X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/369b44b45b100ebccc1e9e9ec79851683e716faa..c812d14677001807a06200e23fed431e7ac774bb:/pod/perlfaq4.pod diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index 5e10a11..7c84de7 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq4 - Data Manipulation ($Revision: 1.44 $, $Date: 2003/07/28 17:35:21 $) +perlfaq4 - Data Manipulation ($Revision: 9681 $) =head1 DESCRIPTION @@ -11,75 +11,93 @@ numbers, dates, strings, arrays, hashes, and miscellaneous data issues. =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? -Internally, your computer represents floating-point 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. +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. +L shows the gory details of number representations and +conversions. -To limit the number of decimal places in your numbers, you -can use the printf or sprintf function. See the -L<"Floating Point Arithmetic"|perlop> for more details. +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 C is most probably working just fine. It's the numbers that +aren't quite what you think. + +First, see the answer to "Why am I getting long decimals +(eg, 19.9499999999999) instead of the numbers I should be getting +(eg, 19.95)?". + +For example, this + + print int(0.6/0.2-2), "\n"; + +will in most computers print 0, not 1, because even such simple +numbers as 0.6 and 0.2 cannot be presented exactly by floating-point +numbers. What you think in the above as 'three' is really more like +2.9999999999999995559. + =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. Octal literals in perl must start with a -leading "0" and hexadecimal literals must start with a leading "0x". +leading C<0> and hexadecimal literals must start with a leading C<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". +conversion takes place. You must explicitly use C or C if you +want the values converted to decimal. C interprets hexadecimal (C<0x350>), +octal (C<0350> or even without the leading C<0>, like C<377>) and binary +(C<0b1010>) numbers, while C only converts hexadecimal ones, with +or without a leading C<0x>, such as C<0x255>, C<3A>, C, or C. The inverse mapping from decimal to octal can be done with either the -"%o" or "%O" sprintf() formats. +<%o> or C<%O> C formats. -This problem shows up most often when people try using chmod(), mkdir(), -umask(), or sysopen(), which by widespread tradition typically take -permissions in octal. +This problem shows up most often when people try using C, +C, C, or C, which by widespread tradition +typically take permissions in octal. - chmod(644, $file); # WRONG - chmod(0644, $file); # right + 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 +C<644>, rather than the intended octal literal C<0644>. The problem can be seen with: - printf("%#o",644); # prints 01204 + 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. +with the following digits restricted to the set C<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 -certain number of digits, sprintf() or printf() is usually the easiest -route. +Remember that C merely truncates toward 0. For rounding to a +certain number of digits, C or C is usually the +easiest route. - printf("%.3f", 3.1415926535); # prints 3.142 + 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. +The C module (part of the standard Perl distribution) +implements C, C, and a number of other mathematical +and trigonometric functions. - use POSIX; - $ceil = ceil(3.5); # 4 - $floor = floor(3.5); # 3 + use POSIX; + $ceil = ceil(3.5); # 4 + $floor = floor(3.5); # 3 -In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex -module. With 5.004, the Math::Trig module (part of the standard Perl +In 5.000 to 5.003 perls, trigonometry was done in the C +module. With 5.004, the C module (part of the standard Perl distribution) implements the trigonometric functions. Internally it -uses the Math::Complex module and some functions can break out from +uses the C module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2. @@ -92,143 +110,148 @@ need yourself. To see why, notice how you'll still have an issue on half-way-point alternation: - for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} + 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 + 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. +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. -=head2 How do I convert between numeric representations? +=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. +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. +Some of the examples later in L use the C +module from CPAN. The reason you might choose C 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: +Using perl's built in conversion of C<0x> notation: - $int = 0xDEADBEEF; - $dec = sprintf("%d", $int); + $dec = 0xDEADBEEF; -Using the hex function: +Using the C function: - $int = hex("DEADBEEF"); - $dec = sprintf("%d", $int); + $dec = hex("DEADBEEF"); -Using pack: +Using C: - $int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); - $dec = sprintf("%d", $int); + $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); -Using the CPAN module Bit::Vector: +Using the CPAN module C: - use Bit::Vector; - $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); - $dec = $vec->to_Dec(); + 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: +Using C: - $hex = sprintf("%X", 3735928559); + $hex = sprintf("%X", 3735928559); # upper case A-F + $hex = sprintf("%x", 3735928559); # lower case a-f -Using unpack +Using C: - $hex = unpack("H*", pack("N", 3735928559)); + $hex = unpack("H*", pack("N", 3735928559)); -Using Bit::Vector +Using C: - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $hex = $vec->to_Hex(); + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $hex = $vec->to_Hex(); -And Bit::Vector supports odd bit counts: +And C supports odd bit counts: - use Bit::Vector; - $vec = Bit::Vector->new_Dec(33, 3735928559); - $vec->Resize(32); # suppress leading 0 if unwanted - $hex = $vec->to_Hex(); + use Bit::Vector; + $vec = Bit::Vector->new_Dec(33, 3735928559); + $vec->Resize(32); # suppress leading 0 if unwanted + $hex = $vec->to_Hex(); =item How do I convert from octal to decimal Using Perl's built in conversion of numbers with leading zeros: - $int = 033653337357; # note the leading 0! - $dec = sprintf("%d", $int); + $dec = 033653337357; # note the leading 0! -Using the oct function: +Using the C function: - $int = oct("33653337357"); - $dec = sprintf("%d", $int); + $dec = oct("33653337357"); -Using Bit::Vector: +Using C: - use Bit::Vector; - $vec = Bit::Vector->new(32); - $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); - $dec = $vec->to_Dec(); + 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: +Using C: - $oct = sprintf("%o", 3735928559); + $oct = sprintf("%o", 3735928559); -Using Bit::Vector +Using C: - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $oct = reverse join('', $vec->Chunk_List_Read(3)); + 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: +the C<0b> notation: $number = 0b10110110; -Using pack and ord +Using C: + + my $input = "10110110"; + $decimal = oct( "0b$input" ); - $decimal = ord(pack('B8', '10110110')); +Using C and C: -Using pack and unpack for larger strings + $decimal = ord(pack('B8', '10110110')); - $int = unpack("N", pack("B32", +Using C and C for larger strings: + + $int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); - $dec = sprintf("%d", $int); + $dec = sprintf("%d", $int); - # substr() is used to left pad a 32 character string with zeros. + # substr() is used to left pad a 32 character string with zeros. -Using Bit::Vector: +Using C: - $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); - $dec = $vec->to_Dec(); + $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); + $dec = $vec->to_Dec(); =item How do I convert from decimal to binary -Using unpack; +Using C (perl 5.6+): + + $bin = sprintf("%b", 3735928559); - $bin = unpack("B*", pack("N", 3735928559)); +Using C: -Using Bit::Vector: + $bin = unpack("B*", pack("N", 3735928559)); - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $bin = $vec->to_Bin(); +Using C: + + use Bit::Vector; + $vec = Bit::Vector->new_Dec(32, -559038737); + $bin = $vec->to_Bin(); The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. @@ -251,16 +274,16 @@ Most problems with C<&> and C<|> arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says: - if ("\020\020" & "\101\101") { - # ... - } + if ("\020\020" & "\101\101") { + # ... + } but a string consisting of two null bytes (the result of C<"\020\020" & "\101\101">) is not a false value in Perl. You need: - if ( ("\020\020" & "\101\101") !~ /[^\000]/) { - # ... - } + if ( ("\020\020" & "\101\101") !~ /[^\000]/) { + # ... + } =head2 How do I multiply matrices? @@ -272,38 +295,38 @@ or the PDL extension (also available from CPAN). To call a function on each element in an array, and collect the results, use: - @results = map { my_func($_) } @array; + @results = map { my_func($_) } @array; For example: - @triple = map { 3 * $_ } @single; + @triple = map { 3 * $_ } @single; To call a function on each element of an array, but ignore the results: - foreach $iterator (@array) { - some_func($iterator); - } + foreach $iterator (@array) { + some_func($iterator); + } To call a function on each integer in a (small) range, you B use: - @results = map { some_func($_) } (5 .. 25); + @results = map { some_func($_) } (5 .. 25); but you should be aware that the C<..> 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)); - } + @results = (); + for ($i=5; $i < 500_005; $i++) { + push(@results, some_func($i)); + } This situation has been fixed in Perl5.005. Use of C<..> in a C loop will iterate over the range, without creating the entire range. - for my $i (5 .. 500_005) { - push(@results, some_func($i)); - } + for my $i (5 .. 500_005) { + push(@results, some_func($i)); + } will not create a list of 500,000 integers. @@ -319,197 +342,172 @@ once at the start of your program to seed the random number generator. BEGIN { srand() if $] < 5.004 } 5.004 and later automatically call C at the beginning. Don't -call C more than once---you make your numbers less random, rather -than more. +call C more than once--you make your numbers less random, +rather than more. Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). see the F article in the "Far More Than You Ever Wanted To Know" -collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of -Tom Phoenix, talks more about this. John von Neumann said, ``Anyone +collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy +of Tom Phoenix, talks more about this. John von Neumann said, "Anyone who attempts to generate random numbers by deterministic means is, of -course, living in a state of sin.'' +course, living in a state of sin." If you want numbers that are more random than C with C -provides, you should also check out the Math::TrulyRandom module from +provides, you should also check out the C module from CPAN. It uses the imperfections in your system's timer to generate random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at -``Numerical Recipes in C'' at http://www.nr.com/ . +"Numerical Recipes in C" at http://www.nr.com/ . =head2 How do I get a random number between X and Y? -Use the following simple function. It selects a random integer between -(and possibly including!) the two given integers, e.g., -C +To get a random number between two values, you can use the C +builtin to get a random number between 0 and 1. From there, you shift +that into the range that you want. + +C returns a number such that C<< 0 <= rand($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. - 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); - } +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 $number = 10 + int rand( 15-10+1 ); + +Hence you derive the following simple function to abstract +that. It selects a random integer between the two given +integers (inclusive), For example: C. + + sub random_int_between { + my($min, $max) = @_; + # Assumes that the two arguments are integers themselves! + return $min if $min == $max; + ($min, $max) = ($max, $min) if $min > $max; + return $min + int rand(1 + $max - $min); + } =head1 Data: Dates =head2 How do I find the day or week of the year? -The localtime function returns the day of the week. Without an +The localtime function returns the day of the year. Without an argument localtime uses the current time. - $day_of_year = (localtime)[7]; - -The POSIX module can also format a date as the day of the year or + $day_of_year = (localtime)[7]; + +The C module can also format a date as the day of the year or week of the year. use POSIX qw/strftime/; my $day_of_year = strftime "%j", localtime; my $week_of_year = strftime "%W", localtime; -To get the day of year for any date, use the Time::Local module to get +To get the day of year for any date, use C's C to get a time in epoch seconds for the argument to localtime. - - use POSIX qw/strftime/; - use Time::Local; - my $week_of_year = strftime "%W", - localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) ); -The Date::Calc module provides two functions for to calculate these. + use POSIX qw/mktime strftime/; + my $week_of_year = strftime "%W", + localtime( mktime( 0, 0, 0, 18, 11, 87 ) ); + +The C module provides two functions to calculate these. 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 { - return int((((localtime(shift || time))[5] + 1999))/100); - } - sub get_millennium { - return 1+int((((localtime(shift || time))[5] + 1899))/1000); - } - -You can also use the POSIX strftime() function which may be a bit -slower but is easier to read and maintain. - - use POSIX qw/strftime/; + sub get_century { + return int((((localtime(shift || time))[5] + 1999))/100); + } - my $week_of_the_year = strftime "%W", localtime; - my $day_of_the_year = strftime "%j", localtime; + sub get_millennium { + return 1+int((((localtime(shift || time))[5] + 1899))/1000); + } -On some systems, the POSIX module's strftime() function has -been extended in a non-standard way to use a C<%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 C module's C 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? -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. +(contributed by brian d foy) + +You could just store all your dates as a number and then subtract. +Life isn't always that simple though. If you want to work with +formatted dates, the C, C, or C +modules can help you. =head2 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 C in the standard -Time::Local module. Otherwise, you should look into the Date::Calc -and Date::Manip modules from CPAN. +C module. Otherwise, you should look into the C +and C modules from CPAN. =head2 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 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 -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.) +(contributed by brian d foy and Dave Cross) -=head2 How do I find yesterday's date? +You can use the C module available on CPAN. Ensure +that you really want to find a Julian day, though, as many people have +different ideas about Julian days. See +http://www.hermetic.ch/cal_stud/jdn.htm for instance. -If you only need to find the date (and not the same time), you -can use the Date::Calc module. +You can also try the C module, which can convert a date/time +to a Julian Day. - use Date::Calc qw(Today Add_Delta_Days); + $ perl -MDateTime -le'print DateTime->today->jd' + 2453401.5 - my @date = Add_Delta_Days( Today(), -1 ); +Or the modified Julian Day - print "@date\n"; + $ perl -MDateTime -le'print DateTime->today->mjd' + 53401 -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. +Or even the day of the year (which is what some people think of as a +Julian day) - 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; - } + $ perl -MDateTime -le'print DateTime->today->doy' + 31 -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. +=head2 How do I find yesterday's date? + +(contributed by brian d foy) + +Use one of the Date modules. The C module makes it simple, and +give you the same time of day, only the day before. + + use DateTime; -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. + my $yesterday = DateTime->now->subtract( days => 1 ); -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). + print "Yesterday was $yesterday\n"; -Note that between 2am and 3am on the day after the time zone switches -off daylight savings time, the exact hour of "yesterday" corresponding -to the current hour is not clearly defined. Note also that if used -between 2am and 3am the day after the change to daylight savings time, -the result will be between 3am and 4am of the previous day; it's -arguable whether this is correct. +You can also use the C module using its C +function. -This sub does not attempt to deal with leap seconds (most things don't). + use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); + my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); + print "@date_time\n"; -=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? +Most people try to use the time rather than the calendar to figure out +dates, but that assumes that days are twenty-four hours each. For +most people, there are two days a year when they aren't: the switch to +and from summer time throws this off. Let the modules do the work. + +=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is -Y2K compliant (whatever that means). The programmers you've hired to +Y2K compliant (whatever that means). The programmers you've hired to use it, however, probably are not. Long answer: The question belies a true understanding of the issue. @@ -532,48 +530,102 @@ C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 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 +not the language. At the risk of inflaming the NRA: "Perl doesn't +break Y2K, people do." See http://www.perl.org/about/y2k.html for a longer exposition. =head1 Data: Strings =head2 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. +(contributed by brian d foy) + +There are many ways to ensure that values are what you expect or +want to accept. Besides the specific examples that we cover in the +perlfaq, you can also look at the modules with "Assert" and "Validate" +in their names, along with other modules such as C. + +Some modules have validation for particular types of input, such +as C, C, C, +and C. =head2 How do I unescape a string? -It depends just what you mean by ``escape''. URL escapes are dealt +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 - s/\\(.)/$1/g; + s/\\(.)/$1/g; This won't expand C<"\n"> or C<"\t"> or any other special escapes. =head2 How do I remove consecutive pairs of characters? -To turn C<"abbcccd"> into C<"abccd">: +(contributed by brian d foy) - s/(.)\1/$1/g; # add /s to include newlines +You can use the substitution operator to find pairs of characters (or +runs of characters) and replace them with a single instance. In this +substitution, we find a character in C<(.)>. The memory parentheses +store the matched character in the back-reference C<\1> and we use +that to require that the same thing immediately follow it. We replace +that part of the string with the character in C<$1>. -Here's a solution that turns "abbcccd" to "abcd": + s/(.)\1/$1/g; - y///cs; # y == tr, but shorter :-) +We can also use the transliteration operator, C. In this +example, the search list side of our C contains nothing, but +the C option complements that so it contains everything. The +replacement list also contains nothing, so the transliteration is +almost a no-op since it won't do any replacements (or more exactly, +replace the character with itself). However, the C option squashes +duplicated and consecutive characters in the string so a character +does not show up next to itself + + my $str = 'Haarlem'; # in the Netherlands + $str =~ tr///cs; # Now Harlem, like in New York =head2 How do I expand function calls in a string? -This is documented in L. 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: +(contributed by brian d foy) + +This is documented in L, and although it's not the easiest +thing to read, it does work. In each of these examples, we call the +function inside the braces used to dereference a reference. If we +have more than one return value, we can construct and dereference an +anonymous array. In this case, we call the function in list context. + + print "The time values are @{ [localtime] }.\n"; + +If we want to call the function in scalar context, we have to do a bit +more work. We can really have any code we like inside the braces, so +we simply have to end with the scalar reference, although how you do +that is up to you, and you can use code inside the braces. Note that +the use of parens creates a list context, so we need C to +force the scalar context on the function: + + print "The time is ${\(scalar localtime)}.\n" + + print "The time is ${ my $x = localtime; \$x }.\n"; - print "My sub returned @{[mysub(1,2,3)]} that time.\n"; +If your function already returns a reference, you don't need to create +the reference yourself. -See also ``How can I expand variables in text strings?'' in this -section of the FAQ. + sub timestamp { my $t = localtime; \$t } + + print "The time is ${ timestamp() }.\n"; + +The C module can also do a lot of magic for you. You can +specify a variable name, in this case C, to set up a tied hash that +does the interpolation for you. It has several other methods to do this +as well. + + use Interpolation E => 'eval'; + print "The time values are $E{localtime()}.\n"; + +In most cases, it is probably easier to simply use string concatenation, +which also forces scalar context. + + print "The time is " . localtime() . ".\n"; =head2 How do I find matching/nesting anything? @@ -581,75 +633,76 @@ 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 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. 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. +C would be needed. But none of these deals with +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. Starting from perl 5.8 the Text::Balanced -is part of the standard distribution. +the CPAN modules C, C, and +C; and the C program. Starting from perl 5.8 +the C 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 - } + 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. + # $_ contains the string to parse + # BEGIN and END are the opening and closing markers for the + # nested text. - @( = ('(',''); - @) = (')',''); - ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; - @$ = (eval{/$re/},$@!~/unmatched/i); - print join("\n",@$[0..$#$]) if( $$[-1] ); + @( = ('(',''); + @) = (')',''); + ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; + @$ = (eval{/$re/},$@!~/unmatched/i); + print join("\n",@$[0..$#$]) if( $$[-1] ); =head2 How do I reverse a string? -Use reverse() in scalar context, as documented in +Use C in scalar context, as documented in L. - $reversed = reverse $string; + $reversed = reverse $string; =head2 How do I expand tabs in a string? You can do it yourself: - 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; + 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 +Or you can just use the C module (part of the standard Perl distribution). - use Text::Tabs; - @expanded_lines = expand(@lines_with_tabs); + use Text::Tabs; + @expanded_lines = expand(@lines_with_tabs); =head2 How do I reformat a paragraph? -Use Text::Wrap (part of the standard Perl distribution): +Use C (part of the standard Perl distribution): - use Text::Wrap; - print wrap("\t", ' ', @paragraphs); + 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). +The paragraphs you give to C should not contain embedded +newlines. C doesn't justify the lines (flush-right). -Or use the CPAN module Text::Autoformat. Formatting files can be easily -done by making a shell alias, like so: +Or use the CPAN module C. 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}' $*" + alias fmt="perl -i -MText::Autoformat -n0777 \ + -e 'print autoformat $_, {all=>1}' $*" -See the documentation for Text::Autoformat to appreciate its many +See the documentation for C to appreciate its many capabilities. =head2 How can I access or change N characters of a string? @@ -660,16 +713,16 @@ and grab the string of length 1. $string = "Just another Perl Hacker"; - $first_char = substr( $string, 0, 1 ); # 'J' + $first_char = substr( $string, 0, 1 ); # 'J' To change part of a string, you can use the optional fourth argument which is the replacement string. - substr( $string, 13, 4, "Perl 5.8.0" ); + substr( $string, 13, 4, "Perl 5.8.0" ); You can also use substr() as an lvalue. - substr( $string, 13, 4 ) = "Perl 5.8.0"; + substr( $string, 13, 4 ) = "Perl 5.8.0"; =head2 How do I change the Nth occurrence of something? @@ -678,29 +731,29 @@ to change the fifth occurrence of C<"whoever"> or C<"whomever"> into C<"whosoever"> or C<"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; + $count = 0; + s{((whom?)ever)}{ + ++$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 C modifier in a C 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"; - } - } + $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: C<"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; + /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; =head2 How can I count the number of occurrences of a substring within a string? @@ -708,9 +761,9 @@ 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: - $string = "ThisXlineXhasXsomeXx'sXinXit"; - $count = ($string =~ tr/X//); - print "There are $count X characters in the string"; + $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 @@ -718,9 +771,9 @@ larger string, C 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"; + $string = "-9 55 48 -2 23 -76 4 14 -44"; + while ($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. @@ -731,27 +784,28 @@ result to a scalar, producing a count of the number of matches. To make the first letter of each word upper case: - $line =~ s/\b(\w)/\U$1/g; + $line =~ s/\b(\w)/\U$1/g; This has the strange effect of turning "C" into "C". 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 - | # or - (\s\w) #preceded by whitespace - ) - /\U$1/xg; - $string =~ /([\w']+)/\u\L$1/g; + $string =~ s/ ( + (^\w) #at the beginning of the line + | # or + (\s\w) #preceded by whitespace + ) + /\U$1/xg; + + $string =~ s/([\w']+)/\u\L$1/g; To make the whole line upper case: - $line = uc($line); + $line = uc($line); To force each word to be lower case, with the first letter upper case: - $line =~ s/(\w+)/\u\L$1/g; + $line =~ s/(\w+)/\u\L$1/g; You can (and probably should) enable locale awareness of those characters by placing a C pragma in your program. @@ -765,83 +819,98 @@ Stop Worrying and Love the Bomb>, for example. Damian Conway's L module provides some smart case transformations: - use Text::Autoformat; - my $x = "Dr. Strangelove or: How I Learned to Stop ". - "Worrying and Love the Bomb"; + 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"; - } + 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. +Several modules can handle this sort of parsing--C, +C, C, and C, 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" + 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 I, to handle these for us. He -suggests (assuming your string is contained in $text): +suggests (assuming your string is contained in C<$text>): - @new = (); - push(@new, $+) while $text =~ m{ - "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes - | ([^,]+),? - | , - }gx; - push(@new, undef) if substr($text,-1,1) eq ','; + @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, C<"like \"this\"">. -Alternatively, the Text::ParseWords module (part of the standard Perl -distribution) lets you say: - - use Text::ParseWords; - @new = quotewords(",", 0, $text); +Alternatively, the C module (part of the standard +Perl distribution) lets you say: -There's also a Text::CSV (Comma-Separated Values) module on CPAN. + use Text::ParseWords; + @new = quotewords(",", 0, $text); =head2 How do I strip blank space from the beginning/end of a string? -Although the simplest approach would seem to be +(contributed by brian d foy) - $string =~ s/^\s*(.*?)\s*$/$1/; +A substitution can do this for you. For a single line, you want to +replace all the leading or trailing whitespace with nothing. You +can do that with a pair of substitutions. -not only is this unnecessarily slow and destructive, it also fails with -embedded newlines. It is much faster to do this operation in two steps: + s/^\s+//; + s/\s+$//; - $string =~ s/^\s+//; - $string =~ s/\s+$//; +You can also write that as a single substitution, although it turns +out the combined statement is slower than the separate ones. That +might not matter to you, though. -Or more nicely written as: + s/^\s+|\s+$//g; - for ($string) { - s/^\s+//; - s/\s+$//; - } +In this regular expression, the alternation matches either at the +beginning or the end of the string since the anchors have a lower +precedence than the alternation. With the C flag, the substitution +makes all possible matches, so it gets both. Remember, the trailing +newline matches the C<\s+>, and the C<$> anchor can match to the +physical end of the string, so the newline disappears too. Just add +the newline to the output, which has the added benefit of preserving +"blank" (consisting entirely of whitespace) lines which the C<^\s+> +would remove all by itself. -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 -values of a hash if you use a slice: + while( <> ) + { + s/^\s+|\s+$//g; + print "$_\n"; + } - # trim whitespace in the scalar, the array, - # and all the values in the hash - foreach ($scalar, @array, @hash{keys %hash}) { - s/^\s+//; - s/\s+$//; - } +For a multi-line string, you can apply the regular expression +to each logical line in the string by adding the C flag (for +"multi-line"). With the C flag, the C<$> matches I an +embedded newline, so it doesn't remove it. It still removes the +newline at the end of the string. + + $string =~ s/^\s+|\s+$//gm; + +Remember that lines consisting entirely of whitespace will disappear, +since the first part of the alternation can match the entire string +and replace it with nothing. If need to keep embedded blank lines, +you have to do a little more work. Instead of matching any whitespace +(since that includes a newline), just match the other whitespace. + + $string =~ s/^[\t\f ]+|[\t\f ]+$//mg; =head2 How do I pad a string with blanks or pad a number with zeroes? @@ -858,20 +927,20 @@ truncate the result. The C function can only pad strings on the 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): + # Left padding a string with blanks (no truncation): $padded = sprintf("%${pad_len}s", $text); $padded = sprintf("%*s", $pad_len, $text); # same thing - # Right padding a string with blanks (no truncation): + # Right padding a string with blanks (no truncation): $padded = sprintf("%-${pad_len}s", $text); $padded = sprintf("%-*s", $pad_len, $text); # same thing - # Left padding a number with 0 (no truncation): + # 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); + # Right padding a string with blanks using pack (will truncate): + $padded = pack("A$pad_len",$text); If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the @@ -880,106 +949,133 @@ not truncate C<$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 ) ); + $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; + $padded = $text . $pad_char x ( $pad_len - length( $text ) ); Left and right padding with any character, modifying C<$text> directly: - substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); - $text .= $pad_char x ( $pad_len - length( $text ) ); + substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); + $text .= $pad_char x ( $pad_len - length( $text ) ); =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, -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; - } +(contributed by brian d foy) + +If you know where the columns that contain the data, you can +use C to extract a single column. + + my $column = substr( $line, $start_column, $length ); + +You can use C if the columns are separated by whitespace or +some other delimiter, as long as whitespace or the delimiter cannot +appear as part of the data. + + my $line = ' fred barney betty '; + my @columns = split /\s+/, $line; + # ( '', 'fred', 'barney', 'betty' ); + + my $line = 'fred||barney||betty'; + my @columns = split /\|/, $line; + # ( 'fred', '', 'barney', '', 'betty' ); + +If you want to work with comma-separated values, don't do this since +that format is a bit more complicated. Use one of the modules that +handle that fornat, such as C, C, or +C. + +If you want to break apart an entire line of fixed columns, you can use +C with the A (ASCII) format. by using a number after the format +specifier, you can denote the column width. See the C and C +entries in L for more details. + + my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); + +Note that spaces in the format argument to C do not denote literal +spaces. If you have space separated data, you may want C instead. =head2 How do I find the soundex value of a string? -Use the standard Text::Soundex module distributed with Perl. -Before you do so, you may want to determine whether `soundex' is in -fact what you think it is. Knuth's soundex algorithm compresses words -into a small space, and so it does not necessarily distinguish between -two words which you might want to appear separately. For example, the -last names `Knuth' and `Kant' are both mapped to the soundex code K530. -If Text::Soundex does not do what you are looking for, you might want -to consider the String::Approx module available at CPAN. +(contributed by brian d foy) + +You can use the Text::Soundex module. If you want to do fuzzy or close +matching, you might also try the C, and +C, and C modules. =head2 How can I expand variables in text strings? -Let's assume that you have a string like: +(contributed by brian d foy) - $text = 'this has a $foo in it and a $bar'; +If you can avoid it, don't, or if you can use a templating system, +such as C or C