NG4R3P

Linux 4gvps.4gvps.com 3.10.0-1127.18.2.vz7.163.46 #1 SMP Fri Nov 20 21:47:55 MSK 2020 x86_64
SOFT : Apache PHP : 7.4.33
/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 X X X perldsc - Perl Data Structures Cookbook =head1 DESCRIPTION The single feature most sorely lacking in the Perl programming language prior to its 5.0 release was complex data structures. Even without direct language support, some valiant programmers did manage to emulate them, but it was hard work and not for the faint of heart. You could occasionally get away with the C<$m{$AoA,$b}> notation borrowed from B in which the keys are actually more like a single concatenated string C<"$AoA$b">, but traversal and sorting were difficult. More desperate programmers even hacked Perl's internal symbol table directly, a strategy that proved hard to develop and maintain--to put it mildly. The 5.0 release of Perl let us have complex data structures. You may now write something like this and all of a sudden, you'd have an array with three dimensions! for $x (1 .. 10) { for $y (1 .. 10) { for $z (1 .. 10) { $AoA[$x][$y][$z] = $x ** $y + $z; } } } Alas, however simple this may appear, underneath it's a much more elaborate construct than meets the eye! How do you print it out? Why can't you say just C? How do you sort it? How can you pass it to a function or get one of these back from a function? Is it an object? Can you save it to disk to read back later? How do you access whole rows or columns of that matrix? Do all the values have to be numeric? As you see, it's quite easy to become confused. While some small portion of the blame for this can be attributed to the reference-based implementation, it's really more due to a lack of existing documentation with examples designed for the beginner. This document is meant to be a detailed but understandable treatment of the many different sorts of data structures you might want to develop. It should also serve as a cookbook of examples. That way, when you need to create one of these complex data structures, you can just pinch, pilfer, or purloin a drop-in example from here. Let's look at each of these possible constructs in detail. There are separate sections on each of the following: =over 5 =item * arrays of arrays =item * hashes of arrays =item * arrays of hashes =item * hashes of hashes =item * more elaborate constructs =back But for now, let's look at general issues common to all these types of data structures. =head1 REFERENCES X X X X The most important thing to understand about all data structures in Perl--including multidimensional arrays--is that even though they might appear otherwise, Perl C<@ARRAY>s and C<%HASH>es are all internally one-dimensional. They can hold only scalar values (meaning a string, number, or a reference). They cannot directly contain other arrays or hashes, but instead contain I to other arrays or hashes. X X You can't use a reference to an array or hash in quite the same way that you would a real array or hash. For C or C++ programmers unused to distinguishing between arrays and pointers to the same, this can be confusing. If so, just think of it as the difference between a structure and a pointer to a structure. You can (and should) read more about references in L. Briefly, references are rather like pointers that know what they point to. (Objects are also a kind of reference, but we won't be needing them right away--if ever.) This means that when you have something which looks to you like an access to a two-or-more-dimensional array and/or hash, what's really going on is that the base type is merely a one-dimensional entity that contains references to the next level. It's just that you can I it as though it were a two-dimensional one. This is actually the way almost all C multidimensional arrays work as well. $array[7][12] # array of arrays $array[7]{string} # array of hashes $hash{string}[7] # hash of arrays $hash{string}{'another string'} # hash of hashes Now, because the top level contains only references, if you try to print out your array in with a simple print() function, you'll get something that doesn't look very nice, like this: @AoA = ( [2, 3], [4, 5, 7], [0] ); print $AoA[1][2]; 7 print @AoA; ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0) That's because Perl doesn't (ever) implicitly dereference your variables. If you want to get at the thing a reference is referring to, then you have to do this yourself using either prefix typing indicators, like C<${$blah}>, C<@{$blah}>, C<@{$blah[$i]}>, or else postfix pointer arrows, like C<$a-E[3]>, C<$h-E{fred}>, or even C<$ob-Emethod()-E[3]>. =head1 COMMON MISTAKES The two most common mistakes made in constructing something like an array of arrays is either accidentally counting the number of elements or else taking a reference to the same memory location repeatedly. Here's the case where you just get the count instead of a nested array: for $i (1..10) { @array = somefunc($i); $AoA[$i] = @array; # WRONG! } That's just the simple case of assigning an array to a scalar and getting its element count. If that's what you really and truly want, then you might do well to consider being a tad more explicit about it, like this: for $i (1..10) { @array = somefunc($i); $counts[$i] = scalar @array; } Here's the case of taking a reference to the same memory location again and again: for $i (1..10) { @array = somefunc($i); $AoA[$i] = \@array; # WRONG! } So, what's the big problem with that? It looks right, doesn't it? After all, I just told you that you need an array of references, so by golly, you've made me one! Unfortunately, while this is true, it's still broken. All the references in @AoA refer to the I, and they will therefore all hold whatever was last in @array! It's similar to the problem demonstrated in the following C program: #include main() { struct passwd *getpwnam(), *rp, *dp; rp = getpwnam("root"); dp = getpwnam("daemon"); printf("daemon name is %s\nroot name is %s\n", dp->pw_name, rp->pw_name); } Which will print daemon name is daemon root name is daemon The problem is that both C and C are pointers to the same location in memory! In C, you'd have to remember to malloc() yourself some new memory. In Perl, you'll want to use the array constructor C<[]> or the hash constructor C<{}> instead. Here's the right way to do the preceding broken code fragments: X<[]> X<{}> for $i (1..10) { @array = somefunc($i); $AoA[$i] = [ @array ]; } The square brackets make a reference to a new array with a I of what's in @array at the time of the assignment. This is what you want. Note that this will produce something similar, but it's much harder to read: for $i (1..10) { @array = 0 .. $i; @{$AoA[$i]} = @array; } Is it the same? Well, maybe so--and maybe not. The subtle difference is that when you assign something in square brackets, you know for sure it's always a brand new reference with a new I of the data. Something else could be going on in this new case with the C<@{$AoA[$i]}> dereference on the left-hand-side of the assignment. It all depends on whether C<$AoA[$i]> had been undefined to start with, or whether it already contained a reference. If you had already populated @AoA with references, as in $AoA[3] = \@another_array; Then the assignment with the indirection on the left-hand-side would use the existing reference that was already there: @{$AoA[3]} = @array; Of course, this I have the "interesting" effect of clobbering @another_array. (Have you ever noticed how when a programmer says something is "interesting", that rather than meaning "intriguing", they're disturbingly more apt to mean that it's "annoying", "difficult", or both? :-) So just remember always to use the array or hash constructors with C<[]> or C<{}>, and you'll be fine, although it's not always optimally efficient. Surprisingly, the following dangerous-looking construct will actually work out fine: for $i (1..10) { my @array = somefunc($i); $AoA[$i] = \@array; } That's because my() is more of a run-time statement than it is a compile-time declaration I. This means that the my() variable is remade afresh each time through the loop. So even though it I as though you stored the same variable reference each time, you actually did not! This is a subtle distinction that can produce more efficient code at the risk of misleading all but the most experienced of programmers. So I usually advise against teaching it to beginners. In fact, except for passing arguments to functions, I seldom like to see the gimme-a-reference operator (backslash) used much at all in code. Instead, I advise beginners that they (and most of the rest of us) should try to use the much more easily understood constructors C<[]> and C<{}> instead of relying upon lexical (or dynamic) scoping and hidden reference-counting to do the right thing behind the scenes. In summary: $AoA[$i] = [ @array ]; # usually best $AoA[$i] = \@array; # perilous; just how my() was that array? @{ $AoA[$i] } = @array; # way too tricky for most programmers =head1 CAVEAT ON PRECEDENCE X X Speaking of things like C<@{$AoA[$i]}>, the following are actually the same thing: X<< -> >> $aref->[2][2] # clear $$aref[2][2] # confusing That's because Perl's precedence rules on its five prefix dereferencers (which look like someone swearing: C<$ @ * % &>) make them bind more tightly than the postfix subscripting brackets or braces! This will no doubt come as a great shock to the C or C++ programmer, who is quite accustomed to using C<*a[i]> to mean what's pointed to by the I element of C. That is, they first take the subscript, and only then dereference the thing at that subscript. That's fine in C, but this isn't C. The seemingly equivalent construct in Perl, C<$$aref[$i]> first does the deref of $aref, making it take $aref as a reference to an array, and then dereference that, and finally tell you the I value of the array pointed to by $AoA. If you wanted the C notion, you'd have to write C<${$AoA[$i]}> to force the C<$AoA[$i]> to get evaluated first before the leading C<$> dereferencer. =head1 WHY YOU SHOULD ALWAYS C If this is starting to sound scarier than it's worth, relax. Perl has some features to help you avoid its most common pitfalls. The best way to avoid getting confused is to start every program like this: #!/usr/bin/perl -w use strict; This way, you'll be forced to declare all your variables with my() and also disallow accidental "symbolic dereferencing". Therefore if you'd done this: my $aref = [ [ "fred", "barney", "pebbles", "bambam", "dino", ], [ "homer", "bart", "marge", "maggie", ], [ "george", "jane", "elroy", "judy", ], ]; print $aref[2][2]; The compiler would immediately flag that as an error I, because you were accidentally accessing C<@aref>, an undeclared variable, and it would thereby remind you to write instead: print $aref->[2][2] =head1 DEBUGGING X X X X X X X X X X Before version 5.002, the standard Perl debugger didn't do a very nice job of printing out complex data structures. With 5.002 or above, the debugger includes several new features, including command line editing as well as the C command to dump out complex data structures. For example, given the assignment to $AoA above, here's the debugger output: DB<1> x $AoA $AoA = ARRAY(0x13b5a0) 0 ARRAY(0x1f0a24) 0 'fred' 1 'barney' 2 'pebbles' 3 'bambam' 4 'dino' 1 ARRAY(0x13b558) 0 'homer' 1 'bart' 2 'marge' 3 'maggie' 2 ARRAY(0x13b540) 0 'george' 1 'jane' 2 'elroy' 3 'judy' =head1 CODE EXAMPLES Presented with little comment (these will get their own manpages someday) here are short code examples illustrating access of various types of data structures. =head1 ARRAYS OF ARRAYS X X =head2 Declaration of an ARRAY OF ARRAYS @AoA = ( [ "fred", "barney" ], [ "george", "jane", "elroy" ], [ "homer", "marge", "bart" ], ); =head2 Generation of an ARRAY OF ARRAYS # reading from file while ( <> ) { push @AoA, [ split ]; } # calling a function for $i ( 1 .. 10 ) { $AoA[$i] = [ somefunc($i) ]; } # using temp vars for $i ( 1 .. 10 ) { @tmp = somefunc($i); $AoA[$i] = [ @tmp ]; } # add to an existing row push @{ $AoA[0] }, "wilma", "betty"; =head2 Access and Printing of an ARRAY OF ARRAYS # one element $AoA[0][0] = "Fred"; # another element $AoA[1][1] =~ s/(\w)/\u$1/; # print the whole thing with refs for $aref ( @AoA ) { print "\t [ @$aref ],\n"; } # print the whole thing with indices for $i ( 0 .. $#AoA ) { print "\t [ @{$AoA[$i]} ],\n"; } # print the whole thing one at a time for $i ( 0 .. $#AoA ) { for $j ( 0 .. $#{ $AoA[$i] } ) { print "elt $i $j is $AoA[$i][$j]\n"; } } =head1 HASHES OF ARRAYS X X =head2 Declaration of a HASH OF ARRAYS %HoA = ( flintstones => [ "fred", "barney" ], jetsons => [ "george", "jane", "elroy" ], simpsons => [ "homer", "marge", "bart" ], ); =head2 Generation of a HASH OF ARRAYS # reading from file # flintstones: fred barney wilma dino while ( <> ) { next unless s/^(.*?):\s*//; $HoA{$1} = [ split ]; } # reading from file; more temps # flintstones: fred barney wilma dino while ( $line = <> ) { ($who, $rest) = split /:\s*/, $line, 2; @fields = split ' ', $rest; $HoA{$who} = [ @fields ]; } # calling a function that returns a list for $group ( "simpsons", "jetsons", "flintstones" ) { $HoA{$group} = [ get_family($group) ]; } # likewise, but using temps for $group ( "simpsons", "jetsons", "flintstones" ) { @members = get_family($group); $HoA{$group} = [ @members ]; } # append new members to an existing family push @{ $HoA{"flintstones"} }, "wilma", "betty"; =head2 Access and Printing of a HASH OF ARRAYS # one element $HoA{flintstones}[0] = "Fred"; # another element $HoA{simpsons}[1] =~ s/(\w)/\u$1/; # print the whole thing foreach $family ( keys %HoA ) { print "$family: @{ $HoA{$family} }\n" } # print the whole thing with indices foreach $family ( keys %HoA ) { print "family: "; foreach $i ( 0 .. $#{ $HoA{$family} } ) { print " $i = $HoA{$family}[$i]"; } print "\n"; } # print the whole thing sorted by number of members foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) { print "$family: @{ $HoA{$family} }\n" } # print the whole thing sorted by number of members and name foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} || $a cmp $b } keys %HoA ) { print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n"; } =head1 ARRAYS OF HASHES X X =head2 Declaration of an ARRAY OF HASHES @AoH = ( { Lead => "fred", Friend => "barney", }, { Lead => "george", Wife => "jane", Son => "elroy", }, { Lead => "homer", Wife => "marge", Son => "bart", } ); =head2 Generation of an ARRAY OF HASHES # reading from file # format: LEAD=fred FRIEND=barney while ( <> ) { $rec = {}; for $field ( split ) { ($key, $value) = split /=/, $field; $rec->{$key} = $value; } push @AoH, $rec; } # reading from file # format: LEAD=fred FRIEND=barney # no temp while ( <> ) { push @AoH, { split /[\s+=]/ }; } # calling a function that returns a key/value pair list, like # "lead","fred","daughter","pebbles" while ( %fields = getnextpairset() ) { push @AoH, { %fields }; } # likewise, but using no temp vars while (<>) { push @AoH, { parsepairs($_) }; } # add key/value to an element $AoH[0]{pet} = "dino"; $AoH[2]{pet} = "santa's little helper"; =head2 Access and Printing of an ARRAY OF HASHES # one element $AoH[0]{lead} = "fred"; # another element $AoH[1]{lead} =~ s/(\w)/\u$1/; # print the whole thing with refs for $href ( @AoH ) { print "{ "; for $role ( keys %$href ) { print "$role=$href->{$role} "; } print "}\n"; } # print the whole thing with indices for $i ( 0 .. $#AoH ) { print "$i is { "; for $role ( keys %{ $AoH[$i] } ) { print "$role=$AoH[$i]{$role} "; } print "}\n"; } # print the whole thing one at a time for $i ( 0 .. $#AoH ) { for $role ( keys %{ $AoH[$i] } ) { print "elt $i $role is $AoH[$i]{$role}\n"; } } =head1 HASHES OF HASHES X X =head2 Declaration of a HASH OF HASHES %HoH = ( flintstones => { lead => "fred", pal => "barney", }, jetsons => { lead => "george", wife => "jane", "his boy" => "elroy", }, simpsons => { lead => "homer", wife => "marge", kid => "bart", }, ); =head2 Generation of a HASH OF HASHES # reading from file # flintstones: lead=fred pal=barney wife=wilma pet=dino while ( <> ) { next unless s/^(.*?):\s*//; $who = $1; for $field ( split ) { ($key, $value) = split /=/, $field; $HoH{$who}{$key} = $value; } # reading from file; more temps while ( <> ) { next unless s/^(.*?):\s*//; $who = $1; $rec = {}; $HoH{$who} = $rec; for $field ( split ) { ($key, $value) = split /=/, $field; $rec->{$key} = $value; } } # calling a function that returns a key,value hash for $group ( "simpsons", "jetsons", "flintstones" ) { $HoH{$group} = { get_family($group) }; } # likewise, but using temps for $group ( "simpsons", "jetsons", "flintstones" ) { %members = get_family($group); $HoH{$group} = { %members }; } # append new members to an existing family %new_folks = ( wife => "wilma", pet => "dino", ); for $what (keys %new_folks) { $HoH{flintstones}{$what} = $new_folks{$what}; } =head2 Access and Printing of a HASH OF HASHES # one element $HoH{flintstones}{wife} = "wilma"; # another element $HoH{simpsons}{lead} =~ s/(\w)/\u$1/; # print the whole thing foreach $family ( keys %HoH ) { print "$family: { "; for $role ( keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; } # print the whole thing somewhat sorted foreach $family ( sort keys %HoH ) { print "$family: { "; for $role ( sort keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; } # print the whole thing sorted by number of members foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) { print "$family: { "; for $role ( sort keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; } # establish a sort order (rank) for each role $i = 0; for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i } # now print the whole thing sorted by number of members foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) { print "$family: { "; # and print these according to rank order for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; } =head1 MORE ELABORATE RECORDS X X X =head2 Declaration of MORE ELABORATE RECORDS Here's a sample showing how to create and use a record whose fields are of many different sorts: $rec = { TEXT => $string, SEQUENCE => [ @old_values ], LOOKUP => { %some_table }, THATCODE => \&some_function, THISCODE => sub { $_[0] ** $_[1] }, HANDLE => \*STDOUT, }; print $rec->{TEXT}; print $rec->{SEQUENCE}[0]; $last = pop @ { $rec->{SEQUENCE} }; print $rec->{LOOKUP}{"key"}; ($first_k, $first_v) = each %{ $rec->{LOOKUP} }; $answer = $rec->{THATCODE}->($arg); $answer = $rec->{THISCODE}->($arg1, $arg2); # careful of extra block braces on fh ref print { $rec->{HANDLE} } "a string\n"; use FileHandle; $rec->{HANDLE}->autoflush(1); $rec->{HANDLE}->print(" a string\n"); =head2 Declaration of a HASH OF COMPLEX RECORDS %TV = ( flintstones => { series => "flintstones", nights => [ qw(monday thursday friday) ], members => [ { name => "fred", role => "lead", age => 36, }, { name => "wilma", role => "wife", age => 31, }, { name => "pebbles", role => "kid", age => 4, }, ], }, jetsons => { series => "jetsons", nights => [ qw(wednesday saturday) ], members => [ { name => "george", role => "lead", age => 41, }, { name => "jane", role => "wife", age => 39, }, { name => "elroy", role => "kid", age => 9, }, ], }, simpsons => { series => "simpsons", nights => [ qw(monday) ], members => [ { name => "homer", role => "lead", age => 34, }, { name => "marge", role => "wife", age => 37, }, { name => "bart", role => "kid", age => 11, }, ], }, ); =head2 Generation of a HASH OF COMPLEX RECORDS # reading from file # this is most easily done by having the file itself be # in the raw data format as shown above. perl is happy # to parse complex data structures if declared as data, so # sometimes it's easiest to do that # here's a piece by piece build up $rec = {}; $rec->{series} = "flintstones"; $rec->{nights} = [ find_days() ]; @members = (); # assume this file in field=value syntax while (<>) { %fields = split /[\s=]+/; push @members, { %fields }; } $rec->{members} = [ @members ]; # now remember the whole thing $TV{ $rec->{series} } = $rec; ########################################################### # now, you might want to make interesting extra fields that # include pointers back into the same data structure so if # change one piece, it changes everywhere, like for example # if you wanted a {kids} field that was a reference # to an array of the kids' records without having duplicate # records and thus update problems. ########################################################### foreach $family (keys %TV) { $rec = $TV{$family}; # temp pointer @kids = (); for $person ( @{ $rec->{members} } ) { if ($person->{role} =~ /kid|son|daughter/) { push @kids, $person; } } # REMEMBER: $rec and $TV{$family} point to same data!! $rec->{kids} = [ @kids ]; } # you copied the array, but the array itself contains pointers # to uncopied objects. this means that if you make bart get # older via $TV{simpsons}{kids}[0]{age}++; # then this would also change in print $TV{simpsons}{members}[2]{age}; # because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2] # both point to the same underlying anonymous hash table # print the whole thing foreach $family ( keys %TV ) { print "the $family"; print " is on during @{ $TV{$family}{nights} }\n"; print "its members are:\n"; for $who ( @{ $TV{$family}{members} } ) { print " $who->{name} ($who->{role}), age $who->{age}\n"; } print "it turns out that $TV{$family}{lead} has "; print scalar ( @{ $TV{$family}{kids} } ), " kids named "; print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } ); print "\n"; } =head1 Database Ties You cannot easily tie a multilevel data structure (such as a hash of hashes) to a dbm file. The first problem is that all but GDBM and Berkeley DB have size limitations, but beyond that, you also have problems with how references are to be represented on disk. One experimental module that does partially attempt to address this need is the MLDBM module. Check your nearest CPAN site as described in L for source code to MLDBM. =head1 SEE ALSO L, L, L, L =head1 AUTHOR Tom Christiansen > Last update: Wed Oct 23 04:57:50 MET DST 1996