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