NG4R3P

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/proc/self/root/usr/share/perl5/pod/

38.135.39.45

[ NAME ]	[ SIZE ]	[ PERM ]	[ DATE ]	[ ACT ]
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a2p.pod	5.962 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl.pod	15.429 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5004delta.pod	54.921 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5005delta.pod	33.48 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5100delta.pod	53.413 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5101delta.pod	42.858 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5120delta.pod	87.193 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5121delta.pod	9.905 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5122delta.pod	9.378 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5123delta.pod	4.004 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5124delta.pod	3.586 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5140delta.pod	140.941 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5141delta.pod	7.779 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5142delta.pod	6.73 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5143delta.pod	7.578 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5160delta.pod	130.52 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5161delta.pod	5.998 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5162delta.pod	3.51 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl5163delta.pod	3.985 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl561delta.pod	121.786 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl56delta.pod	104.685 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl581delta.pod	37.168 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl582delta.pod	4.365 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl583delta.pod	6.187 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl584delta.pod	7.19 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl585delta.pod	5.751 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl586delta.pod	4.542 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl587delta.pod	8.161 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl588delta.pod	24.68 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl589delta.pod	52.637 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perl58delta.pod	112.215 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlaix.pod	18.768 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlamiga.pod	6.869 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlapi.pod	315.465 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlapio.pod	18.878 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlartistic.pod	6.846 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlbeos.pod	2.867 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlbook.pod	7.192 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlboot.pod	0.179 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlbot.pod	0.178 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlbs2000.pod	7.733 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlcall.pod	54.026 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlce.pod	8.723 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlcheat.pod	4.395 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlclib.pod	7.504 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlcn.pod	4.815 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlcommunity.pod	6.293 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlcygwin.pod	27.17 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldata.pod	36.331 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldbmfilter.pod	4.863 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldebguts.pod	36.789 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldebtut.pod	20.786 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldebug.pod	38.151 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldelta.pod	3.985 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldgux.pod	2.754 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldiag.pod	207.818 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldos.pod	10.275 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldsc.pod	24.837 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perldtrace.pod	6.209 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlebcdic.pod	67.605 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlembed.pod	35.209 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlepoc.pod	3.572 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlexperiment.pod	4.843 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq.pod	22.007 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq1.pod	13.519 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq2.pod	9.278 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq3.pod	37.464 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq4.pod	87.393 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq5.pod	54.113 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq6.pod	38.664 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq7.pod	36.352 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq8.pod	48.279 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfaq9.pod	14.709 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfork.pod	12.782 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlform.pod	16.287 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfreebsd.pod	1.554 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlfunc.pod	338.432 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlgit.pod	29.755 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlglossary.pod	110.655 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlgpl.pod	13.537 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlguts.pod	111.664 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhack.pod	35.03 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhacktips.pod	45.498 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhacktut.pod	6.068 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhaiku.pod	1.469 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhist.pod	43.319 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhpux.pod	28.065 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlhurd.pod	1.94 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlintern.pod	42.532 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlinterp.pod	30.002 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlintro.pod	22.085 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perliol.pod	33.027 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlipc.pod	70.165 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlirix.pod	4.291 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perljp.pod	7.572 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlko.pod	7.521 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perllexwarn.pod	14.612 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perllinux.pod	1.453 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perllocale.pod	51.435 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perllol.pod	10.935 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmacos.pod	0.978 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmacosx.pod	10.399 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmod.pod	24.041 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmodinstall.pod	12.412 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmodlib.pod	78.491 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmodstyle.pod	20.757 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmpeix.pod	14.236 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlmroapi.pod	3.128 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlnetware.pod	6.348 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlnewmod.pod	10.952 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlnumber.pod	8.156 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlobj.pod	33.653 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlootut.pod	25.604 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlop.pod	121.731 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlopenbsd.pod	1.176 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlopentut.pod	37.533 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlos2.pod	90.533 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlos390.pod	15.2 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlos400.pod	4.513 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlpacktut.pod	49.834 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlperf.pod	50.048 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlplan9.pod	5.005 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlpod.pod	21.27 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlpodspec.pod	66.203 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlpolicy.pod	19.726 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlport.pod	82.635 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlpragma.pod	5.105 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlqnx.pod	4.145 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlre.pod	100.761 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlreapi.pod	25.168 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlrebackslash.pod	25.644 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlrecharclass.pod	34.188 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlref.pod	28.319 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlreftut.pod	18.232 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlreguts.pod	35.995 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlrequick.pod	17.499 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlreref.pod	14.19 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlretut.pod	115.128 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlriscos.pod	1.493 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlrun.pod	49.575 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsec.pod	22.768 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsolaris.pod	28.63 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsource.pod	6.194 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlstyle.pod	8.416 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsub.pod	55.153 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsymbian.pod	15.439 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlsyn.pod	41.044 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlthrtut.pod	45.407 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltie.pod	37.02 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltoc.pod	638.995 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltodo.pod	0.354 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltooc.pod	0.179 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltoot.pod	0.179 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltrap.pod	40.281 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltru64.pod	7.552 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perltw.pod	5.151 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlunicode.pod	70.891 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlunifaq.pod	13.314 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perluniintro.pod	35.443 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perluniprops.pod	229.736 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlunitut.pod	7.756 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlutil.pod	9.679 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perluts.pod	3.105 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlvar.pod	69.188 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlvmesa.pod	3.881 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlvms.pod	51.333 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlvos.pod	5.823 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlwin32.pod	34.581 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlxs.pod	71.663 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlxstut.pod	48.517 KB	-rw-r--r--	2023-10-25 12:41	R E G D
perlxstypemap.pod	22.966 KB	-rw-r--r--	2023-10-25 12:41	R E G D
REQUEST EXIT

=head1 NAME perlfaq4 - Data Manipulation =head1 DESCRIPTION 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)? For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (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 shows the gory details of number representations and conversions. To limit the number of decimal places in your numbers, you can use the C or C function. See L 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? (contributed by brian d foy) You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10: my $string = '0644'; print $string + 0; # prints 644 print $string + 44; # prints 688, certainly not octal! This problem usually involves one of the Perl built-ins that has the same name a Unix command that uses octal numbers as arguments on the command line. In this example, C on the command line knows that its first argument is octal because that's what it does: %prompt> chmod 644 file If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers either by prefixing the digits with a C<0> or using C: chmod( 0644, $filename ); # right, has leading zero chmod( oct(644), $filename ); # also correct The problem comes in when you take your numbers from something that Perl thinks is a string, such as a command line argument in C<@ARGV>: chmod( $ARGV[0], $filename ); # wrong, even if "0644" chmod( oct($ARGV[0]), $filename ); # correct, treat string as octal You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format: printf "0%o %d", $number, $number; =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? 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 The L module (part of the standard Perl distribution) implements C, C, and a number of other mathematical and trigonometric functions. use POSIX; my $ceil = ceil(3.5); # 4 my $floor = floor(3.5); # 3 In 5.000 to 5.003 perls, trigonometry was done in the L module. With 5.004, the L module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the L module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2. Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system of rounding is being used by Perl, but instead to implement the rounding function you need yourself. To see why, notice how you'll still have an issue on half-way-point alternation: for (my $i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0 Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32-bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed. =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 later in L use the L module from CPAN. The reason you might choose L 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 C<0x> notation: my $dec = 0xDEADBEEF; Using the C function: my $dec = hex("DEADBEEF"); Using C: my $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); Using the CPAN module C: use Bit::Vector; my $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); my $dec = $vec->to_Dec(); =item How do I convert from decimal to hexadecimal Using C: my $hex = sprintf("%X", 3735928559); # upper case A-F my $hex = sprintf("%x", 3735928559); # lower case a-f Using C: my $hex = unpack("H*", pack("N", 3735928559)); Using L: use Bit::Vector; my $vec = Bit::Vector->new_Dec(32, -559038737); my $hex = $vec->to_Hex(); And L supports odd bit counts: use Bit::Vector; my $vec = Bit::Vector->new_Dec(33, 3735928559); $vec->Resize(32); # suppress leading 0 if unwanted my $hex = $vec->to_Hex(); =item How do I convert from octal to decimal Using Perl's built in conversion of numbers with leading zeros: my $dec = 033653337357; # note the leading 0! Using the C function: my $dec = oct("33653337357"); Using L: use Bit::Vector; my $vec = Bit::Vector->new(32); $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); my $dec = $vec->to_Dec(); =item How do I convert from decimal to octal Using C: my $oct = sprintf("%o", 3735928559); Using L: use Bit::Vector; my $vec = Bit::Vector->new_Dec(32, -559038737); my $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 C<0b> notation: my $number = 0b10110110; Using C: my $input = "10110110"; my $decimal = oct( "0b$input" ); Using C and C: my $decimal = ord(pack('B8', '10110110')); Using C and C for larger strings: my $int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); my $dec = sprintf("%d", $int); # substr() is used to left-pad a 32-character string with zeros. Using L: my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); my $dec = $vec->to_Dec(); =item How do I convert from decimal to binary Using C (perl 5.6+): my $bin = sprintf("%b", 3735928559); Using C: my $bin = unpack("B*", pack("N", 3735928559)); Using L: use Bit::Vector; my $vec = Bit::Vector->new_Dec(32, -559038737); my $bin = $vec->to_Bin(); The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. =back =head2 Why doesn't & work the way I want it to? The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string C<"3"> is the bit pattern C<00110011>). The operators work with the binary form of a number (the number C<3> is treated as the bit pattern C<00000011>). So, saying C<11 & 3> performs the "and" operation on numbers (yielding C<3>). Saying C<"11" & "3"> performs the "and" operation on strings (yielding C<"1">). Most problems with C<&> and C<|> arise because the programmer thinks they have a number but really it's a string or vice versa. To avoid this, stringify the arguments explicitly (using C<""> or C) or convert them to numbers explicitly (using C<0+$arg>). The rest arise because the programmer says: 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]/) { # ... } =head2 How do I multiply matrices? Use the L or L modules (available from CPAN) or the L extension (also available from CPAN). =head2 How do I perform an operation on a series of integers? To call a function on each element in an array, and collect the results, use: my @results = map { my_func($_) } @array; For example: my @triple = map { 3 * $_ } @single; To call a function on each element of an array, but ignore the results: foreach my $iterator (@array) { some_func($iterator); } To call a function on each integer in a (small) range, you B use: my @results = map { some_func($_) } (5 .. 25); but you should be aware that in this form, the C<..> operator creates a list of all integers in the range, which can take a lot of memory for large ranges. However, the problem does not occur when using C<..> within a C loop, because in that case the range operator is optimized to I over the range, without creating the entire list. So my @results = (); for my $i (5 .. 500_005) { push(@results, some_func($i)); } or even push(@results, some_func($_)) for 5 .. 500_005; will not create an intermediate list of 500,000 integers. =head2 How can I output Roman numerals? Get the L 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 than more. Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). The F article in the "Far More Than You Ever Wanted To Know" collection in L, 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." Perl relies on the underlying system for the implementation of C and C; on some systems, the generated numbers are not random enough (especially on Windows : see L). Several CPAN modules in the C

namespace implement better
pseudorandom generators; see for example
L ("Mersenne Twister", fast), or
L (uses the imperfections in the system's
timer to generate random numbers, which is rather slow).
More algorithms for random numbers are described in
"Numerical Recipes in C" at L =head2 How do I get a random number between X and Y?

To get a random number between two values, you can use the C built-in 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 .

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 ); # ( 10,11,12,13,14, or 15 )

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 day of the year is in the list returned
by the C function. Without an
argument C uses the current time.

    my $day_of_year = (localtime)[7];

The L 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 L's C to get
a time in epoch seconds for the argument to C .

    use POSIX qw/mktime strftime/;
    my $week_of_year = strftime "%W",
        localtime( mktime( 0, 0, 0, 18, 11, 87 ) );

You can also use L, which comes with Perl and provides a
C that returns an object:

    use Time::Piece;
    my $day_of_year  = localtime->yday;
    my $week_of_year = localtime->week;

The L module provides two functions to calculate these, too:

    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);
    }

On some systems, the L 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 determine reliably the current century or
millennium.

=head2 How can I compare two dates and find the difference?

(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.

The L module, which comes with Perl, replaces L with a version that returns an object. It also overloads the comparison
operators so you can compare them directly:

    use Time::Piece;
    my $date1 = localtime( $some_time );
    my $date2 = localtime( $some_other_time );

    if( $date1 < $date2 ) {
        print "The date was in the past\n";
    }

You can also get differences with a subtraction, which returns a
L object:

    my $diff = $date1 - $date2;
    print "The difference is ", $date_diff->days, " days\n";

If you want to work with formatted dates, the L,
L, or L 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
L module. Otherwise, you should look into the L,
L, and L modules from CPAN.

=head2 How can I find the Julian Day?

(contributed by brian d foy and Dave Cross)

You can use the L module, part of the Standard Library,
which can convert a date/time to a Julian Day:

    $ perl -MTime::Piece -le 'print localtime->julian_day'
    2455607.7959375

Or the modified Julian Day:

    $ perl -MTime::Piece -le 'print localtime->mjd'
    55607.2961226851

Or even the day of the year (which is what some people think of as a
Julian day):

    $ perl -MTime::Piece -le 'print localtime->yday'
    45

You can also do the same things with the L module:

    $ perl -MDateTime -le'print DateTime->today->jd'
    2453401.5
    $ perl -MDateTime -le'print DateTime->today->mjd'
    53401
    $ perl -MDateTime -le'print DateTime->today->doy'
    31

You can use the L 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 L for instance):

    $  perl -MTime::JulianDay -le 'print local_julian_day( time )'
    55608

=head2 How do I find yesterday's date?
X X X X X X X X X X (contributed by brian d foy)

To do it correctly, you can use one of the C modules since they
work with calendars instead of times. The L module makes it
simple, and give you the same time of day, only the day before,
despite daylight saving time changes:

    use DateTime;

    my $yesterday = DateTime->now->subtract( days => 1 );

    print "Yesterday was $yesterday\n";

You can also use the L module using its C function.

    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";

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. For example, the rest of the
suggestions will be wrong sometimes:

Starting with Perl 5.10, L and L are part
of the standard distribution, so you might think that you could do
something like this:

    use Time::Piece;
    use Time::Seconds;

    my $yesterday = localtime() - ONE_DAY; # WRONG
    print "Yesterday was $yesterday\n";

The L module exports a new 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? (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"; If your function already returns a reference, you don't need to create the reference yourself. 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? 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. For nested patterns and/or balanced expressions, see the so-called L<< (?PARNO)|perlre/C<(?PARNO)> C<(?-PARNO)> C<(?+PARNO)> C<(?R)> C<(?0)> >> construct (available since perl 5.10). The CPAN module L can help to build such regular expressions (see in particular L and L). More complex cases will require to write a parser, probably using a parsing module from CPAN, like L, L, L, L, or L. =head2 How do I reverse a string? Use C in scalar context, as documented in L. my $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; Or you can just use the L module (part of the standard Perl distribution). use Text::Tabs; my @expanded_lines = expand(@lines_with_tabs); =head2 How do I reformat a paragraph? Use L (part of the standard Perl distribution): use Text::Wrap; print wrap("\t", ' ', @paragraphs); The paragraphs you give to L should not contain embedded newlines. L doesn't justify the lines (flush-right). Or use the CPAN module L. 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 L 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. my $string = "Just another Perl Hacker"; my $first_char = substr( $string, 0, 1 ); # 'J' To change part of a string, you can use the optional fourth argument which is the replacement string. substr( $string, 13, 4, "Perl 5.8.0" ); You can also use substr() as an lvalue. substr( $string, 13, 4 ) = "Perl 5.8.0"; =head2 How do I change the Nth occurrence of something? You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of 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; 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"; } } 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; =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 count of a certain single character (X) within a string, you can use the C function like so: my $string = "ThisXlineXhasXsomeXx'sXinXit"; my $count = ($string =~ tr/X//); print "There are $count X characters in the string"; This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, 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: my $string = "-9 55 48 -2 23 -76 4 14 -44"; my $count = 0; while ($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. my $count = () = $string =~ /-\d+/g; =head2 How do I capitalize all the words on one line? X X X X (contributed by brian d foy) Damian Conway's L handles all of the thinking for you. 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"; } How do you want to capitalize those words? FRED AND BARNEY'S LODGE # all uppercase Fred And Barney's Lodge # title case Fred and Barney's Lodge # highlight case It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait for it.... If you answered 5 you're right. Perl words are groups of C<\w+>, but that's not what you want to capitalize. How is Perl supposed to know not to capitalize that C after the apostrophe? You could try a regular expression: $string =~ s/ ( (^\w) #at the beginning of the line | # or (\s\w) #preceded by whitespace ) /\U$1/xg; $string =~ s/([\w']+)/\u\L$1/g; Now, what if you don't want to capitalize that "and"? Just use L and get on with the next problem. :) =head2 How can I split a [character]-delimited string except when inside [character]? Several modules can handle this sort of parsing--L, L, L, and L, among others. Take the example case of trying to split a string that is comma-separated into its different fields. You can't use C because you shouldn't split if the comma is inside quotes. For example, take a data line like this: SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of I, to handle these for us. He suggests (assuming your string is contained in C<$text>): my @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 L module (part of the standard Perl distribution) lets you say: use Text::ParseWords; @new = quotewords(",", 0, $text); For parsing or generating CSV, though, using L rather than implementing it yourself is highly recommended; you'll save yourself odd bugs popping up later by just using code which has already been tried and tested in production for years. =head2 How do I strip blank space from the beginning/end of a string? (contributed by brian d foy) A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions: s/^\s+//; s/\s+$//; 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: s/^\s+|\s+$//g; In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the C flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the C<\s+>, and the C<$> anchor can match to the absolute 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: while( <> ) { s/^\s+|\s+$//g; print "$_\n"; } 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. This pattern 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 you 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? 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 character string constant instead of the C<$pad_char> variable if you know what it is in advance. And in the same way you can use an integer in place of C<$pad_len> if you know the pad length in advance. The simplest method uses the C function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The 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): my $padded = sprintf("%${pad_len}s", $text); my $padded = sprintf("%*s", $pad_len, $text); # same thing # Right padding a string with blanks (no truncation): my $padded = sprintf("%-${pad_len}s", $text); my $padded = sprintf("%-*s", $pad_len, $text); # same thing # Left padding a number with 0 (no truncation): my $padded = sprintf("%0${pad_len}d", $num); my $padded = sprintf("%0*d", $pad_len, $num); # same thing # Right padding a string with blanks using pack (will truncate): my $padded = pack("A$pad_len",$text); If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the C operator and combine that with C<$text>. These methods do not truncate C<$text>. Left and right padding with any character, creating a new string: my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; my $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 ) ); =head2 How do I extract selected columns from a string? (contributed by brian d foy) If you know 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 format, such as L, L, or L. 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? (contributed by brian d foy) You can use the C module. If you want to do fuzzy or close matching, you might also try the L, and L, and L modules. =head2 How can I expand variables in text strings? (contributed by brian d foy) If you can avoid it, don't, or if you can use a templating system, such as L or L