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

NNAAMMEE
       perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date:
       1999/01/08 04:26:39 $)

DDEESSCCRRIIPPTTIIOONN
       The section of the FAQ answers question related to the
       manipulation of data as numbers, dates, strings, arrays,
       hashes, and miscellaneous data issues.

DDaattaa:: NNuummbbeerrss
       WWhhyy aamm II ggeettttiinngg lloonngg ddeecciimmaallss ((eegg,, 1199..99449999999999999999999999))
       iinnsstteeaadd ooff tthhee nnuummbbeerrss II sshhoouulldd bbee ggeettttiinngg ((eegg,, 1199..9955))??

       The infinite set that a mathematician thinks of as the
       real numbers can only be approximate on a computer, since
       the computer only has a finite number of bits to store an
       infinite number of, um, numbers.

       Internally, your computer represents floating-point
       numbers in binary.  Floating-point numbers read in from a
       file or appearing as literals in your program are
       converted from their decimal floating-point representation
       (eg, 19.95) to the internal binary representation.

       However, 19.95 can't be precisely represented as a binary
       floating-point number, just like 1/3 can't be exactly
       represented as a decimal floating-point number.  The
       computer's binary representation of 19.95, therefore,
       isn't exactly 19.95.

       When a floating-point number gets printed, the binary
       floating-point representation is converted back to
       decimal.  These decimal numbers are displayed in either
       the format you specify with printf(), or the current
       output format for numbers (see the section on $# in the
       perlvar manpage if you use print.  $# has a different
       default value in Perl5 than it did in Perl4.  Changing $#
       yourself is deprecated.

       This affects aallll computer languages that represent decimal
       floating-point numbers in binary, not just Perl.  Perl
       provides arbitrary-precision decimal numbers with the
       Math::BigFloat module (part of the standard Perl
       distribution), but mathematical operations are
       consequently slower.

       To get rid of the superfluous digits, just use a format
       (eg, printf("%.2f", 19.95)) to get the required precision.
       See the section on Floating-point Arithmetic in the perlop
       manpage.

       WWhhyy iissnn''tt mmyy ooccttaall ddaattaa iinntteerrpprreetteedd ccoorrrreeccttllyy??

       Perl only understands octal and hex numbers as such when
       they occur as literals in your program.  If they are read
       in from somewhere and assigned, no automatic conversion
       takes place.  You must explicitly use oct() or hex() if
       you want the values converted.  oct() interprets both hex
       ("0x350") numbers and octal ones ("0350" or even without
       the leading "0", like "377"), while hex() only converts
       hexadecimal ones, with or without a leading "0x", like
       "0x255", "3A", "ff", or "deadbeef".

       This problem shows up most often when people try using
       chmod(), mkdir(), umask(), or sysopen(), which all want
       permissions in octal.

           chmod(644,  $file); # WRONG -- perl -w catches this
           chmod(0644, $file); # right

       DDooeess PPeerrll hhaavvee aa 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;
           $ceil   = ceil(3.5);                        # 4
           $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 rounding is being used by Perl,
       but to instead implement the rounding function you need
       yourself.

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

           for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}

           0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
           0.8 0.8 0.9 0.9 1.0 1.0

       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.

       HHooww ddoo II ccoonnvveerrtt bbiittss iinnttoo iinnttss??

       To turn a string of 1s and 0s like 10110110 into a scalar
       containing its binary value, use the pack() function
       (documented in the section on pack in the perlfunc
       manpage):

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

       Here's an example of going the other way:

           $binary_string = join('', unpack('B*', "\x29"));

       WWhhyy ddooeessnn''tt &<b>& wwoorrkk tthhee wwaayy II wwaanntt iitt ttoo??

       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 1).  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.  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]/) {
               # ...
           }

       HHooww ddoo II mmuullttiippllyy mmaattrriicceess??

       Use the Math::Matrix or Math::MatrixReal modules
       (available from CPAN) or the PDL extension (also available
       from CPAN).

       HHooww ddoo II ppeerrffoorrmm aann ooppeerraattiioonn oonn aa sseerriieess ooff iinntteeggeerrss??

       To call a function on each element in an array, and
       collect the results, use:

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

       For example:

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

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

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

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

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

       but you should be aware that the .. operator creates an
       array of all integers in the range.  This can take a lot
       of memory for large ranges.  Instead use:

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

       HHooww ccaann II oouuttppuutt RRoommaann nnuummeerraallss??

       Get the http://www.perl.com/CPAN/modules/by-module/Roman
       module.

       WWhhyy aarreenn''tt mmyy rraannddoomm nnuummbbeerrss rraannddoomm??

       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.  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 :-).
       http://www.perl.com/CPAN/doc/FMTEYEWTK/random, 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.''

       If you want numbers that are more random than rand with
       srand provides, you should also check out the
       Math::TrulyRandom module from CPAN.  It uses the
       imperfections in your system's timer to generate random
       numbers, but this takes quite a while.  If you want a
       better pseudorandom generator than comes with your
       operating system, look at ``Numerical Recipes in C'' at
       http://www.nr.com/ .

DDaattaa:: DDaatteess
       HHooww ddoo II ffiinndd tthhee wweeeekk--ooff--tthhee--yyeeaarr//ddaayy--ooff--tthhee--yyeeaarr??

       The day of the year is in the array returned by
       localtime() (see the section on localtime in the perlfunc
       manpage):

           $day_of_year = (localtime(time()))[7];

       or more legibly (in 5.004 or higher):

           use Time::localtime;
           $day_of_year = localtime(time())->yday;

       You can find the week of the year by dividing this by 7:

           $week_of_year = int($day_of_year / 7);

       Of course, this believes that weeks start at zero.  The
       Date::Calc module from CPAN has a lot of date calculation
       functions, including day of the year, week of the year,
       and so on.   Note that not all businesses consider ``week
       1'' to be the same; for example, American businesses often
       consider the first week with a Monday in it to be Work
       Week #1, despite ISO 8601, which considers WW1 to be the
       first week with a Thursday in it.

       HHooww ccaann II ccoommppaarree ttwwoo ddaatteess aanndd ffiinndd tthhee ddiiffffeerreennccee??

       If you're storing your dates as epoch seconds then simply
       subtract one from the other.  If you've got a structured
       date (distinct year, day, month, hour, minute, seconds
       values) then use one of the Date::Manip and Date::Calc
       modules from CPAN.

       HHooww ccaann II ttaakkee aa ssttrriinngg aanndd ttuurrnn iitt iinnttoo eeppoocchh sseeccoonnddss??

       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 and Date::Manip
       modules from CPAN.

       HHooww ccaann II ffiinndd tthhee JJuulliiaann DDaayy??

       Neither Date::Manip nor Date::Calc deal with Julian days.
       Instead, there is an example of Julian date calculation
       that should help you in Time::JulianDay (part of the Time-
       modules bundle) which can be found at
       http://www.perl.com/CPAN/modules/by-module/Time/.

       HHooww ddoo II ffiinndd yyeesstteerrddaayy''ss ddaattee??

       The time() function returns the current time in seconds
       since the epoch.  Take one day off that:

           $yesterday = time() - ( 24 * 60 * 60 );

       Then you can pass this to localtime() and get the
       individual year, month, day, hour, minute, seconds values.

       DDooeess PPeerrll hhaavvee aa yyeeaarr 22000000 pprroobblleemm??  IIss PPeerrll YY22KK
       ccoommpplliiaanntt??

       Short answer: No, Perl does not have a Year 2000 problem.
       Yes, Perl is Y2K compliant (whatever that means).  The
       programmers you've hired to use it, however, probably are
       not.

       Long answer: The question belies a true understanding of
       the issue.  Perl is just as Y2K compliant as your
       pencil--no more, and no less.  Can you use your pencil to
       write a non-Y2K-compliant memo?  Of course you can.  Is
       that the pencil's fault?  Of course it isn't.

       The date and time functions supplied with perl (gmtime and
       localtime) supply adequate information to determine the
       year well beyond 2000 (2038 is when trouble strikes for
       32-bit machines).  The year returned by these functions
       when used in an array context is the year minus 1900.  For
       years between 1910 and 1999 this happens to be a 2-digit
       decimal number. To avoid the year 2000 problem simply do
       not treat the year as a 2-digit number.  It isn't.

       When gmtime() and localtime() are used in scalar context
       they return a timestamp string that contains a fully-
       expanded year.  For example, $timestamp =
       gmtime(1005613200) sets $timestamp to "Tue Nov 13 01:00:00
       2001".  There's no year 2000 problem here.

       That doesn't mean that Perl can't be used to create non-
       Y2K compliant programs.  It can.  But so can your pencil.
       It's the fault of the user, not the language.  At the risk
       of inflaming the NRA: ``Perl doesn't break Y2K, people
       do.''  See http://language.perl.com/news/y2k.html for a
       longer exposition.

DDaattaa:: SSttrriinnggss
       HHooww ddoo II vvaalliiddaattee iinnppuutt??

       The answer to this question is usually a regular
       expression, perhaps with auxiliary logic.  See the more
       specific questions (numbers, mail addresses, etc.) for
       details.

       HHooww ddoo II uunneessccaappee aa ssttrriinngg??

       It depends just what you mean by ``escape''.  URL escapes
       are dealt with in the perlfaq9 manpage.  Shell escapes
       with the backslash (\) character are removed with:

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

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

       HHooww ddoo II rreemmoovvee ccoonnsseeccuuttiivvee ppaaiirrss ooff cchhaarraacctteerrss??

       To turn "abbcccd" into "abccd":

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

       HHooww ddoo II eexxppaanndd ffuunnccttiioonn ccaallllss iinn aa ssttrriinngg??

       This is documented in the perlref manpage.  In general,
       this is fraught with quoting and readability problems, but
       it is possible.  To interpolate a subroutine call (in list
       context) into a string:

           print "My sub returned @{[mysub(1,2,3)]} that time.\n";

       If you prefer scalar context, similar chicanery is also
       useful for arbitrary expressions:

           print "That yields ${\($n + 5)} widgets\n";

       Version 5.004 of Perl had a bug that gave list context to
       the expression in ${...}, but this is fixed in version
       5.005.

       See also ``How can I expand variables in text strings?''
       in this section of the FAQ.

       HHooww ddoo II ffiinndd mmaattcchhiinngg//nneessttiinngg aannyytthhiinngg??

       This isn't something that can be done in one regular
       expression, no matter how complicated.  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.  But none of these deals with nested patterns, nor
       can they.  For that you'll have to write a parser.

       If you are serious about writing a parser, there are a
       number of modules or oddities that will make your life a
       lot easier.  There is the CPAN module Parse::RecDescent,
       the standard module Text::Balanced, the byacc program, the
       CPAN module Parse::Yapp, and Mark-Jason Dominus's
       excellent py tool at http://www.plover.com/~mjd/perl/py/ .

       One simple destructive, inside-out approach that you might
       try is to pull out the smallest nesting parts one at a
       time:

           while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
               # do something with $1
           }

       A more complicated and sneaky approach is to make Perl's
       regular expression engine do it for you.  This is courtesy
       Dean Inada, and rather has the nature of an Obfuscated
       Perl Contest entry, but it really does work:

           # $_ contains the string to parse
           # BEGIN and END are the opening and closing markers for the
           # nested text.

           @( = ('(','');
           @) = (')','');
           ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
           @$ = (eval{/$re/},$@!~/unmatched/);
           print join("\n",@$[0..$#$]) if( $$[-1] );

       HHooww ddoo II rreevveerrssee aa ssttrriinngg??

       Use reverse() in scalar context, as documented in the
       reverse entry in the perlfunc manpage.

           $reversed = reverse $string;

       HHooww ddoo II eexxppaanndd ttaabbss iinn aa ssttrriinngg??

       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;
           @expanded_lines = expand(@lines_with_tabs);

       HHooww ddoo II rreeffoorrmmaatt aa ppaarraaggrraapphh??

       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).

       HHooww ccaann II aacccceessss//cchhaannggee tthhee ffiirrsstt NN lleetttteerrss ooff aa ssttrriinngg??

       There are many ways.  If you just want to grab a copy, use
       substr():

           $first_byte = substr($a, 0, 1);

       If you want to modify part of a string, the simplest way
       is often to use substr() as an lvalue:

           substr($a, 0, 3) = "Tom";

       Although those with a pattern matching kind of thought
       process will likely prefer:

           $a =~ s/^.../Tom/;

       HHooww ddoo II cchhaannggee tthhee NNtthh ooccccuurrrreennccee ooff ssoommeetthhiinngg??

       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.

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

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

           $WANT = 3;
           $count = 0;
           while (/(\w+)\s+fish\b/gi) {
               if (++$count == $WANT) {
                   print "The third fish is a $1 one.\n";
                   # Warning: don't `last' out of this loop
               }
           }

       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;

       HHooww ccaann II ccoouunntt tthhee nnuummbbeerr ooff ooccccuurrrreenncceess ooff aa ssuubbssttrriinngg
       wwiitthhiinn aa ssttrriinngg??

       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:

           $string = "ThisXlineXhasXsomeXx'sXinXit";
           $count = ($string =~ tr/X//);
           print "There are $count X charcters 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:

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

       HHooww ddoo II ccaappiittaalliizzee aallll tthhee wwoorrddss oonn oonnee lliinnee??

       To make the first letter of each word upper case:

               $line =~ s/\b(\w)/\U$1/g;

       This has the strange effect of turning "don't do it" into
       "Don'T Do It".  Sometimes you might want this, instead
       (Suggested by Brian Foy):

           $string =~ s/ (
                        (^\w)    #at the beginning of the line
                          |      # or
                        (\s\w)   #preceded by whitespace
                          )
                       /\U$1/xg;
           $string =~ /([\w']+)/\u\L$1/g;

       To make the whole line upper case:

               $line = uc($line);

       To force each word to be lower case, with the first letter
       upper case:

               $line =~ s/(\w+)/\u\L$1/g;

       You can (and probably should) enable locale awareness of
       those characters by placing a use locale pragma in your
       program.  See the perllocale manpage for endless details
       on locales.

       This is sometimes referred to as putting something into
       "title case", but that's not quite accurate.  Consdier the
       proper capitalization of the movie Dr. Strangelove or: How
       I Learned to Stop Worrying and Love the Bomb, for example.

       HHooww ccaann II sspplliitt aa [[cchhaarraacctteerr]] ddeelliimmiitteedd ssttrriinngg eexxcceepptt wwhheenn
       iinnssiiddee [[cchhaarraacctteerr]]?? ((CCoommmmaa--sseeppaarraatteedd ffiilleess))

       Take the example case of trying to split a string that is
       comma-separated into its different fields.  (We'll pretend
       you said comma-separated, not comma-delimited, which is
       different and almost never what you mean.) 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 a highly recommended book on regular
       expressions, to handle these for us.  He suggests
       (assuming your string is contained in $text):

            @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\"".  Unescaping them is a
       task addressed earlier in this section.

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

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

       There's also a Text::CSV module on CPAN.

       HHooww ddoo II ssttrriipp bbllaannkk ssppaaccee ffrroomm tthhee bbeeggiinnnniinngg//eenndd ooff aa
       ssttrriinngg??

       Although the simplest approach would seem to be:

           $string =~ s/^\s*(.*?)\s*$/$1/;

       This is unnecessarily slow, destructive, and fails with
       embedded newlines.  It is much better faster to do this in
       two steps:

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

       Or more nicely written as:

           for ($string) {
               s/^\s+//;
               s/\s+$//;
           }

       This idiom takes advantage of the foreach loop's aliasing
       behavior to factor out common code.  You can do this on
       several strings at once, or arrays, or even the values of
       a hash if you use a slide:

           # trim whitespace in the scalar, the array,
           # and all the values in the hash
           foreach ($scalar, @array, @hash{keys %hash}) {
               s/^\s+//;
               s/\s+$//;
           }

       HHooww ddoo II ppaadd aa ssttrriinngg wwiitthh bbllaannkkss oorr ppaadd aa nnuummbbeerr wwiitthh
       zzeerrooeess??

       (This answer contributed by Uri Guttman)

       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.

       The simplest method use the sprintf function. It can pad
       on the left or right with blanks and on the left with
       zeroes.

           # Left padding with blank:
           $padded = sprintf( "%${pad_len}s", $text ) ;

           # Right padding with blank:
           $padded = sprintf( "%${pad_len}s", $text ) ;

           # Left padding with 0:
           $padded = sprintf( "%0${pad_len}d", $num ) ;

       If you need to pad with a character other than blank or
       zero you can use one of the following methods.

       These methods generate a pad string with the x operator
       and concatenate that with the original text.

       Left and right padding with any character:

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

       Or you can left or right pad $text directly:

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

       HHooww ddoo II eexxttrraacctt sseelleecctteedd ccoolluummnnss ffrroomm aa ssttrriinngg??

       Use substr() or unpack(), both documented in the perlfunc
       manpage.  If you prefer thinking in terms of columns
       instead of widths, you can use this kind of thing:

           # determine the unpack format needed to split Linux ps output
           # arguments are cut columns
           my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

           sub cut2fmt {
               my(@positions) = @_;
               my $template  = '';
               my $lastpos   = 1;
               for my $place (@positions) {
                   $template .= "A" . ($place - $lastpos) . " ";
                   $lastpos   = $place;
               }
               $template .= "A*";
               return $template;
           }

       HHooww ddoo II ffiinndd tthhee ssoouunnddeexx vvaalluuee ooff aa ssttrriinngg??

       Use the standard Text::Soundex module distributed with
       perl.

       HHooww ccaann II eexxppaanndd vvaarriiaabblleess iinn tteexxtt ssttrriinnggss??

       Let's assume that you have a string like:

           $text = 'this has a $foo in it and a $bar';

       If those were both global variables, then this would
       suffice:

           $text =~ s/\$(\w+)/${$1}/g;  # no /e needed

       But since they are probably lexicals, or at least, they
       could be, you'd have to do this:

           $text =~ s/(\$\w+)/$1/eeg;
           die if $@;                  # needed /ee, not /e

       It's probably better in the general case to treat those
       variables as entries in some special hash.  For example:

           %user_defs = (
               foo  => 23,
               bar  => 19,
           );
           $text =~ s/\$(\w+)/$user_defs{$1}/g;

       See also ``How do I expand function calls in a string?''
       in this section of the FAQ.

       WWhhaatt''ss wwrroonngg wwiitthh aallwwaayyss qquuoottiinngg """"$$vvaarrss""""??

       The problem is that those double-quotes force
       stringification, coercing numbers and references into
       strings, even when you don't want them to be.  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
           $new = "$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;
           $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.

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

       WWhhyy ddoonn''tt mmyy <<b><<<b><HHEERREE ddooccuummeennttss wwoorrkk??

       Check for these three things:

       1. There must be no space after the << part.

       2. There (probably) should be a semicolon at the end.

       3. You can't (easily) have any space in front of the tag.

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

           # all in one
           ($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.

           ($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 off.  Otherwise, it takes the amount of
       leading white space found on the first line and removes
       that much off each subsequent line.

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

       This works with leading special strings, dynamically
       determined:

           $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 white space, with
       remaining indentation correctly preserved:

           $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

DDaattaa:: AArrrraayyss
       WWhhaatt iiss tthhee ddiiffffeerreennccee bbeettwweeeenn aa lliisstt aanndd aann aarrrraayy??

       An array has a changeable length.  A list does not.  An
       array is something you can push or pop, while a list is a
       set of values.  Some people make the distinction that a
       list is a value while an array is a variable.  Subroutines
       are passed and return lists, you put things into list
       context, you initialize arrays with lists, and you
       foreach() across a list.  @ variables are arrays,
       anonymous arrays are arrays, arrays in scalar context
       behave like the number of elements in them, subroutines
       access their arguments through the array @_,
       push/pop/shift only work on arrays.

       As a side note, there's no such thing as a list in scalar
       context.  When you say

           $scalar = (2, 5, 7, 9);

       you're using the comma operator in scalar context, so it
       evaluates the left hand side, then evaluates and returns
       the left hand side.  This causes the last value to be
       returned: 9.

       WWhhaatt iiss tthhee ddiiffffeerreennccee bbeettwweeeenn $$aarrrraayy[1] and @array[1]?

       The former is a scalar value, the latter an array slice,
       which makes it a list with one (scalar) value.  You should
       use $ when you want a scalar value (most of the time) and
       @ when you want a list with one scalar value in it (very,
       very rarely; nearly never, in fact).

       Sometimes it doesn't make a difference, but sometimes it
       does.  For example, compare:

           $good[0] = `some program that outputs several lines`;

       with

           @bad[0]  = `same program that outputs several lines`;

       The --ww flag will warn you about these matters.

       HHooww ccaann II eexxttrraacctt jjuusstt tthhee uunniiqquuee eelleemmeennttss ooff aann aarrrraayy??

       There are several possible ways, depending on whether the
       array is ordered and whether you wish to preserve the
       ordering.

       a) If @in is sorted, and you want @out to be sorted: (this
           assumes all true values in the array)

               $prev = 'nonesuch';
               @out = grep($_ ne $prev && ($prev = $_), @in);

           This is nice in that it doesn't use much extra memory,
           simulating uniq(1)'s behavior of removing only
           adjacent duplicates.  It's less nice in that it won't
           work with false values like undef, 0, or ""; "0 but
           true" is ok, though.

       b) If you don't know whether @in is sorted:

               undef %saw;
               @out = grep(!$saw{$_}++, @in);

       c) Like (b), but @in contains only small integers:

               @out = grep(!$saw[$_]++, @in);

       d) A way to do (b) without any loops or greps:

               undef %saw;
               @saw{@in} = ();
               @out = sort keys %saw;  # remove sort if undesired

       e) Like (d), but @in contains only small positive
           integers:

               undef @ary;
               @ary[@in] = @in;
               @out = @ary;

       But perhaps you should have been using a hash all along,
       eh?

       HHooww ccaann II tteellll wwhheetthheerr aa lliisstt oorr aarrrraayy ccoonnttaaiinnss aa cceerrttaaiinn
       eelleemmeenntt??

       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 keep an associative array
       lying about whose keys are the first array's values.

           @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
           undef %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:

           @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
           undef @is_tiny_prime;
           for (@primes) { $is_tiny_prime[$_] = 1; }

       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:

           @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.

       Please do not use

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

       or worse yet

           $is_there = grep /$whatever/, @array;

       These are slow (checks every element even if the first
       matches), inefficient (same reason), and potentially buggy
       (what if there are regexp characters in $whatever?).  If
       you're only testing once, then use:

           $is_there = 0;
           foreach $elt (@array) {
               if ($elt eq $elt_to_find) {
                   $is_there = 1;
                   last;
               }
           }
           if ($is_there) { ... }

       HHooww ddoo II ccoommppuuttee tthhee ddiiffffeerreennccee ooff ttwwoo aarrrraayyss??  HHooww ddoo II
       ccoommppuuttee tthhee iinntteerrsseeccttiioonn ooff ttwwoo aarrrraayyss??

       Use a hash.  Here's code to do both and more.  It assumes
       that each element is unique in a given array:

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

       HHooww ddoo II tteesstt wwhheetthheerr ttwwoo aarrrraayyss oorr hhaasshheess aarree eeqquuaall??

       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) = @_;
               local $^W = 0;  # silence spurious -w undef complaints
               return 0 unless @$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);
           @a = @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);

           %a = %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.

       HHooww ddoo II ffiinndd tthhee ffiirrsstt aarrrraayy eelleemmeenntt ffoorr wwhhiicchh aa
       ccoonnddiittiioonn iiss ttrruuee??

       You can use this if you care about the index:

           for ($i= 0; $i < @array; $i++) {
               if ($array[$i] eq "Waldo") {
                   $found_index = $i;
                   last;
               }
           }

       Now $found_index has what you want.

       HHooww ddoo II hhaannddllee lliinnkkeedd lliissttss??

       In general, you usually don't need a linked list in Perl,
       since with regular arrays, you can push and pop or shift
       and unshift at either end, or you can use splice to add
       and/or remove arbitrary number of elements at arbitrary
       points.  Both pop and shift are both O(1) operations on
       perl's dynamic arrays.  In the absence of shifts and pops,
       push in general needs to reallocate on the order every
       log(N) times, and unshift will need to copy pointers each
       time.

       If you really, really wanted, you could use structures as
       described in the perldsc manpage or the perltoot manpage
       and do just what the algorithm book tells you to do.  For
       example, imagine a list node like this:

           $node = {
               VALUE => 42,
               LINK  => undef,
           };

       You could walk the list this way:

           print "List: ";
           for ($node = $head;  $node; $node = $node->{LINK}) {
               print $node->{VALUE}, " ";
           }
           print "\n";

       You could grow the list this way:

           my ($head, $tail);
           $tail = append($head, 1);       # grow a new head
           for $value ( 2 .. 10 ) {
               $tail = append($tail, $value);
           }

           sub append {
               my($list, $value) = @_;
               my $node = { VALUE => $value };
               if ($list) {
                   $node->{LINK} = $list->{LINK};
                   $list->{LINK} = $node;
               } else {
                   $_[0] = $node;      # replace caller's version
               }
               return $node;
           }

       But again, Perl's built-in are virtually always good
       enough.

       HHooww ddoo II hhaannddllee cciirrccuullaarr lliissttss??

       Circular lists could be handled in the traditional fashion
       with linked lists, or you could just do something like
       this with an array:

           unshift(@array, pop(@array));  # the last shall be first
           push(@array, shift(@array));   # and vice versa

       HHooww ddoo II sshhuuffffllee aann aarrrraayy rraannddoommllyy??

       Use this:

           # fisher_yates_shuffle( \@array ) :
           # generate a random permutation of @array in place
           sub fisher_yates_shuffle {
               my $array = shift;
               my $i;
               for ($i = @$array; --$i; ) {
                   my $j = int rand ($i+1);
                   next if $i == $j;
                   @$array[$i,$j] = @$array[$j,$i];
               }
           }

           fisher_yates_shuffle( \@array );    # permutes @array in place

       You've probably seen shuffling algorithms that works 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.

       HHooww ddoo II pprroocceessss//mmooddiiffyy eeaacchh eelleemmeenntt ooff aann aarrrraayy??

       Use for/foreach:

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

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

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

       If you want to do the same thing to modify the values of
       the hash, you may not use the values function, oddly
       enough.  You need a slice:

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

       HHooww ddoo II sseelleecctt aa rraannddoomm eelleemmeenntt ffrroomm aann aarrrraayy??

       Use the rand() function (see the rand entry in the
       perlfunc manpage):

           # at the top of the program:
           srand;                      # not needed for 5.004 and later

           # then later on
           $index   = rand @array;
           $element = $array[$index];

       Make sure you only call srand once per program, if then.
       If you are calling it more than once (such as before each
       call to rand), you're almost certainly doing something
       wrong.

       HHooww ddoo II ppeerrmmuuttee NN eelleemmeennttss ooff aa lliisstt??

       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 should work on any
       list:

           #!/usr/bin/perl -n
           # tsc-permute: permute each word of input
           permute([split], []);
           sub permute {
               my @items = @{ $_[0] };
               my @perms = @{ $_[1] };
               unless (@items) {
                   print "@perms\n";
               } else {
                   my(@newitems,@newperms,$i);
                   foreach $i (0 .. $#items) {
                       @newitems = @items;
                       @newperms = @perms;
                       unshift(@newperms, splice(@newitems, $i, 1));
                       permute([@newitems], [@newperms]);
                   }
               }
           }

       HHooww ddoo II ssoorrtt aann aarrrraayy bbyy ((aannyytthhiinngg))??

       Supply a comparison function to sort() (described in the
       sort entry in the perlfunc manpage):

           @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.

           @idx = ();
           for (@data) {
               ($item) = /\d+\s*(\S+)/;
               push @idx, uc($item);
           }
           @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:

           @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.

           @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 http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for
       more about this approach.

       See also the question below on sorting hashes.

       HHooww ddoo II mmaanniippuullaattee aarrrraayyss ooff bbiittss??

       Use pack() and unpack(), or else vec() and the bitwise
       operations.

       For example, this sets $vec to have bit N set if $ints[N]
       was set:

           $vec = '';
           foreach(@ints) { vec($vec,$_,1) = 1 }

       And 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.)

       Here's a demo on how to use vec():

           # vec demo
           $vector = "\xff\x0f\xef\xfe";
           print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
               unpack("N", $vector), "\n";
           $is_set = vec($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";
           }

       WWhhyy ddooeess 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 the defined entry in the perlfunc
       manpage in the 5.004 release or later of Perl for more
       detail.

DDaattaa:: HHaasshheess ((AAssssoocciiaattiivvee AArrrraayyss))
       HHooww ddoo II pprroocceessss aann eennttiirree hhaasshh??

       Use the each() function (see the each entry in the
       perlfunc manpage) if you don't care whether it's sorted:

           while ( ($key, $value) = each %hash) {
               print "$key = $value\n";
           }

       If you want it sorted, you'll have to use foreach() on the
       result of sorting the keys as shown in an earlier
       question.

       WWhhaatt hhaappppeennss iiff II aadddd oorr rreemmoovvee kkeeyyss ffrroomm aa hhaasshh wwhhiillee
       iitteerraattiinngg oovveerr iitt??

       Don't do that.

       HHooww ddoo II llooookk uupp aa hhaasshh eelleemmeenntt bbyy vvaalluuee??

       Create a reverse hash:

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

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

           while (($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.

       HHooww ccaann II kknnooww hhooww mmaannyy eennttrriieess aarree iinn aa hhaasshh??

       If you mean how many keys, then all you have to do is take
       the scalar sense of the keys() function:

           $num_keys = scalar keys %hash;

       In void context it just resets the iterator, which is
       faster for tied hashes.

       HHooww ddoo II ssoorrtt aa hhaasshh ((ooppttiioonnaallllyy bbyy vvaalluuee iinnsstteeaadd ooff kkeeyy))??

       Internally, hashes are stored in a way that prevents you
       from imposing an order on key-value pairs.  Instead, you
       have to sort a list of the keys or values:

           @keys = sort keys %hash;    # sorted by key
           @keys = sort {
                           $hash{$a} cmp $hash{$b}
                   } keys %hash;       # and by value

       Here we'll do a reverse numeric sort by value, and if two
       keys are identical, sort by length of key, and if that
       fails, by straight ASCII comparison of the keys (well,
       possibly modified by your locale -- see the perllocale
       manpage).

           @keys = sort {
                       $hash{$b} <=> $hash{$a}
                                 ||
                       length($b) <=> length($a)
                                 ||
                             $a cmp $b
           } keys %hash;

       HHooww ccaann II aallwwaayyss kkeeeepp mmyy hhaasshh ssoorrtteedd??

       You can look into using the DB_File module and tie() using
       the $DB_BTREE hash bindings as documented in the section
       on In Memory Databases in the DB_File manpage.  The
       Tie::IxHash module from CPAN might also be instructive.

       WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn """"ddeelleettee"""" aanndd """"uunnddeeff""""
       wwiitthh hhaasshheess??

       Hashes are 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 $key is present in the
       array, exists($key) will return true.  The value for a
       given key can be undef, in which case $array{$key} will be
       undef while $exists{$key} will return true.  This
       corresponds to ($key, undef) being in the hash.

       Pictures help...  here's the %ary table:

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

       And these conditions hold

               $ary{'a'}                       is true
               $ary{'d'}                       is false
               defined $ary{'d'}               is true
               defined $ary{'a'}               is true
               exists $ary{'a'}                is true (perl5 only)
               grep ($_ eq 'a', keys %ary)     is true

       If you now say

               undef $ary{'a'}

       your table now reads:

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

       and these conditions now hold; changes in caps:

               $ary{'a'}                       is FALSE
               $ary{'d'}                       is false
               defined $ary{'d'}               is true
               defined $ary{'a'}               is FALSE
               exists $ary{'a'}                is true (perl5 only)
               grep ($_ eq 'a', keys %ary)     is true

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

       Now, consider this:

               delete $ary{'a'}

       your table now reads:

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

       and these conditions now hold; changes in caps:

               $ary{'a'}                       is false
               $ary{'d'}                       is false
               defined $ary{'d'}               is true
               defined $ary{'a'}               is false
               exists $ary{'a'}                is FALSE (perl5 only)
               grep ($_ eq 'a', keys %ary)     is FALSE

       See, the whole entry is gone!

       WWhhyy ddoonn''tt mmyy ttiieedd hhaasshheess mmaakkee tthhee ddeeffiinneedd//eexxiissttss
       ddiissttiinnccttiioonn??

       They may or may not implement the EXISTS() and DEFINED()
       methods differently.  For example, there isn't the concept
       of undef with hashes that are tied to DBM* files. This
       means the true/false tables above will give different
       results when used on such a hash.  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.

       HHooww ddoo II rreesseett aann each() operation part-way through?

       Using keys %hash in scalar context returns the number of
       keys in the hash and resets the iterator associated with
       the hash.  You may need to do this if you use last to exit
       a loop early so that when you re-enter it, the hash
       iterator has been reset.

       HHooww ccaann II ggeett tthhee uunniiqquuee kkeeyyss ffrroomm ttwwoo hhaasshheess??

       First you extract the keys from the hashes into arrays,
       and then solve the uniquifying the array problem described
       above.  For example:

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

       Or more succinctly:

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

       Or if you really want to save space:

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

       HHooww ccaann II ssttoorree aa mmuullttiiddiimmeennssiioonnaall aarrrraayy iinn aa DDBBMM ffiillee??

       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.

       HHooww ccaann II mmaakkee mmyy hhaasshh rreemmeemmbbeerr tthhee oorrddeerr II ppuutt eelleemmeennttss
       iinnttoo iitt??

       Use the Tie::IxHash from CPAN.

           use Tie::IxHash;
           tie(%myhash, Tie::IxHash);
           for ($i=0; $i<20; $i++) {
               $myhash{$i} = 2*$i;
           }
           @keys = keys %myhash;
           # @keys = (0,1,2,3,...)

       WWhhyy ddooeess ppaassssiinngg aa ssuubbrroouuttiinnee aann uunnddeeffiinneedd eelleemmeenntt iinn aa
       hhaasshh ccrreeaattee iitt??

       If you say something like:

           somefunc($hash{"nonesuch key here"});

       Then that element "autovivifies"; that is, it springs into
       existence whether you store something there or not.
       That's because functions get scalars passed in by
       reference.  If somefunc() modifies $_[0], it has to be
       ready to write it back into the caller's version.

       This has been fixed as of perl5.004.

       Normally, merely accessing a key's value for a nonexistent
       key does not cause that key to be forever there.  This is
       different than awk's behavior.

       HHooww ccaann II mmaakkee tthhee PPeerrll eeqquuiivvaalleenntt ooff aa CC ssttrruuccttuurree//CC++++
       ccllaassss//hhaasshh oorr aarrrraayy ooff hhaasshheess oorr aarrrraayyss??

       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 the perlref manpage and the
       upcoming the perlreftut manpage.  Examples of complex data
       structures are given in the perldsc manpage and the
       perllol manpage.  Examples of structures and object-
       oriented classes are in the perltoot manpage.

       HHooww ccaann II uussee aa rreeffeerreennccee aass aa hhaasshh kkeeyy??

       You can't do this directly, but you could use the standard
       Tie::Refhash module distributed with perl.

DDaattaa:: MMiisscc
       HHooww ddoo II hhaannddllee bbiinnaarryy ddaattaa ccoorrrreeccttllyy??

       Perl is binary clean, so this shouldn't be a problem.  For
       example, this works fine (assuming the files are found):

           if (`cat /vmunix` =~ /gzip/) {
               print "Your kernel is GNU-zip enabled!\n";
           }

       On some legacy systems, however, you have to play tedious
       games with "text" versus "binary" files.  See the section
       on binmode in the perlfunc manpage, or the upcoming the
       perlopentut manpage manpage.

       If you're concerned about 8-bit ASCII data, then see the
       perllocale manpage.

       If you want to deal with multibyte characters, however,
       there are some gotchas.  See the section on Regular
       Expressions.

       HHooww ddoo II ddeetteerrmmiinnee wwhheetthheerr aa ssccaallaarr iiss aa
       nnuummbbeerr//wwhhoollee//iinntteeggeerr//ffllooaatt??

       Assuming that you don't care about IEEE notations like
       "NaN" or "Infinity", you probably just want to use a
       regular expression.

          if (/\D/)            { print "has nondigits\n" }
          if (/^\d+$/)         { print "is a whole number\n" }
          if (/^-?\d+$/)       { print "is an integer\n" }
          if (/^[+-]?\d+$/)    { print "is a +/- integer\n" }
          if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
          if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" }
          if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                               { print "a C float" }

       If you're on a POSIX system, Perl's supports the
       POSIX::strtod function.  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 }

       Or you could check out String::Scanf which can be found at
       http://www.perl.com/CPAN/modules/by-module/String/.  The
       POSIX module (part of the standard Perl distribution)
       provides the strtol and strtod for converting strings to
       double and longs, respectively.

       HHooww ddoo II kkeeeepp ppeerrssiisstteenntt ddaattaa aaccrroossss pprrooggrraamm ccaallllss??

       For some specific applications, you can use one of the DBM
       modules.  See the AnyDBM_File manpage.  More generically,
       you should consult the FreezeThaw, Storable, or
       Class::Eroot modules from CPAN.  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

       HHooww ddoo II pprriinntt oouutt oorr ccooppyy aa rreeccuurrssiivvee ddaattaa ssttrruuccttuurree??

       The Data::Dumper module on CPAN (or the 5.005 release of
       Perl) is great for printing out data structures.  The
       Storable module, found on CPAN, 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) };

       HHooww ddoo II ddeeffiinnee mmeetthhooddss ffoorr eevveerryy ccllaassss//oobbjjeecctt??

       Use the UNIVERSAL class (see the UNIVERSAL manpage).

       HHooww ddoo II vveerriiffyy aa ccrreeddiitt ccaarrdd cchheecckkssuumm??

       Get the Business::CreditCard module from CPAN.

       HHooww ddoo II ppaacckk aarrrraayyss ooff ddoouubblleess oorr ffllooaattss ffoorr XXSS ccooddee??

       The kgbpack.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.

AAUUTTHHOORR AANNDD CCOOPPYYRRIIGGHHTT
       Copyright (c) 1997-1999 Tom Christiansen and Nathan
       Torkington.  All rights reserved.

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic Licence.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright
       holder.

       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.

27/Mar/1999            perl 5.005, patch 03                     1