X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=6a882c53ff5b2c6718b0d0549ddd9a51b60ed1dc;hb=938c8732ceb115a707f725327a631eb35319ba87;hp=112b1edabf4de7bb6c205f5cb4cfa242cbfeb65e;hpb=90fdbbb75512007b809510f03fc507ff5c145e16;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index 112b1ed..6a882c5 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,64 +1,83 @@ =head1 NAME -perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) +perlfaq4 - Data Manipulation ($Revision: 1.54 $, $Date: 2003/11/30 00:50:08 $) =head1 DESCRIPTION -The section of the FAQ answers question related to the manipulation -of data as numbers, dates, strings, arrays, hashes, and miscellaneous -data issues. +This section of the FAQ answers questions related to manipulating +numbers, dates, strings, arrays, hashes, and miscellaneous data issues. =head1 Data: Numbers =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? -The infinite set that a mathematician thinks of as the real numbers can -only be approximate on a computer, since the computer only has a finite -number of bits to store an infinite number of, um, numbers. - -Internally, your computer represents floating-point numbers in binary. -Floating-point numbers read in from a file or appearing as literals -in your program are converted from their decimal floating-point -representation (eg, 19.95) to the internal binary representation. - -However, 19.95 can't be precisely represented as a binary -floating-point number, just like 1/3 can't be exactly represented as a -decimal floating-point number. The computer's binary representation -of 19.95, therefore, isn't exactly 19.95. - -When a floating-point number gets printed, the binary floating-point -representation is converted back to decimal. These decimal numbers -are displayed in either the format you specify with printf(), or the -current output format for numbers (see L if you use -print. C<$#> has a different default value in Perl5 than it did in -Perl4. Changing C<$#> yourself is deprecated.) - -This affects B 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, -C) to get the required precision. -See L. +Internally, your computer represents floating-point numbers +in binary. Digital (as in powers of two) computers cannot +store all numbers exactly. Some real numbers lose precision +in the process. This is a problem with how computers store +numbers and affects all computer languages, not just Perl. + +L 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 +L<"Floating Point Arithmetic"|perlop> for more details. + + printf "%.2f", 10/3; + + my $number = sprintf "%.2f", 10/3; + +=head2 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. =head2 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". +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 literals must start with a leading "0x". +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 to decimal. oct() interprets hex ("0x350"), +octal ("0350" or even without the leading "0", like "377") and binary +("0b1010") numbers, while hex() only converts hexadecimal ones, with +or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". +The inverse mapping from decimal to octal can be done with either the +"%o" or "%O" sprintf() formats. This problem shows up most often when people try using chmod(), mkdir(), -umask(), or sysopen(), which all want permissions in octal. +umask(), or sysopen(), which by widespread tradition typically take +permissions in octal. - chmod(644, $file); # WRONG -- perl -w catches this + chmod(644, $file); # WRONG chmod(0644, $file); # right +Note the mistake in the first line was specifying the decimal 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 C - 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 following digits restricted to the set 0..7. + =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? Remember that int() merely truncates toward 0. For rounding to a @@ -75,7 +94,7 @@ functions. $ceil = ceil(3.5); # 4 $floor = floor(3.5); # 3 -In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex +In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from @@ -93,7 +112,7 @@ alternation: for ($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 + 0.0 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. @@ -101,24 +120,143 @@ 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. -=head2 How do I convert bits into ints? +=head2 How do I convert between numeric representations/bases/radixes? + +As always with Perl there is more than one way to do it. Below +are a few examples of approaches to making common conversions +between number representations. This is intended to be representational +rather than exhaustive. + +Some of the examples 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 operations, and for at least some +programmers the notation might be familiar. + +=over 4 + +=item 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(); + +=item 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: -To turn a string of 1s and 0s like C<10110110> into a scalar containing -its binary value, use the pack() and unpack() functions (documented in -L and L): + use Bit::Vector; + $vec = Bit::Vector->new_Dec(33, 3735928559); + $vec->Resize(32); # suppress leading 0 if unwanted + $hex = $vec->to_Hex(); - $decimal = unpack('c', pack('B8', '10110110')); +=item How do I convert from octal to decimal -This packs the string C<10110110> into an eight bit binary structure. -This is then unpacked as a character, which returns its ordinal value. +Using Perl's built in conversion of numbers with leading zeros: -This does the same thing: + $dec = 033653337357; # note the leading 0! + +Using the oct function: + + $dec = oct("33653337357"); + +Using Bit::Vector: + + use Bit::Vector; + $vec = Bit::Vector->new(32); + $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); + $dec = $vec->to_Dec(); + +=item 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)); + +=item 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')); -Here's an example of going the other way: +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. - $binary_string = unpack('B*', "\x29"); +Using Bit::Vector: + + $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); + $dec = $vec->to_Dec(); + +=item 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: + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $bin = $vec->to_Bin(); + +The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) +are left as an exercise to the inclined reader. + +=back =head2 Why doesn't & work the way I want it to? @@ -129,7 +267,7 @@ C<00110011>). The operators work with the binary form of a number (the number C<3> is treated as the bit pattern C<00000011>). So, saying C<11 & 3> performs the "and" operation on numbers (yielding -C<1>). Saying C<"11" & "3"> performs the "and" operation on strings +C<3>). Saying C<"11" & "3"> performs the "and" operation on strings (yielding C<"1">). Most problems with C<&> and C<|> arise because the programmer thinks @@ -194,21 +332,25 @@ will not create a list of 500,000 integers. =head2 How can I output Roman numerals? -Get the http://www.perl.com/CPAN/modules/by-module/Roman module. +Get the http://www.cpan.org/modules/by-module/Roman module. =head2 Why aren't my random numbers random? If you're using a version of Perl before 5.004, you must call C once at the start of your program to seed the random number generator. + + BEGIN { srand() if $] < 5.004 } + 5.004 and later automatically call C at the beginning. Don't -call C more than once--you make your numbers less random, rather +call C more than once---you make your numbers less random, rather than more. Computers are good at being predictable and bad at being random -(despite appearances caused by bugs in your programs :-). -http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom -Phoenix, talks more about this.. John von Neumann said, ``Anyone who -attempts to generate random numbers by deterministic means is, of +(despite appearances caused by bugs in your programs :-). see the +F article in the "Far More Than You Ever Wanted To Know" +collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of +Tom Phoenix, talks more about this. John von Neumann said, ``Anyone +who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.'' If you want numbers that are more random than C with C @@ -218,48 +360,78 @@ random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at ``Numerical Recipes in C'' at http://www.nr.com/ . +=head2 How do I get a random number between X and Y? + +C returns a number such that +C<< 0 <= rand($x) < $x >>. Thus what you want to have perl +figure out is a random number in the range from 0 to the +difference between your I and I. + +That is, to get a number between 10 and 15, inclusive, you +want a random number between 0 and 5 that you can then add +to 10. + + my $number = 10 + int rand( 15-10+1 ); + +Hence you derive the following simple function to abstract +that. It selects a random integer between the two given +integers (inclusive), For example: C. + + sub random_int_in ($$) { + my($min, $max) = @_; + # Assumes that the two arguments are integers themselves! + return $min if $min == $max; + ($min, $max) = ($max, $min) if $min > $max; + return $min + int rand(1 + $max - $min); + } + =head1 Data: Dates -=head2 How do I find the week-of-the-year/day-of-the-year? +=head2 How do I find the day or week of the year? + +The localtime function returns the day of the week. Without an +argument localtime uses the current time. -The day of the year is in the array returned by localtime() (see -L): + $day_of_year = (localtime)[7]; - $day_of_year = (localtime(time()))[7]; +The POSIX module can also format a date as the day of the year or +week of the year. -or more legibly (in 5.004 or higher): + use POSIX qw/strftime/; + my $day_of_year = strftime "%j", localtime; + my $week_of_year = strftime "%W", localtime; - use Time::localtime; - $day_of_year = localtime(time())->yday; +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. -You can find the week of the year by dividing this by 7: + use POSIX qw/strftime/; + use Time::Local; + my $week_of_year = strftime "%W", + localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) ); - $week_of_year = int($day_of_year / 7); +The Date::Calc module provides two functions for to calculate these. -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. + use Date::Calc; + my $day_of_year = Day_of_Year( 1987, 12, 18 ); + my $week_of_year = Week_of_Year( 1987, 12, 18 ); =head2 How do I find the current century or millennium? Use the following simple functions: - sub get_century { + sub get_century { return int((((localtime(shift || time))[5] + 1999))/100); - } - sub get_millennium { + } + 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<%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. +On some systems, the POSIX module's strftime() function has +been extended in a non-standard way to use a C<%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. =head2 How can I compare two dates and find the difference? @@ -285,76 +457,80 @@ and Date::Manip modules from CPAN. 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 I 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 +Before you immerse yourself too deeply in this, be sure to verify that +it is the I 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 -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.) +modules 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.) =head2 How do I find yesterday's date? -The C function returns the current time in seconds since the -epoch. Take twenty-four hours off that: +If you only need to find the date (and not the same time), you +can use the Date::Calc module. - $yesterday = time() - ( 24 * 60 * 60 ); + use Date::Calc qw(Today Add_Delta_Days); -Then you can pass this to C and get the individual year, -month, day, hour, minute, seconds values. + my @date = Add_Delta_Days( Today(), -1 ); -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. + 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 summer time +throws this off. Russ Allbery offers this solution. 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 - # This code is in the public domain + 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). + + =head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? @@ -398,7 +574,7 @@ addresses, etc.) for details. It depends just what you mean by ``escape''. URL escapes are dealt with in L. Shell escapes with the backslash (C<\>) -character are removed with: +character are removed with s/\\(.)/$1/g; @@ -422,14 +598,6 @@ 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 ${\($n + 5)} widgets\n"; - -Version 5.004 of Perl had a bug that gave list context to the -expression in C<${...}>, but this is fixed in version 5.005. - See also ``How can I expand variables in text strings?'' in this section of the FAQ. @@ -440,20 +608,22 @@ matter how complicated. To find something between two single characters, a pattern like C will get the intervening bits in $1. For multiple ones, then something more like C would be needed. But none of these deals with -nested patterns, nor can they. For that you'll have to write a -parser. +nested patterns. For balanced expressions using C<(>, C<{>, C<[> +or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see +L. For other cases, 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. +and the byacc program. Starting from perl 5.8 the Text::Balanced +is part of the standard distribution. 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 $1 - } + } A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and @@ -467,7 +637,7 @@ really does work: @( = ('(',''); @) = (')',''); ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; - @$ = (eval{/$re/},$@!~/unmatched/); + @$ = (eval{/$re/},$@!~/unmatched/i); print join("\n",@$[0..$#$]) if( $$[-1] ); =head2 How do I reverse a string? @@ -499,22 +669,33 @@ Use Text::Wrap (part of the standard Perl distribution): The paragraphs you give to Text::Wrap should not contain embedded newlines. Text::Wrap doesn't justify the lines (flush-right). -=head2 How can I access/change the first N letters of a string? +Or use the CPAN module Text::Autoformat. Formatting files can be easily +done by making a shell alias, like so: + + alias fmt="perl -i -MText::Autoformat -n0777 \ + -e 'print autoformat $_, {all=>1}' $*" + +See the documentation for Text::Autoformat to appreciate its many +capabilities. + +=head2 How can I access or change N characters of a string? + +You can access the first characters of a string with substr(). +To get the first character, for example, start at position 0 +and grab the string of length 1. -There are many ways. If you just want to grab a copy, use -substr(): - $first_byte = substr($a, 0, 1); + $string = "Just another Perl Hacker"; + $first_char = substr( $string, 0, 1 ); # 'J' -If you want to modify part of a string, the simplest way is often to -use substr() as an lvalue: +To change part of a string, you can use the optional fourth +argument which is the replacement string. - substr($a, 0, 3) = "Tom"; + substr( $string, 13, 4, "Perl 5.8.0" ); -Although those with a pattern matching kind of thought process will -likely prefer: +You can also use substr() as an lvalue. - $a =~ s/^.../Tom/; + substr( $string, 13, 4 ) = "Perl 5.8.0"; =head2 How do I change the Nth occurrence of something? @@ -549,7 +730,7 @@ repetition count and repeated pattern like this: =head2 How can I count the number of occurrences of a substring within a string? -There are a number of ways, with varying efficiency: If you want a +There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the C function like so: @@ -567,6 +748,11 @@ integers: while ($string =~ /-\d+/g) { $count++ } print "There are $count negative numbers in the string"; +Another version uses a global match in list context, then assigns the +result to a scalar, producing a count of the number of matches. + + $count = () = $string =~ /-\d+/g; + =head2 How do I capitalize all the words on one line? To make the first letter of each word upper case: @@ -574,8 +760,8 @@ To make the first letter of each word upper case: $line =~ s/\b(\w)/\U$1/g; This has the strange effect of turning "C" into "C". Sometimes you might want this, instead (Suggested by brian d. -foy): +Do It>". Sometimes you might want this. Other times you might need a +more thorough solution (Suggested by brian d foy): $string =~ s/ ( (^\w) #at the beginning of the line @@ -602,20 +788,34 @@ case", but that's not quite accurate. Consider the proper capitalization of the movie I, for example. -=head2 How can I split a [character] delimited string except when inside -[character]? (Comma-separated files) +Damian Conway's L module provides some smart +case transformations: -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 C because you shouldn't split if the comma is inside -quotes. For example, take a data line like this: + use Text::Autoformat; + my $x = "Dr. Strangelove or: How I Learned to Stop ". + "Worrying and Love the Bomb"; + + print $x, "\n"; + for my $style (qw( sentence title highlight )) + { + print autoformat($x, { case => $style }), "\n"; + } + +=head2 How can I split a [character] delimited string except when inside [character]? + +Several modules can handle this sort of pasing---Text::Balanced, +Text::CVS, Text::CVS_XS, and Text::ParseWords, among others. + +Take the example case of trying to split a string that is +comma-separated into its different fields. You can't use C +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 +problem. Thankfully, we have Jeffrey Friedl, author of +I, to handle these for us. He suggests (assuming your string is contained in $text): @new = (); @@ -628,8 +828,7 @@ suggests (assuming your string is contained in $text): If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, -C<"like \"this\"">. Unescaping them is a task addressed earlier in -this section. +C<"like \"this\"">. Alternatively, the Text::ParseWords module (part of the standard Perl distribution) lets you say: @@ -637,15 +836,15 @@ distribution) lets you say: use Text::ParseWords; @new = quotewords(",", 0, $text); -There's also a Text::CSV module on CPAN. +There's also a Text::CSV (Comma-Separated Values) module on CPAN. =head2 How do I strip blank space from the beginning/end of a string? -Although the simplest approach would seem to be: +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 +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: $string =~ s/^\s+//; @@ -660,10 +859,10 @@ Or more nicely written as: This idiom takes advantage of the C loop's aliasing behavior to factor out common code. You can do this -on several strings at once, or arrays, or even the +on several strings at once, or arrays, or even the values of a hash if you use a slice: - # trim whitespace in the scalar, the array, + # trim whitespace in the scalar, the array, # and all the values in the hash foreach ($scalar, @array, @hash{keys %hash}) { s/^\s+//; @@ -672,9 +871,6 @@ values of a hash if you use a slice: =head2 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, C<$pad_len> is the length to which you wish to pad the string, C<$text> or C<$num> contains the string to be padded, and C<$pad_char> contains the padding character. You can use a single @@ -689,13 +885,16 @@ right with blanks and it will truncate the result to a maximum length of C<$pad_len>. # Left padding a string with blanks (no truncation): - $padded = sprintf("%${pad_len}s", $text); + $padded = sprintf("%${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); + $padded = sprintf("%-${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); + # 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",$text); @@ -718,19 +917,19 @@ Left and right padding with any character, modifying C<$text> directly: =head2 How do I extract selected columns from a string? Use substr() or unpack(), both documented in L. -If you prefer thinking in terms of columns instead of widths, +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 { + sub cut2fmt { my(@positions) = @_; my $template = ''; my $lastpos = 1; for my $place (@positions) { - $template .= "A" . ($place - $lastpos) . " "; + $template .= "A" . ($place - $lastpos) . " "; $lastpos = $place; } $template .= "A*"; @@ -740,7 +939,7 @@ you can use this kind of thing: =head2 How do I find the soundex value of a string? Use the standard Text::Soundex module distributed with Perl. -But before you do so, you may want to determine whether `soundex' is in +Before you do so, you may want to determine whether `soundex' is in fact what you think it is. Knuth's soundex algorithm compresses words into a small space, and so it does not necessarily distinguish between two words which you might want to appear separately. For example, the @@ -768,7 +967,7 @@ be, you'd have to do this: It's probably better in the general case to treat those variables as entries in some special hash. For example: - %user_defs = ( + %user_defs = ( foo => 23, bar => 19, ); @@ -779,10 +978,10 @@ of the FAQ. =head2 What's wrong with always quoting "$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 +The problem is that those double-quotes force stringification-- +coercing numbers and references into strings--even when you +don't want them to be strings. Think of it this way: double-quote +expansion is used to produce new strings. If you already have a string, why do you need more? If you get used to writing odd things like these: @@ -813,27 +1012,27 @@ that actually do care about the difference between a string and a number, such as the magical C<++> autoincrement operator or the syscall() function. -Stringification also destroys arrays. +Stringification also destroys arrays. @lines = `command`; print "@lines"; # WRONG - extra blanks print @lines; # right -=head2 Why don't my <EHERE documents work? Check for these three things: =over 4 -=item 1. There must be no space after the << part. +=item There must be no space after the EE part. -=item 2. There (probably) should be a semicolon at the end. +=item There (probably) should be a semicolon at the end. -=item 3. You can't (easily) have any space in front of the tag. +=item You can't (easily) have any space in front of the tag. =back -If you want to indent the text in the here document, you +If you want to indent the text in the here document, you can do this: # all in one @@ -843,7 +1042,7 @@ can do this: HERE_TARGET But the HERE_TARGET must still be flush against the margin. -If you want that indented also, you'll have to quote +If you want that indented also, you'll have to quote in the indentation. ($quote = <<' FINIS') =~ s/^\s+//gm; @@ -852,18 +1051,18 @@ in the indentation. 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--/; + $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 +if so, strips that substring off. Otherwise, it takes the amount of leading +whitespace found on the first line and removes that much off each subsequent line. sub fix { local $_ = shift; - my ($white, $leader); # common white space and common leading string + my ($white, $leader); # common whitespace and common leading string if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { ($white, $leader) = ($2, quotemeta($1)); } else { @@ -886,7 +1085,7 @@ This works with leading special strings, dynamically determined: @@@ } MAIN_INTERPRETER_LOOP -Or with a fixed amount of leading white space, with remaining +Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved: $poem = fix< 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 C<@_>, push/pop/shift only work +access their arguments through the array C<@_>, and push/pop/shift only work on arrays. As a side note, there's no such thing as a list in scalar context. @@ -924,7 +1123,7 @@ last value to be returned: 9. =head2 What is the difference between $array[1] and @array[1]? -The former is a scalar value, the latter an array slice, which makes +The former is a scalar value; the latter an array slice, 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). @@ -938,7 +1137,7 @@ with @bad[0] = `same program that outputs several lines`; -The C pragma and the B<-w> flag will warn you about these +The C pragma and the B<-w> flag will warn you about these matters. =head2 How can I remove duplicate elements from a list or array? @@ -948,33 +1147,43 @@ ordered and whether you wish to preserve the ordering. =over 4 -=item a) If @in is sorted, and you want @out to be sorted: +=item a) + +If @in is sorted, and you want @out to be sorted: (this assumes all true values in the array) - $prev = 'nonesuch'; - @out = grep($_ ne $prev && ($prev = $_), @in); + $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 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. +uniq(1)'s behavior of removing only adjacent duplicates. The ", 1" +guarantees that the expression is true (so that grep picks it up) +even if the $_ is 0, "", or undef. + +=item b) -=item b) If you don't know whether @in is sorted: +If you don't know whether @in is sorted: undef %saw; @out = grep(!$saw{$_}++, @in); -=item c) Like (b), but @in contains only small integers: +=item c) + +Like (b), but @in contains only small integers: @out = grep(!$saw[$_]++, @in); -=item d) A way to do (b) without any loops or greps: +=item d) + +A way to do (b) without any loops or greps: undef %saw; @saw{@in} = (); @out = sort keys %saw; # remove sort if undesired -=item e) Like (d), but @in contains only small positive integers: +=item e) + +Like (d), but @in contains only small positive integers: undef @ary; @ary[@in] = @in; @@ -984,7 +1193,7 @@ nice in that it won't work with false values like undef, 0, or ""; But perhaps you should have been using a hash all along, eh? -=head2 How can I tell whether a list or array contains a certain element? +=head2 How can I tell whether a certain element is contained in a list or array? Hearing the word "in" is an Idication that you probably should have used a hash, not a list or array, to store your data. Hashes are @@ -992,11 +1201,11 @@ 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. +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/; - undef %is_blue; + %is_blue = (); for (@blues) { $is_blue{$_} = 1 } Now you can check whether $is_blue{$some_color}. It might have been a @@ -1006,7 +1215,7 @@ 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; + @is_tiny_prime = (); for (@primes) { $is_tiny_prime[$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes; @@ -1023,11 +1232,11 @@ Now check whether C is true for some C<$n>. Please do not use - $is_there = grep $_ eq $whatever, @array; + ($is_there) = grep $_ eq $whatever, @array; or worse yet - $is_there = grep /$whatever/, @array; + ($is_there) = grep /$whatever/, @array; These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are @@ -1057,7 +1266,7 @@ each element is unique in a given array: } Note that this is the I, that is, all elements in -either A or in B, but not in both. Think of it as an xor operation. +either A or in B but not in both. Think of it as an xor operation. =head2 How do I test whether two arrays or hashes are equal? @@ -1084,8 +1293,8 @@ like this one. It uses the CPAN module FreezeThaw: @a = @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", - cmpStr(\@a, \@b) == 0 - ? "the same" + cmpStr(\@a, \@b) == 0 + ? "the same" : "different"; This approach also works for comparing hashes. Here @@ -1095,7 +1304,7 @@ we'll demonstrate two different answers: %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); $a{EXTRA} = \%b; - $b{EXTRA} = \%a; + $b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; @@ -1110,16 +1319,37 @@ an exercise to the reader. =head2 How do I find the first array element for which a condition is true? -You can use this if you care about the index: +To find the first array element which satisfies a condition, you can +use the first() function in the List::Util module, which comes with +Perl 5.8. This example finds the first element that contains "Perl". - for ($i= 0; $i < @array; $i++) { - if ($array[$i] eq "Waldo") { - $found_index = $i; - last; - } - } + use List::Util qw(first); + + my $element = first { /Perl/ } @array; + +If you cannot use List::Util, you can make your own loop to do the +same thing. Once you find the element, you stop the loop with last. -Now C<$found_index> has what you want. + 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 = $i; + last; + } + } =head2 How do I handle linked lists? @@ -1148,7 +1378,7 @@ You could walk the list this way: } print "\n"; -You could grow the list this way: +You could add to the list this way: my ($head, $tail); $tail = append($head, 1); # grow a new head @@ -1180,24 +1410,36 @@ lists, or you could just do something like this with an array: =head2 How do I shuffle an array randomly? -Use this: +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. - # 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 $deck = shift; # $deck is a reference to an array + my $i = @$deck; + while ($i--) { my $j = int rand ($i+1); - next if $i == $j; - @$array[$i,$j] = @$array[$j,$i]; + @$deck[$i,$j] = @$deck[$j,$i]; } } - fisher_yates_shuffle( \@array ); # permutes @array in place + # shuffle my mpeg collection + # + my @mpeg =