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perlfaq4




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)?

       Internally, your computer represents floating-point num­
       bers 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 com­
       puters store numbers and affects all computer languages,
       not just Perl.

       perlnumber show 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 the "Floating
       Point Arithmetic" 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 above item "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?

       Perl only understands octal and hex numbers as such when
       they occur as literals in your program.  Octal literals in
       perl must start with a leading "0" and hexadecimal liter­
       als must start with a leading "0x".  If they are read in
           chmod(644,  $file); # WRONG
           chmod(0644, $file); # right

       Note the mistake in the first line was specifying the dec­
       imal literal 644, rather than the intended octal literal
       0644.  The problem can be seen with:

           printf("%#o",644); # prints 01204

       Surely you had not intended "chmod(01204, $file);" - did
       you?  If you want to use numeric literals as arguments to
       chmod() et al. then please try to express them as octal
       constants, that is with a leading zero and with the fol­
       lowing digits restricted to the set 0..7.

       Does Perl have a round() function?  What about ceil() and
       floor()?  Trig functions?

       Remember that int() merely truncates toward 0.  For round­
       ing 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 mathe­
       matical 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::Com­
       plex 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 impli­
       cations, 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

       intended to be representational rather than exhaustive.

       Some of the examples below 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 opera­
       tions, 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:

               $dec = 0xDEADBEEF;

           Using the hex function:

               $dec = hex("DEADBEEF");

           Using pack:

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

           Using the CPAN module Bit::Vector:

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

       How do I convert from decimal to hexadecimal
           Using sprintf:

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

           Using unpack:

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

           Using Bit::Vector:

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

           And Bit::Vector supports odd bit counts:

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

       How do I convert from octal to decimal
               $dec = $vec->to_Dec();

       How do I convert from decimal to octal
           Using sprintf:

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

           Using Bit::Vector:

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

               $number = 0b10110110;

           Using oct:

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

           Using pack and ord:

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

           Using pack and unpack for larger strings:

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

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

           Using Bit::Vector:

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

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

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

           Using unpack:

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

           Using Bit::Vector:

       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 num­
       bers (yielding 3).  Saying "11" & "3" performs the "and"
       operation on strings (yielding "1").

       Most problems with "&" and "|" arise because the program­
       mer 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]/) {
               # ...
           }

       How do I multiply matrices?

       Use the Math::Matrix or Math::MatrixReal modules (avail­
       able 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 col­
       lect 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
       can use:

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

       the entire range.

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

       will not create a list of 500,000 integers.

       How can I output Roman numerals?

       Get the http://www.cpan.org/modules/by-module/Roman mod­
       ule.

       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 begin­
       ning.  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 pro­
       grams :-).  see 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.''

       If you want numbers that are more random than "rand" with
       "srand" provides, you should also check out the Math::Tru­
       lyRandom 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/ .

       How do I get a random number between X and Y?

       Use the following simple function.  It selects a random
       integer between (and possibly including!) the two given
       integers, e.g., "random_int_in(50,120)"

          sub random_int_in ($$) {
            my($min, $max) = @_;
             # Assumes that the two arguments are integers themselves!

       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 the Time::Local
       module to get a time in epoch seconds for the argument to
       localtime.

               use POSIX qw/strftime/;
               use Time::Local;
               my $week_of_year = strftime "%W",
                       localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );

       The Date::Calc module provides two functions for to calcu­
       late these.

               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);
           }

       You can also use the POSIX strftime() function which may
       be a bit slower but is easier to read and maintain.

               use POSIX qw/strftime/;

               my $week_of_the_year = strftime "%W", localtime;
               my $day_of_the_year  = strftime "%j", localtime;

       On some systems, the POSIX module's strftime() function
       has been extended in a non-standard way to use a %C for­
       mat, 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 reliably determine the current century or mil­
       lennium.

       How can I compare two dates and find the difference?

       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.  Other­
       wise, you should look into the Date::Calc and Date::Manip
       modules from CPAN.

       How can I find the Julian Day?

       Use the Time::JulianDay module (part of the Time-modules
       bundle available from CPAN.)

       Before you immerse yourself too deeply in this, be sure to
       verify that it is the Julian Day you really want.  Are you
       interested in a way of getting serial days so that you
       just can tell how many days they are apart or so that you
       can do also other date arithmetic?  If you are interested
       in performing date arithmetic, this can be done using mod­
       ules Date::Manip or Date::Calc.

       There is too many details and much confusion on this issue
       to cover in this FAQ, but the term is applied (correctly)
       to a calendar now supplanted by the Gregorian Calendar,
       with the Julian Calendar failing to adjust properly for
       leap years on centennial years (among other annoyances).
       The term is also used (incorrectly) to mean: [1] days in
       the Gregorian Calendar; and [2] days since a particular
       starting time or `epoch', usually 1970 in the Unix world
       and 1980 in the MS-DOS/Windows world.  If you find that it
       is not the first meaning that you really want, then check
       out the Date::Manip and Date::Calc modules.  (Thanks to
       David Cassell for most of this text.)

       How do I find yesterday's date?

       If you only need to find the date (and not the same time),
       you can use the Date::Calc module.

               use Date::Calc qw(Today Add_Delta_Days);

               my @date = Add_Delta_Days( Today(), -1 );

               print "@date\n";

       Most people try to use the time rather than the calendar
       to figure out dates, but that assumes that your days are
       twenty-four hours each.  For most people, there are two
       days a year when they aren't: the switch to and from sum­
       mer time throws this off. Russ Allbery offers this solu­
       tion.

           sub yesterday {

       crossed, and the correction will subtract 0.  If $tdst is
       1 and $ndst is 0, subtract an hour more from yesterday's
       time since we gained an extra hour while going off day­
       light savings time.  If $tdst is 0 and $ndst is 1, sub­
       tract a negative hour (add an hour) to yesterday's time
       since we lost an hour.

       All of this is because during those days when one switches
       off or onto DST, a "day" isn't 24 hours long; it's either
       23 or 25.

       The explicit settings of $ndst and $tdst are necessary
       because localtime only says it returns the system tm
       struct, and the system tm struct at least on Solaris
       doesn't guarantee any particular positive value (like,
       say, 1) for isdst, just a positive value.  And that value
       can potentially be negative, if DST information isn't
       available (this sub just treats those cases like no DST).

       Note that between 2am and 3am on the day after the time
       zone switches off daylight savings time, the exact hour of
       "yesterday" corresponding to the current hour is not
       clearly defined.  Note also that if used between 2am and
       3am the day after the change to daylight savings time, the
       result will be between 3am and 4am of the previous day;
       it's arguable whether this is correct.

       This sub does not attempt to deal with leap seconds (most
       things don't).

       Does Perl have a Year 2000 problem?  Is Perl Y2K compli­
       ant?

       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 pen­
       cil--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 a list 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.


Data: Strings

       How do I validate input?

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

       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 back­
       slash ("\") 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?

       To turn "abbcccd" into "abccd":

           s/(.)\1/$1/g;       # add /s to include newlines

       Here's a solution that turns "abbcccd" to "abcd":

           y///cs;     # y == tr, but shorter :-)

       How do I expand function calls in a string?

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

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

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

       How do I find matching/nesting anything?

       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.  For balanced expressions
       using "(", "{", "[" or "<" as delimiters, use the CPAN
       module Regexp::Common, or see "(??{ code })" in perlre.
       For other cases, you'll have to write a parser.
           }

       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/i);
           print join("\n",@$[0..$#$]) if( $$[-1] );

       How do I reverse a string?

       Use reverse() in scalar context, as documented in
       "reverse" in perlfunc.

           $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;
           @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}' $*"
               $string = "Just another Perl Hacker";
           $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:


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

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

       How do I capitalize all the words on one line?

       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.
       Other times you might need a more thorough solution (Sug­
       gested by brian d 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 perllocale for endless details on locales.

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

       Damian Conway's Text::Autoformat module provides some
       smart case transformations:

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

           print $x, "\n";

       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):

            @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 quota­
       tion-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);

       There's also a Text::CSV (Comma-Separated Values) module
       on CPAN.

       How do I strip blank space from the beginning/end of a
       string?

       Although the simplest approach would seem to be

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

       not only is this unnecessarily slow and destructive, it
       also fails with embedded newlines.  It is much faster to
       do this operation in two steps:

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

       Or more nicely written as:

           for ($string) {
           }

       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):
               $padded = sprintf("%${pad_len}s", $text);
               $padded = sprintf("%*s", $pad_len, $text);  # same thing

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

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

           # Right padding a string with blanks using pack (will truncate):
           $padded = pack("A$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:

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

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

       How do I find the soundex value of a string?

       Use the standard Text::Soundex module distributed with
       Perl.  Before you do so, you may want to determine whether
       `soundex' is in fact what you think it is.  Knuth's
       soundex algorithm compresses words into a small space, and
       so it does not necessarily distinguish between two words
       which you might want to appear separately.  For example,
       the last names `Knuth' and `Kant' are both mapped to the
       soundex code K530.  If Text::Soundex does not do what you
       are looking for, you might want to consider the
       String::Approx module available at CPAN.

       How can I expand variables in text strings?

       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 suf­
       fice:

           $text =~ s/\$(\w+)/${$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.

       What's wrong with always quoting "$vars"?
       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 opera­
       tions 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

       Why don't my <<HERE documents work?

       Check for these three things:

       There must be no space after the << part.
       There (probably) should be a semicolon at the end.
       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 mar­
       gin.  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

               local $_ = shift;
               my ($white, $leader);  # common whitespace and common leading string
               if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
                   ($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 whitespace, with remain­
       ing 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


Data: Arrays

       What is the difference between a list and an array?

       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 con­
       text, 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 num­
       ber of elements in them, subroutines access their argu­
       ments through the array @_, and push/pop/shift only work
       on arrays.
       making 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 "use warnings" pragma and the -w flag will warn you
       about these matters.

       How can I remove duplicate elements from a list or array?

       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 = "not equal to $in[0]";
               @out = grep($_ ne $prev && ($prev = $_, 1), @in);

           This is nice in that it doesn't use much extra memory,
           simulating uniq(1)'s behavior of removing only adja­
           cent duplicates.  The ", 1" guarantees that the
           expression is true (so that grep picks it up) even if
           the $_ is 0, "", or undef.

       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 inte­
           gers:

       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 arbi­
       trary string values, the fastest way is probably to invert
       the original array and maintain a hash whose keys are the
       first array's values.

           @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
           %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 sim­
       ple indexed array.  This kind of an array will take up
       less space:

           @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
           @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:

           @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 regex characters in $whatever?).  If
       you're only testing once, then use:

           $is_there = 0;
           foreach $elt (@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;
           }

       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);
           @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",

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

       If you want the array index, you can iterate through the
       indices and check 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?

       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 perldsc or perltoot and do just what the
       algorithm book tells you to do.  For example, imagine a
       list node like this:

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

           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.

       How do I handle circular lists?

       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

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

           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
               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
           }

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

               @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 $orbit ( values %orbits ) {
               ($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):

       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 embod­
       ied in the permute() function is discussed in Volume 4
       (still unpublished) of Knuth's The Art of Computer Pro­
       gramming and will work on any list:

               #!/usr/bin/perl -n
               # Fischer-Kause 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;

       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
           }
           @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 the sort artitcle 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 below on sorting hashes.

       How do I manipulate arrays of bits?

       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 }

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

                   # 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 sug­
       gestion from Benjamin Goldberg:

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

       Or use the CPAN module Bit::Vector:

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

       Use the each() function (see "each" in perlfunc) 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 ques­
       tion.

       What happens if I add or remove keys from a hash while
       iterating over it?

       Don't do that. :-)

       [lwall] In Perl 4, you were not allowed to modify a hash
       at all while iterating over it.  In Perl 5 you can delete
       from it, but you still can't add to it, because that might
       cause a doubling of the hash table, in which half the
           %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.  If it does worry you, you can always
       reverse the hash into a hash of arrays instead:

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

       How can I know how many entries are in a hash?

       If you mean how many keys, then all you have to do is use
       the keys() function in a scalar context:

           $num_keys = keys %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)?

       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, or if that
       fails, by straight ASCII comparison of the keys (well,
       possibly modified by your locale--see perllocale).

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

       although the value can be any kind of scalar: string, num­
       ber, 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 $hash{'a'}                is true (Perl5 only)
               grep ($_ 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 $hash{'a'}                is true (Perl5 only)
               grep ($_ eq 'a', keys %hash)     is true

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

       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 $hash{'a'}                is FALSE (Perl5 only)
               grep ($_ eq 'a', keys %hash)     is FALSE

       See, the whole entry is gone!

       Why don't my tied hashes make the defined/exists distinc­
       tion?

       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?

       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.

       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:

           %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)) {

       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?

       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 refer­
       ence.  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.

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

       How can I use a reference as a hash key?
               print "Your kernel is GNU-zip enabled!\n";
           }

       On less elegant (read: Byzantine) systems, however, you
       have to play tedious games with "text" versus "binary"
       files.  See "binmode" in perlfunc or perlopentut.

       If you're concerned about 8-bit ASCII data, then see perl­
       locale.

       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 num­
       ber/whole/integer/float?

       Assuming that you don't care about IEEE notations like
       "NaN" or "Infinity", you probably just want to use a regu­
       lar 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+)$/) { print "is a decimal number\n" }
          if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
                               { print "a C float\n" }

       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 determin­
       ing 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 avail­
       able from the CPAN.

       If you're on a POSIX system, Perl supports the
       "POSIX::strtod" function.  Its semantics are somewhat cum­
       bersome, 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?''

               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. The POSIX module (part of the standard Perl
       distribution) provides the "strtod" and "strtol" for con­
       verting strings to double and longs, respectively.

       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), pro­
       vides 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?

       Use the UNIVERSAL class (see UNIVERSAL).


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Tork­
       ington.  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.

perl v5.8.1                 2003-09-02                PERLFAQ4(1)

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