements are in reverse-sorted order then the array
       is reversed, making it sorted, and it returns false; otherwise the next permutation is
       returned.

       "NextPermute" uses string order and "NextPermuteNum" numeric order, so you can enumerate
       all the permutations of 0..9 like this:

           use Algorithm::Loops qw(NextPermuteNum);

           my @list= 0..9;
           do { print "@list\n" } while NextPermuteNum @list;

   How do I sort an array by (anything)?
       Supply a comparison function to sort() (described in "sort" in perlfunc):

           @list = sort { $a <=> $b } @list;

       The default sort function is cmp, string comparison, which would sort "(1, 2, 10)" into
       "(1, 10, 2)". "<=>", used above, is the numerical comparison operator.

       If you have a complicated function needed to pull out the part you want to sort on, then
       don't do it inside the sort function. Pull it out first, because the sort BLOCK can be
       called many times for the same element. Here's an example of how to pull out the first
       word after the first number on each item, and then sort those words case-insensitively.

           my @idx;
           for (@data) {
               my $item;
               ($item) = /\d+\s*(\S+)/;
               push @idx, uc($item);
           }
           my @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];

       which could also be written this way, using a trick that's come to be known as the
       Schwartzian Transform:

           my @sorted = map  { $_->[0] }
               sort { $a->[1] cmp $b->[1] }
               map  { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;

       If you need to sort on several fields, the following paradigm is useful.

           my @sorted = sort {
               field1($a) <=> field1($b) ||
               field2($a) cmp field2($b) ||
               field3($a) cmp field3($b)
           } @data;

       This can be conveniently combined with precalculation of keys as given above.

       See the sort article in the "Far More Than You Ever Wanted To Know" collection in
       <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> for more about this approach.

       See also the question later in perlfaq4 on sorting hashes.

   How do I manipulate arrays of bits?
       Use "pack()" and "unpack()", or else "vec()" and the bitwise operations.

       For example, you don't have to store individual bits in an array (which would mean that
       you're wasting a lot of space). To convert an array of bits to a string, use "vec()" to
       set the right bits. This sets $vec to have bit N set only if $ints[N] was set:

           my @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
           my $vec = '';
           foreach( 0 .. $#ints ) {
               vec($vec,$_,1) = 1 if $ints[$_];
           }

       The string $vec only takes up as many bits as it needs. For instance, if you had 16
       entries in @ints, $vec only needs two bytes to store them (not counting the scalar
       variable overhead).

       Here's how, given a vector in $vec, you can get those bits into your @ints array:

           sub bitvec_to_list {
               my $vec = shift;
               my @ints;
               # Find null-byte density then select best algorithm
               if ($vec =~ tr/\0// / length $vec > 0.95) {
                   use integer;
                   my $i;

                   # This method is faster with mostly null-bytes
                   while($vec =~ /[^\0]/g ) {
                       $i = -9 + 8 * pos $vec;
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                       push @ints, $i if vec($vec, ++$i, 1);
                   }
               }
               else {
                   # This method is a fast general algorithm
                   use integer;
                   my $bits = unpack "b*", $vec;
                   push @ints, 0 if $bits =~ s/^(\d)// && $1;
                   push @ints, pos $bits while($bits =~ /1/g);
               }

               return \@ints;
           }

       This method gets faster the more sparse the bit vector is.  (Courtesy of Tim Bunce and
       Winfried Koenig.)

       You can make the while loop a lot shorter with this suggestion from Benjamin Goldberg:

           while($vec =~ /[^\0]+/g ) {
               push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
           }

       Or use the CPAN module Bit::Vector:

           my $vector = Bit::Vector->new($num_of_bits);
           $vector->Index_List_Store(@ints);
           my @ints = $vector->Index_List_Read();

       Bit::Vector provides efficient methods for bit vector, sets of small integers and "big
       int" math.

       Here's a more extensive illustration using vec():

           # vec demo
           my $vector = "\xff\x0f\xef\xfe";
           print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
           unpack("N", $vector), "\n";
           my $is_set = vec($vector, 23, 1);
           print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
           pvec($vector);

           set_vec(1,1,1);
           set_vec(3,1,1);
           set_vec(23,1,1);

           set_vec(3,1,3);
           set_vec(3,2,3);
           set_vec(3,4,3);
           set_vec(3,4,7);
           set_vec(3,8,3);
           set_vec(3,8,7);

           set_vec(0,32,17);
           set_vec(1,32,17);

           sub set_vec {
               my ($offset, $width, $value) = @_;
               my $vector = '';
               vec($vector, $offset, $width) = $value;
               print "offset=$offset width=$width value=$value\n";
               pvec($vector);
           }

           sub pvec {
               my $vector = shift;
               my $bits = unpack("b*", $vector);
               my $i = 0;
               my $BASE = 8;

               print "vector length in bytes: ", length($vector), "\n";
               @bytes = unpack("A8" x length($vector), $bits);
               print "bits are: @bytes\n\n";
           }

   Why does defined() return true on empty arrays and hashes?
       The short story is that you should probably only use defined on scalars or functions, not
       on aggregates (arrays and hashes). See "defined" in perlfunc in the 5.004 release or later
       of Perl for more detail.

Data: Hashes (Associative Arrays)
   How do I process an entire hash?
       (contributed by brian d foy)

       There are a couple of ways that you can process an entire hash. You can get a list of
       keys, then go through each key, or grab a one key-value pair at a time.

       To go through all of the keys, use the "keys" function. This extracts all of the keys of
       the hash and gives them back to you as a list. You can then get the value through the
       particular key you're processing:

           foreach my $key ( keys %hash ) {
               my $value = $hash{$key}
               ...
           }

       Once you have the list of keys, you can process that list before you process the hash
       elements. For instance, you can sort the keys so you can process them in lexical order:

           foreach my $key ( sort keys %hash ) {
               my $value = $hash{$key}
               ...
           }

       Or, you might want to only process some of the items. If you only want to deal with the
       keys that start with "text:", you can select just those using "grep":

           foreach my $key ( grep /^text:/, keys %hash ) {
               my $value = $hash{$key}
               ...
           }

       If the hash is very large, you might not want to create a long list of keys. To save some
       memory, you can grab one key-value pair at a time using "each()", which returns a pair you
       haven't seen yet:

           while( my( $key, $value ) = each( %hash ) ) {
               ...
           }

       The "each" operator returns the pairs in apparently random order, so if ordering matters
       to you, you'll have to stick with the "keys" method.

       The "each()" operator can be a bit tricky though. You can't add or delete keys of the hash
       while you're using it without possibly skipping or re-processing some pairs after Perl
       internally rehashes all of the elements. Additionally, a hash has only one iterator, so if
       you mix "keys", "values", or "each" on the same hash, you risk resetting the iterator and
       messing up your processing. See the "each" entry in perlfunc for more details.

   How do I merge two hashes?
       (contributed by brian d foy)

       Before you decide to merge two hashes, you have to decide what to do if both hashes
       contain keys that are the same and if you want to leave the original hashes as they were.

       If you want to preserve the original hashes, copy one hash (%hash1) to a new hash
       (%new_hash), then add the keys from the other hash (%hash2 to the new hash. Checking that
       the key already exists in %new_hash gives you a chance to decide what to do with the
       duplicates:

           my %new_hash = %hash1; # make a copy; leave %hash1 alone

           foreach my $key2 ( keys %hash2 ) {
               if( exists $new_hash{$key2} ) {
                   warn "Key [$key2] is in both hashes!";
                   # handle the duplicate (perhaps only warning)
                   ...
                   next;
               }
               else {
                   $new_hash{$key2} = $hash2{$key2};
               }
           }

       If you don't want to create a new hash, you can still use this looping technique; just
       change the %new_hash to %hash1.

           foreach my $key2 ( keys %hash2 ) {
               if( exists $hash1{$key2} ) {
                   warn "Key [$key2] is in both hashes!";
                   # handle the duplicate (perhaps only warning)
                   ...
                   next;
               }
               else {
                   $hash1{$key2} = $hash2{$key2};
               }
             }

       If you don't care that one hash overwrites keys and values from the other, you could just
       use a hash slice to add one hash to another. In this case, values from %hash2 replace
       values from %hash1 when they have keys in common:

           @hash1{ keys %hash2 } = values %hash2;

   What happens if I add or remove keys from a hash while iterating over it?
       (contributed by brian d foy)

       The easy answer is "Don't do that!"

       If you iterate through the hash with each(), you can delete the key most recently returned
       without worrying about it. If you delete or add other keys, the iterator may skip or
       double up on them since perl may rearrange the hash table. See the entry for "each()" in
       perlfunc.

   How do I look up a hash element by value?
       Create a reverse hash:

           my %by_value = reverse %by_key;
           my $key = $by_value{$value};

       That's not particularly efficient. It would be more space-efficient to use:

           while (my ($key, $value) = each %by_key) {
               $by_value{$value} = $key;
           }

       If your hash could have repeated values, the methods above will only find one of the
       associated keys.  This may or may not worry you. If it does worry you, you can always
       reverse the hash into a hash of arrays instead:

           while (my ($key, $value) = each %by_key) {
                push @{$key_list_by_value{$value}}, $key;
           }

   How can I know how many entries are in a hash?
       (contributed by brian d foy)

       This is very similar to "How do I process an entire hash?", also in perlfaq4, but a bit
       simpler in the common cases.

       You can use the "keys()" built-in function in scalar context to find out have many entries
       you have in a hash:

           my $key_count = keys %hash; # must be scalar context!

       If you want to find out how many entries have a defined value, that's a bit different. You
       have to check each value. A "grep" is handy:

           my $defined_value_count = grep { defined } values %hash;

       You can use that same structure to count the entries any way that you like. If you want
       the count of the keys with vowels in them, you just test for that instead:

           my $vowel_count = grep { /[aeiou]/ } keys %hash;

       The "grep" in scalar context returns the count. If you want the list of matching items,
       just use it in list context instead:

           my @defined_values = grep { defined } values %hash;

       The "keys()" function also resets the iterator, which means that you may see strange
       results if you use this between uses of other hash operators such as "each()".

   How do I sort a hash (optionally by value instead of key)?
       (contributed by brian d foy)

       To sort a hash, start with the keys. In this example, we give the list of keys to the sort
       function which then compares them ASCIIbetically (which might be affected by your locale
       settings). The output list has the keys in ASCIIbetical order. Once we have the keys, we
       can go through them to create a report which lists the keys in ASCIIbetical order.

           my @keys = sort { $a cmp $b } keys %hash;

           foreach my $key ( @keys ) {
               printf "%-20s %6d\n", $key, $hash{$key};
           }

       We could get more fancy in the "sort()" block though. Instead of comparing the keys, we
       can compute a value with them and use that value as the comparison.

       For instance, to make our report order case-insensitive, we use "lc" to lowercase the keys
       before comparing them:

           my @keys = sort { lc $a cmp lc $b } keys %hash;

       Note: if the computation is expensive or the hash has many elements, you may want to look
       at the Schwartzian Transform to cache the computation results.

       If we want to sort by the hash value instead, we use the hash key to look it up. We still
       get out a list of keys, but this time they are ordered by their value.

           my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;

       From there we can get more complex. If the hash values are the same, we can provide a
       secondary sort on the hash key.

           my @keys = sort {
               $hash{$a} <=> $hash{$b}
                   or
               "\L$a" cmp "\L$b"
           } keys %hash;

   How can I always keep my hash sorted?
       You can look into using the "DB_File" module and "tie()" using the $DB_BTREE hash bindings
       as documented in "In Memory Databases" in DB_File. The Tie::IxHash module from CPAN might
       also be instructive. Although this does keep your hash sorted, you might not like the
       slowdown you suffer from the tie interface. Are you sure you need to do this? :)

   What's the difference between "delete" and "undef" with hashes?
       Hashes contain pairs of scalars: the first is the key, the second is the value. The key
       will be coerced to a string, although the value can be any kind of scalar: string, number,
       or reference. If a key $key is present in %hash, "exists($hash{$key})" will return true.
       The value for a given key can be "undef", in which case $hash{$key} will be "undef" while
       "exists $hash{$key}" will return true. This corresponds to ($key, "undef") being in the
       hash.

       Pictures help... Here's the %hash table:

             keys  values
           +------+------+
           |  a   |  3   |
           |  x   |  7   |
           |  d   |  0   |
           |  e   |  2   |
           +------+------+

       And these conditions hold

           $hash{'a'}                       is true
           $hash{'d'}                       is false
           defined $hash{'d'}               is true
           defined $hash{'a'}               is true
           exists $hash{'a'}                is true (Perl 5 only)
           grep ($_ eq 'a', keys %hash)     is true

       If you now say

           undef $hash{'a'}

       your table now reads:

             keys  values
           +------+------+
           |  a   | undef|
           |  x   |  7   |
           |  d   |  0   |
           |  e   |  2   |
           +------+------+

       and these conditions now hold; changes in caps:

           $hash{'a'}                       is FALSE
           $hash{'d'}                       is false
           defined $hash{'d'}               is true
           defined $hash{'a'}               is FALSE
           exists $hash{'a'}                is true (Perl 5 only)
           grep ($_ eq 'a', keys %hash)     is true

       Notice the last two: you have an undef value, but a defined key!

       Now, consider this:

           delete $hash{'a'}

       your table now reads:

             keys  values
           +------+------+
           |  x   |  7   |
           |  d   |  0   |
           |  e   |  2   |
           +------+------+

       and these conditions now hold; changes in caps:

           $hash{'a'}                       is false
           $hash{'d'}                       is false
           defined $hash{'d'}               is true
           defined $hash{'a'}               is false
           exists $hash{'a'}                is FALSE (Perl 5 only)
           grep ($_ eq 'a', keys %hash)     is FALSE

       See, the whole entry is gone!

   Why don't my tied hashes make the defined/exists distinction?
       This depends on the tied hash's implementation of EXISTS().  For example, there isn't the
       concept of undef with hashes that are tied to DBM* files. It also means that exists() and
       defined() do the same thing with a DBM* file, and what they end up doing is not what they
       do with ordinary hashes.

   How do I reset an each() operation part-way through?
       (contributed by brian d foy)

       You can use the "keys" or "values" functions to reset "each". To simply reset the iterator
       used by "each" without doing anything else, use one of them in void context:

           keys %hash; # resets iterator, nothing else.
           values %hash; # resets iterator, nothing else.

       See the documentation for "each" in perlfunc.

   How can I get the unique keys from two hashes?
       First you extract the keys from the hashes into lists, then solve the "removing
       duplicates" problem described above. For example:

           my %seen = ();
           for my $element (keys(%foo), keys(%bar)) {
               $seen{$element}++;
           }
           my @uniq = keys %seen;

       Or more succinctly:

           my @uniq = keys %{{%foo,%bar}};

       Or if you really want to save space:

           my %seen = ();
           while (defined ($key = each %foo)) {
               $seen{$key}++;
           }
           while (defined ($key = each %bar)) {
               $seen{$key}++;
           }
           my @uniq = keys %seen;

   How can I store a multidimensional array in a DBM file?
       Either stringify the structure yourself (no fun), or else get the MLDBM (which uses
       Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File. You
       might also try DBM::Deep, but it can be a bit slow.

   How can I make my hash remember the order I put elements into it?
       Use the Tie::IxHash from CPAN.

           use Tie::IxHash;

           tie my %myhash, 'Tie::IxHash';

           for (my $i=0; $i<20; $i++) {
               $myhash{$i} = 2*$i;
           }

           my @keys = keys %myhash;
           # @keys = (0,1,2,3,...)

   Why does passing a subroutine an undefined element in a hash create it?
       (contributed by brian d foy)

       Are you using a really old version of Perl?

       Normally, accessing a hash key's value for a nonexistent key will not create the key.

           my %hash  = ();
           my $value = $hash{ 'foo' };
           print "This won't print\n" if exists $hash{ 'foo' };

       Passing $hash{ 'foo' } to a subroutine used to be a special case, though.  Since you could
       assign directly to $_[0], Perl had to be ready to make that assignment so it created the
       hash key ahead of time:

           my_sub( $hash{ 'foo' } );
           print "This will print before 5.004\n" if exists $hash{ 'foo' };

           sub my_sub {
               # $_[0] = 'bar'; # create hash key in case you do this
               1;
           }

       Since Perl 5.004, however, this situation is a special case and Perl creates the hash key
       only when you make the assignment:

           my_sub( $hash{ 'foo' } );
           print "This will print, even after 5.004\n" if exists $hash{ 'foo' };

           sub my_sub {
               $_[0] = 'bar';
           }

       However, if you want the old behavior (and think carefully about that because it's a weird
       side effect), you can pass a hash slice instead.  Perl 5.004 didn't make this a special
       case:

           my_sub( @hash{ qw/foo/ } );

   How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or
       arrays?
       Usually a hash ref, perhaps like this:

           $record = {
               NAME   => "Jason",
               EMPNO  => 132,
               TITLE  => "deputy peon",
               AGE    => 23,
               SALARY => 37_000,
               PALS   => [ "Norbert", "Rhys", "Phineas"],
           };

       References are documented in perlref and perlreftut.  Examples of complex data structures
       are given in perldsc and perllol. Examples of structures and object-oriented classes are
       in perlootut.

   How can I use a reference as a hash key?
       (contributed by brian d foy and Ben Morrow)

       Hash keys are strings, so you can't really use a reference as the key.  When you try to do
       that, perl turns the reference into its stringified form (for instance,
       "HASH(0xDEADBEEF)"). From there you can't get back the reference from the stringified
       form, at least without doing some extra work on your own.

       Remember that the entry in the hash will still be there even if the referenced variable
       goes out of scope, and that it is entirely possible for Perl to subsequently allocate a
       different variable at the same address. This will mean a new variable might accidentally
       be associated with the value for an old.

       If you have Perl 5.10 or later, and you just want to store a value against the reference
       for lookup later, you can use the core Hash::Util::Fieldhash module. This will also handle
       renaming the keys if you use multiple threads (which causes all variables to be
       reallocated at new addresses, changing their stringification), and garbage-collecting the
       entries when the referenced variable goes out of scope.

       If you actually need to be able to get a real reference back from each hash entry, you can
       use the Tie::RefHash module, which does the required work for you.

   How can I check if a key exists in a multilevel hash?
       (contributed by brian d foy)

       The trick to this problem is avoiding accidental autovivification. If you want to check
       three keys deep, you might naively try this:

           my %hash;
           if( exists $hash{key1}{key2}{key3} ) {
               ...;
           }

       Even though you started with a completely empty hash, after that call to "exists" you've
       created the structure you needed to check for "key3":

           %hash = (
                     'key1' => {
                                 'key2' => {}
                               }
                   );

       That's autovivification. You can get around this in a few ways. The easiest way is to just
       turn it off. The lexical "autovivification" pragma is available on CPAN. Now you don't add
       to the hash:

           {
               no autovivification;
               my %hash;
               if( exists $hash{key1}{key2}{key3} ) {
                   ...;
               }
           }

       The Data::Diver module on CPAN can do it for you too. Its "Dive" subroutine can tell you
       not only if the keys exist but also get the value:

           use Data::Diver qw(Dive);

           my @exists = Dive( \%hash, qw(key1 key2 key3) );
           if(  ! @exists  ) {
               ...; # keys do not exist
           }
           elsif(  ! defined $exists[0]  ) {
               ...; # keys exist but value is undef
           }

       You can easily do this yourself too by checking each level of the hash before you move
       onto the next level. This is essentially what Data::Diver does for you:

           if( check_hash( \%hash, qw(key1 key2 key3) ) ) {
               ...;
           }

           sub check_hash {
              my( $hash, @keys ) = @_;

              return unless @keys;

              foreach my $key ( @keys ) {
                  return unless eval { exists $hash->{$key} };
                  $hash = $hash->{$key};
               }

              return 1;
           }

   How can I prevent addition of unwanted keys into a hash?
       Since version 5.8.0, hashes can be restricted to a fixed number of given keys. Methods for
       creating and dealing with restricted hashes are exported by the Hash::Util module.

Data: Misc
   How do I handle binary data correctly?
       Perl is binary-clean, so it can handle binary data just fine.  On Windows or DOS, however,
       you have to use "binmode" for binary files to avoid conversions for line endings. In
       general, you should use "binmode" any time you want to work with binary data.

       Also see "binmode" in perlfunc or perlopentut.

       If you're concerned about 8-bit textual data then see perllocale.  If you want to deal
       with multibyte characters, however, there are some gotchas. See the section on Regular
       Expressions.

   How do I determine whether a scalar is a number/whole/integer/float?
       Assuming that you don't care about IEEE notations like "NaN" or "Infinity", you probably
       just want to use a regular expression (see also perlretut and perlre):

           use 5.010;

           if ( /\D/ )
               { say "\thas nondigits"; }
           if ( /^\d+\z/ )
               { say "\tis a whole number"; }
           if ( /^-?\d+\z/ )
               { say "\tis an integer"; }
           if ( /^[+-]?\d+\z/ )
               { say "\tis a +/- integer"; }
           if ( /^-?(?:\d+\.?|\.\d)\d*\z/ )
               { say "\tis a real number"; }
           if ( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i )
               { say "\tis a C float" }

       There are also some commonly used modules for the task.  Scalar::Util (distributed with
       5.8) provides access to perl's internal function "looks_like_number" for determining
       whether a variable looks like a number. Data::Types exports functions that validate data
       types using both the above and other regular expressions. Thirdly, there is Regexp::Common
       which has regular expressions to match various types of numbers. Those three modules are
       available from the CPAN.

       If you're on a POSIX system, Perl supports the "POSIX::strtod" function for converting
       strings to doubles (and also "POSIX::strtol" for longs). Its semantics are somewhat
       cumbersome, so here's a "getnum" wrapper function for more convenient access. This
       function takes a string and returns the number it found, or "undef" for input that isn't a
       C float. The "is_numeric" function is a front end to "getnum" if you just want to say, "Is
       this a float?"

           sub getnum {
               use POSIX qw(strtod);
               my $str = shift;
               $str =~ s/^\s+//;
               $str =~ s/\s+$//;
               $! = 0;
               my($num, $unparsed) = strtod($str);
               if (($str eq '') || ($unparsed != 0) || $!) {
                       return undef;
               }
               else {
                   return $num;
               }
           }

           sub is_numeric { defined getnum($_[0]) }

       Or you could check out the String::Scanf module on the CPAN instead.

   How do I keep persistent data across program calls?
       For some specific applications, you can use one of the DBM modules.  See AnyDBM_File. More
       generically, you should consult the FreezeThaw or Storable modules from CPAN. Starting
       from Perl 5.8, Storable is part of the standard distribution. Here's one example using
       Storable's "store" and "retrieve" functions:

           use Storable;
           store(\%hash, "filename");

           # later on...
           $href = retrieve("filename");        # by ref
           %hash = %{ retrieve("filename") };   # direct to hash

   How do I print out or copy a recursive data structure?
       The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great for printing out
       data structures. The Storable module on CPAN (or the 5.8 release of Perl), provides a
       function called "dclone" that recursively copies its argument.

           use Storable qw(dclone);
           $r2 = dclone($r1);

       Where $r1 can be a reference to any kind of data structure you'd like.  It will be deeply
       copied. Because "dclone" takes and returns references, you'd have to add extra punctuation
       if you had a hash of arrays that you wanted to copy.

           %newhash = %{ dclone(\%oldhash) };

   How do I define methods for every class/object?
       (contributed by Ben Morrow)

       You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be very careful to
       consider the consequences of doing this: adding methods to every object is very likely to
       have unintended consequences. If possible, it would be better to have all your object
       inherit from some common base class, or to use an object system like Moose that supports
       roles.

   How do I verify a credit card checksum?
       Get the Business::CreditCard module from CPAN.

   How do I pack arrays of doubles or floats for XS code?
       The arrays.h/arrays.c code in the PGPLOT module on CPAN does just this.  If you're doing a
       lot of float or double processing, consider using the PDL module from CPAN instead--it
       makes number-crunching easy.

       See <https://metacpan.org/release/PGPLOT> for the code.

AUTHOR AND COPYRIGHT
       Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other authors as noted.
       All rights reserved.

       This documentation is free; you can redistribute it and/or modify it under the same terms
       as Perl itself.

       Irrespective of its distribution, all code examples in this file are hereby placed into
       the public domain. You are permitted and encouraged to use this code in your own programs
       for fun or for profit as you see fit. A simple comment in the code giving credit would be
       courteous but is not required.

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