The OpenNET Project / Index page

[ новости /+++ | форум | теги | ]

Интерактивная система просмотра системных руководств (man-ов)

 ТемаНаборКатегория 
 
 [Cписок руководств | Печать]

perlfaq4 (1)
  • >> perlfaq4 (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlfaq4 (1) ( Разные man: Команды и прикладные программы пользовательского уровня )
  • 
    
    

    NAME

         perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date:
         1999/05/23 20:37:49 $)
    
    
    

    DESCRIPTION

         The section of the FAQ answers question related to the
         manipulation of data as 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)?
    
         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 all 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 Floating-point Arithmetic entry in the perlop manpage.
    
    
    
         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.  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
    
    
         Does Perl have a round() function?  What about ceil() and
         floor()?  Trig functions?
    
         Remember that int() merely truncates toward 0.  For rounding
         to a certain number of digits, sprintf() or printf() is
         usually the easiest route.
    
             printf("%.3f", 3.1415926535);       # prints 3.142
    
         The POSIX module (part of the standard Perl distribution)
         implements ceil(), floor(), and a number of other
         mathematical and trigonometric functions.
    
             use POSIX;
             $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.
    
         How do I convert bits into ints?
    
         To turn a string of 1s and 0s like `10110110' into a scalar
         containing its binary value, use the pack() and unpack()
         functions (documented in the pack entry in the perlfunc
         manpage and the unpack entry in the perlfunc manpage):
    
             $decimal = unpack('c', pack('B8', '10110110'));
    
         This packs the string `10110110' into an eight bit binary
         structure.  This is then unpacked as a character, which
         returns its ordinal value.
    
         This does the same thing:
    
             $decimal = ord(pack('B8', '10110110'));
    
         Here's an example of going the other way:
    
             $binary_string = unpack('B*', "\x29");
    
    
         Why doesn't & work the way I want it to?
    
         The behavior of binary arithmetic operators depends on
         whether they're used on numbers or strings.  The operators
         treat a string as a series of bits and work with that (the
         string `"3"' is the bit pattern `00110011').  The operators
         work with the binary form of a number (the number `3' is
         treated as the bit pattern `00000011').
    
         So, saying `11 & 3' performs the "and" operation on numbers
         (yielding `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]/) {
                 # ...
             }
    
    
         How do I multiply matrices?
    
         Use the Math::Matrix or Math::MatrixReal modules (available
         from CPAN) or the PDL extension (also available from CPAN).
    
         How do I perform an operation on a series of integers?
    
         To call a function on each element in an array, and collect
         the results, use:
    
             @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);
    
         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));
             }
    
         This situation has been fixed in Perl5.005. Use of `..' in a
         `for' loop will iterate over the range, without creating 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.perl.com/CPAN/modules/by-module/Roman
         module.
    
         Why aren't my random numbers random?
    
         If you're using a version of Perl before 5.004, you must
         call `srand' once at the start of your program to seed the
         random number generator.  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/ .
    
    
    

    Data: Dates

         How do I find the week-of-the-year/day-of-the-year?
    
         The day of the year is in the array returned by localtime()
         (see the localtime entry 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.
    
         How do I find the current century or millennium?
    
         Use the following simple functions:
    
             sub get_century    {
                 return int((((localtime(shift || time))[5] + 1999))/100);
             }
             sub get_millennium {
                 return 1+int((((localtime(shift || time))[5] + 1899))/1000);
             }
    
         On some systems, you'll find that the POSIX module's
         strftime() function has been extended in a non-standard way
         to use a `%C' format, which they sometimes claim is the
         "century".  It isn't, because on most such systems, this is
         only the first two digits of the four-digit year, and thus
         cannot be used to reliably determine the current century or
         millennium.
    
         How can I compare two dates and find the difference?
    
         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 for reasons of accessibility, simplicity, and
         efficiency, merely use either timelocal or timegm (from the
         Time::Local module in the standard distribution) to reduce
         structured dates to epoch seconds.  However, if you don't
         know the precise format of your dates, then you should
         probably use either of the Date::Manip and Date::Calc
         modules from CPAN before you go hacking up your own parsing
         routine to handle arbitrary date formats.
    
         How can I take a string and turn it into epoch seconds?
    
         If it's a regular enough string that it always has the same
         format, you can split it up and pass the parts to
         `timelocal' in the standard Time::Local module.  Otherwise,
         you should look into the Date::Calc 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 they
         really just interested in a way of getting serial days so
         that they can do date arithmetic?  If you are interested in
         performing date arithmetic, this can be done using either
         Date::Manip or Date::Calc, without converting to Julian Day
         first.
    
         There is too 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?
    
         The `time()' function returns the current time in seconds
         since the epoch.  Take twenty-four hours 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.
    
         Note very carefully that the code above 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
         summer time throws this off.  A solution to this issue is
         offered by Russ Allbery.
    
    
    
             sub yesterday {
                 my $now  = defined $_[0] ? $_[0] : time;
                 my $then = $now - 60 * 60 * 24;
                 my $ndst = (localtime $now)[8] > 0;
                 my $tdst = (localtime $then)[8] > 0;
                 $then - ($tdst - $ndst) * 60 * 60;
             }
             # Should give you "this time yesterday" in seconds since epoch relative to
             # the first argument or the current time if no argument is given and
             # suitable for passing to localtime or whatever else you need to do with
             # it.  $ndst is whether we're currently in daylight savings time; $tdst is
             # whether the point 24 hours ago was in daylight savings time.  If $tdst
             # and $ndst are the same, a boundary wasn't 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
             # daylight savings time.  If $tdst is 0 and $ndst is 1, subtract 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).
             #
             # Copyright relinquished 1999 by Russ Allbery <[email protected]>
             # This code is in the public domain
    
    
         Does Perl have a Year 2000 problem?  Is Perl Y2K compliant?
    
         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.
    
    
    

    Data: Strings

         How do I validate input?
    
         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.
    
         How do I unescape a string?
    
         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.
    
         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 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.
    
         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, 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 are the CPAN modules Parse::RecDescent,
         Parse::Yapp, and Text::Balanced; and the byacc program.
    
         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] );
    
    
         How do I reverse a string?
    
         Use reverse() in scalar context, as documented in the
         reverse entry in the perlfunc manpage.
    
             $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).
    
         How can I access/change the first N letters of a string?
    
         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/;
    
    
         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:
    
             $string = "ThisXlineXhasXsomeXx'sXinXit";
             $count = ($string =~ tr/X//);
             print "There are $count X characters in the string";
    
         This is fine if you are just looking for a single character.
         However, if you are trying to count multiple character
         substrings within a larger string, `tr///' won't work.  What
         you can do is wrap a while() loop around a global pattern
         match.  For example, let's count negative integers:
    
             $string = "-9 55 48 -2 23 -76 4 14 -44";
             while ($string =~ /-\d+/g) { $count++ }
             print "There are $count negative numbers in the string";
    
    
         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, instead
         (Suggested 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 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.  Consider the
         proper capitalization of the movie Dr. Strangelove or: How I
         Learned to Stop Worrying and Love the Bomb, for example.
    
         How can I split a [character] delimited string except when
         inside
    
         [character]? (Comma-separated files)" [character]? (Comma-
         separated files)" 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.
    
         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 ($string) {
                 s/^\s+//;
                 s/\s+$//;
             }
    
         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 slice:
    
             # trim whitespace in the scalar, the array,
             # and all the values in the hash
             foreach ($scalar, @array, @hash{keys %hash}) {
                 s/^\s+//;
                 s/\s+$//;
             }
    
    
         How do I pad a string with blanks or pad a number with
         zeroes?
    
         (This answer contributed by Uri Guttman, with kibitzing from
         Bart Lateur.)
    
         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);
    
             # Right padding a string with blanks (no truncation):
             $padded = sprintf("%-${pad_len}s", $text);
    
             # Left padding a number with 0 (no truncation):
             $padded = sprintf("%0${pad_len}d", $num);
    
             # 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 ) );
    
    
         How do I extract selected columns from a string?
    
         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;
             }
    
    
         How do I find the soundex value of a string?
    
         Use the standard Text::Soundex module distributed with Perl.
         But 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
         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.
    
         What's wrong with always quoting "$vars"?
    
         The problem is that those double-quotes force
         stringification, coercing numbers and references into
         strings, even when you don't want them to be.  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
    
    
         Why don't my <<HERE documents work?
    
         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
    
    
    
    

    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 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
         uses the scalar comma operator.  There never was a list
         there at all!  This causes the last value to be returned: 9.
    
         What is the difference between $array[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 `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.
    
    assumes all true values in the array)
         a) If @in is sorted, and you want @out to be sorted: (this
                 $prev = 'nonesuch';
                 @out = grep($_ 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 = grep {defined} @ary;
    
    
         But perhaps you should have been using a hash all along, eh?
    
         How can I tell whether a list or array contains a certain
         element?
    
         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 }
             # 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 $elt (@array) {
                 if ($elt eq $elt_to_find) {
                     $is_there = 1;
                     last;
                 }
             }
             if ($is_there) { ... }
    
    
         How do I compute the difference of two arrays?  How do I
         compute the intersection of two arrays?
    
         Use a hash.  Here's code to do both and more.  It assumes
         that each element is unique in a given array:
    
    
    
             @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",
                 cmpStr(\%a, \%b) == 0 ? "the same" : "different";
    
             printf "a and b contain %s hashes\n",
                 cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
    
         The first reports that both those the hashes contain the
         same data, while the second reports that they do not.  Which
         you prefer is left as an exercise to the reader.
    
         How do I find the first array element for which a condition
         is true?
    
         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.
    
         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 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.
    
         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?
    
         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 work using
         splice, randomly picking another element to swap the current
         element with:
    
             srand;
             @new = ();
             @old = 1 .. 10;  # just a demo
             while (@old) {
                 push(@new, splice(@old, rand @old, 1));
             }
    
         This is bad because splice is already O(N), and since you do
         it N times, you just invented a quadratic algorithm; that
         is, O(N**2).  This does not scale, although Perl is so
         efficient that you probably won't notice this until you have
         rather largish arrays.
    
         How do I process/modify each element of an array?
    
         Use `for'/`foreach':
    
             for (@lines) {
                 s/foo/bar/;     # change that word
                 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;
             }
    
    
         How do I select a random element from an array?
    
         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.
    
         How do I permute N elements of a list?
    
         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]);
                     }
                 }
             }
    
    
         How do I sort an array by (anything)?
    
         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.
    
         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 }
    
         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";
             }
    
    
         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 the defined entry in the perlfunc manpage 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 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.
    
         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 entries get
         copied up to the new top half of the table, at which point
         you've totally bamboozled the iterator code.  Even if the
         table doesn't double, there's no telling whether your new
         entry will be inserted before or after the current iterator
         position.
    
         Either treasure up your changes and make them after the
         iterator finishes, or use keys to fetch all the old keys at
         once, and iterate over the list of keys.
    
         How do I look up a hash element by value?
    
         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.  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 take
         the scalar sense of the keys() function:
    
             $num_keys = scalar keys %hash;
    
         In void context, the keys() function just resets the
         iterator, which is faster for tied hashes than would be
         iterating through the whole hash, one key-value pair at a
         time.
    
         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, 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;
    
    
         How can I always keep my hash sorted?
    
         You can look into using the DB_File module and tie() using
         the $DB_BTREE hash bindings as documented in the In Memory
         Databases entry in the DB_File manpage.  The Tie::IxHash
         module from CPAN might also be instructive.
    
         What's the difference between "delete" and "undef" with
         hashes?
    
         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!
    
         Why don't my tied hashes make the defined/exists
         distinction?
    
         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.
    
         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)) {
                 $seen{$key}++;
             }
             @uniq = keys %seen;
    
    
         How can I store a multidimensional array in a DBM file?
    
         Either stringify the structure yourself (no fun), or else
         get the MLDBM (which uses Data::Dumper) module from CPAN and
         layer it on top of either DB_File or GDBM_File.
    
         How can I make my hash remember the order I put elements
         into it?
    
         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,...)
    
    
         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 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.
    
    
    
         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 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.
    
         How can I use a reference as a hash key?
    
         You can't do this directly, but you could use the standard
         Tie::Refhash module distributed with Perl.
    
    
    

    Data: Misc

         How do I handle binary data correctly?
    
         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 less elegant (read: Byzantine) systems, however, you have
         to play tedious games with "text" versus "binary" files.
         See the binmode entry in the perlfunc manpage or the
         perlopentut manpage.  Most of these ancient-thinking systems
         are curses out of Microsoft, who seem to be committed to
         putting the backward into backward compatibility.
    
         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.
    
    
    
         How do I determine whether a scalar is a
         number/whole/integer/float?
    
         Assuming that you don't care about IEEE notations like "NaN"
         or "Infinity", you probably just want to use a regular
         expression.
    
            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($_[0]) }
    
         Or you could check out the String::Scanf module on CPAN
         instead.  The POSIX module (part of the standard Perl
         distribution) provides the `strtol' and `strtod' for
         converting 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 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
    
    
         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, 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) };
    
    
         How do I define methods for every class/object?
    
         Use the UNIVERSAL class (see the UNIVERSAL manpage).
    
         How do I verify a credit card checksum?
    
         Get the Business::CreditCard module from CPAN.
    
         How do I pack arrays of doubles or floats for XS code?
    
         The 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.
    
    
    

    AUTHOR AND COPYRIGHT

         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 License.  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.
    
    
    
    


    Поиск по тексту MAN-ов: 




    Партнёры:
    PostgresPro
    Inferno Solutions
    Hosting by Hoster.ru
    Хостинг:

    Закладки на сайте
    Проследить за страницей
    Created 1996-2024 by Maxim Chirkov
    Добавить, Поддержать, Вебмастеру