[15957] | 1 | =head1 NAME
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| 2 | X<data structure> X<complex data structure> X<struct>
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| 3 |
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| 4 | perldsc - Perl Data Structures Cookbook
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| 5 |
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| 6 | =head1 DESCRIPTION
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| 7 |
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| 8 | The single feature most sorely lacking in the Perl programming language
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| 9 | prior to its 5.0 release was complex data structures. Even without direct
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| 10 | language support, some valiant programmers did manage to emulate them, but
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| 11 | it was hard work and not for the faint of heart. You could occasionally
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| 12 | get away with the C<$m{$AoA,$b}> notation borrowed from B<awk> in which the
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| 13 | keys are actually more like a single concatenated string C<"$AoA$b">, but
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| 14 | traversal and sorting were difficult. More desperate programmers even
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| 15 | hacked Perl's internal symbol table directly, a strategy that proved hard
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| 16 | to develop and maintain--to put it mildly.
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| 17 |
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| 18 | The 5.0 release of Perl let us have complex data structures. You
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| 19 | may now write something like this and all of a sudden, you'd have an array
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| 20 | with three dimensions!
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| 21 |
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| 22 | for $x (1 .. 10) {
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| 23 | for $y (1 .. 10) {
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| 24 | for $z (1 .. 10) {
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| 25 | $AoA[$x][$y][$z] =
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| 26 | $x ** $y + $z;
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| 27 | }
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| 28 | }
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| 29 | }
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| 30 |
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| 31 | Alas, however simple this may appear, underneath it's a much more
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| 32 | elaborate construct than meets the eye!
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| 33 |
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| 34 | How do you print it out? Why can't you say just C<print @AoA>? How do
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| 35 | you sort it? How can you pass it to a function or get one of these back
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| 36 | from a function? Is it an object? Can you save it to disk to read
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| 37 | back later? How do you access whole rows or columns of that matrix? Do
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| 38 | all the values have to be numeric?
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| 39 |
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| 40 | As you see, it's quite easy to become confused. While some small portion
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| 41 | of the blame for this can be attributed to the reference-based
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| 42 | implementation, it's really more due to a lack of existing documentation with
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| 43 | examples designed for the beginner.
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| 44 |
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| 45 | This document is meant to be a detailed but understandable treatment of the
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| 46 | many different sorts of data structures you might want to develop. It
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| 47 | should also serve as a cookbook of examples. That way, when you need to
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| 48 | create one of these complex data structures, you can just pinch, pilfer, or
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| 49 | purloin a drop-in example from here.
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| 50 |
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| 51 | Let's look at each of these possible constructs in detail. There are separate
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| 52 | sections on each of the following:
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| 53 |
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| 54 | =over 5
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| 55 |
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| 56 | =item * arrays of arrays
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| 57 |
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| 58 | =item * hashes of arrays
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| 59 |
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| 60 | =item * arrays of hashes
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| 61 |
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| 62 | =item * hashes of hashes
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| 63 |
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| 64 | =item * more elaborate constructs
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| 65 |
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| 66 | =back
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| 67 |
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| 68 | But for now, let's look at general issues common to all
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| 69 | these types of data structures.
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| 70 |
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| 71 | =head1 REFERENCES
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| 72 | X<reference> X<dereference> X<dereferencing> X<pointer>
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| 73 |
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| 74 | The most important thing to understand about all data structures in Perl
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| 75 | -- including multidimensional arrays--is that even though they might
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| 76 | appear otherwise, Perl C<@ARRAY>s and C<%HASH>es are all internally
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| 77 | one-dimensional. They can hold only scalar values (meaning a string,
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| 78 | number, or a reference). They cannot directly contain other arrays or
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| 79 | hashes, but instead contain I<references> to other arrays or hashes.
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| 80 | X<multidimensional array> X<array, multidimensional>
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| 81 |
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| 82 | You can't use a reference to an array or hash in quite the same way that you
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| 83 | would a real array or hash. For C or C++ programmers unused to
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| 84 | distinguishing between arrays and pointers to the same, this can be
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| 85 | confusing. If so, just think of it as the difference between a structure
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| 86 | and a pointer to a structure.
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| 87 |
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| 88 | You can (and should) read more about references in the perlref(1) man
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| 89 | page. Briefly, references are rather like pointers that know what they
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| 90 | point to. (Objects are also a kind of reference, but we won't be needing
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| 91 | them right away--if ever.) This means that when you have something which
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| 92 | looks to you like an access to a two-or-more-dimensional array and/or hash,
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| 93 | what's really going on is that the base type is
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| 94 | merely a one-dimensional entity that contains references to the next
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| 95 | level. It's just that you can I<use> it as though it were a
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| 96 | two-dimensional one. This is actually the way almost all C
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| 97 | multidimensional arrays work as well.
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| 98 |
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| 99 | $array[7][12] # array of arrays
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| 100 | $array[7]{string} # array of hashes
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| 101 | $hash{string}[7] # hash of arrays
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| 102 | $hash{string}{'another string'} # hash of hashes
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| 103 |
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| 104 | Now, because the top level contains only references, if you try to print
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| 105 | out your array in with a simple print() function, you'll get something
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| 106 | that doesn't look very nice, like this:
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| 107 |
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| 108 | @AoA = ( [2, 3], [4, 5, 7], [0] );
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| 109 | print $AoA[1][2];
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| 110 | 7
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| 111 | print @AoA;
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| 112 | ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)
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| 113 |
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| 114 |
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| 115 | That's because Perl doesn't (ever) implicitly dereference your variables.
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| 116 | If you want to get at the thing a reference is referring to, then you have
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| 117 | to do this yourself using either prefix typing indicators, like
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| 118 | C<${$blah}>, C<@{$blah}>, C<@{$blah[$i]}>, or else postfix pointer arrows,
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| 119 | 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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| 120 |
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| 121 | =head1 COMMON MISTAKES
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| 122 |
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| 123 | The two most common mistakes made in constructing something like
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| 124 | an array of arrays is either accidentally counting the number of
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| 125 | elements or else taking a reference to the same memory location
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| 126 | repeatedly. Here's the case where you just get the count instead
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| 127 | of a nested array:
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| 128 |
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| 129 | for $i (1..10) {
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| 130 | @array = somefunc($i);
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| 131 | $AoA[$i] = @array; # WRONG!
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| 132 | }
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| 133 |
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| 134 | That's just the simple case of assigning an array to a scalar and getting
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| 135 | its element count. If that's what you really and truly want, then you
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| 136 | might do well to consider being a tad more explicit about it, like this:
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| 137 |
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| 138 | for $i (1..10) {
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| 139 | @array = somefunc($i);
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| 140 | $counts[$i] = scalar @array;
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| 141 | }
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| 142 |
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| 143 | Here's the case of taking a reference to the same memory location
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| 144 | again and again:
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| 145 |
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| 146 | for $i (1..10) {
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| 147 | @array = somefunc($i);
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| 148 | $AoA[$i] = \@array; # WRONG!
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| 149 | }
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| 150 |
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| 151 | So, what's the big problem with that? It looks right, doesn't it?
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| 152 | After all, I just told you that you need an array of references, so by
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| 153 | golly, you've made me one!
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| 154 |
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| 155 | Unfortunately, while this is true, it's still broken. All the references
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| 156 | in @AoA refer to the I<very same place>, and they will therefore all hold
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| 157 | whatever was last in @array! It's similar to the problem demonstrated in
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| 158 | the following C program:
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| 159 |
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| 160 | #include <pwd.h>
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| 161 | main() {
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| 162 | struct passwd *getpwnam(), *rp, *dp;
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| 163 | rp = getpwnam("root");
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| 164 | dp = getpwnam("daemon");
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| 165 |
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| 166 | printf("daemon name is %s\nroot name is %s\n",
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| 167 | dp->pw_name, rp->pw_name);
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| 168 | }
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| 169 |
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| 170 | Which will print
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| 171 |
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| 172 | daemon name is daemon
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| 173 | root name is daemon
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| 174 |
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| 175 | The problem is that both C<rp> and C<dp> are pointers to the same location
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| 176 | in memory! In C, you'd have to remember to malloc() yourself some new
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| 177 | memory. In Perl, you'll want to use the array constructor C<[]> or the
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| 178 | hash constructor C<{}> instead. Here's the right way to do the preceding
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| 179 | broken code fragments:
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| 180 | X<[]> X<{}>
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| 181 |
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| 182 | for $i (1..10) {
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| 183 | @array = somefunc($i);
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| 184 | $AoA[$i] = [ @array ];
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| 185 | }
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| 186 |
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| 187 | The square brackets make a reference to a new array with a I<copy>
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| 188 | of what's in @array at the time of the assignment. This is what
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| 189 | you want.
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| 190 |
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| 191 | Note that this will produce something similar, but it's
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| 192 | much harder to read:
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| 193 |
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| 194 | for $i (1..10) {
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| 195 | @array = 0 .. $i;
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| 196 | @{$AoA[$i]} = @array;
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| 197 | }
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| 198 |
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| 199 | Is it the same? Well, maybe so--and maybe not. The subtle difference
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| 200 | is that when you assign something in square brackets, you know for sure
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| 201 | it's always a brand new reference with a new I<copy> of the data.
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| 202 | Something else could be going on in this new case with the C<@{$AoA[$i]}}>
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| 203 | dereference on the left-hand-side of the assignment. It all depends on
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| 204 | whether C<$AoA[$i]> had been undefined to start with, or whether it
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| 205 | already contained a reference. If you had already populated @AoA with
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| 206 | references, as in
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| 207 |
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| 208 | $AoA[3] = \@another_array;
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| 209 |
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| 210 | Then the assignment with the indirection on the left-hand-side would
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| 211 | use the existing reference that was already there:
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| 212 |
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| 213 | @{$AoA[3]} = @array;
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| 214 |
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| 215 | Of course, this I<would> have the "interesting" effect of clobbering
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| 216 | @another_array. (Have you ever noticed how when a programmer says
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| 217 | something is "interesting", that rather than meaning "intriguing",
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| 218 | they're disturbingly more apt to mean that it's "annoying",
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| 219 | "difficult", or both? :-)
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| 220 |
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| 221 | So just remember always to use the array or hash constructors with C<[]>
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| 222 | or C<{}>, and you'll be fine, although it's not always optimally
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| 223 | efficient.
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| 224 |
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| 225 | Surprisingly, the following dangerous-looking construct will
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| 226 | actually work out fine:
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| 227 |
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| 228 | for $i (1..10) {
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| 229 | my @array = somefunc($i);
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| 230 | $AoA[$i] = \@array;
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| 231 | }
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| 232 |
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| 233 | That's because my() is more of a run-time statement than it is a
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| 234 | compile-time declaration I<per se>. This means that the my() variable is
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| 235 | remade afresh each time through the loop. So even though it I<looks> as
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| 236 | though you stored the same variable reference each time, you actually did
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| 237 | not! This is a subtle distinction that can produce more efficient code at
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| 238 | the risk of misleading all but the most experienced of programmers. So I
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| 239 | usually advise against teaching it to beginners. In fact, except for
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| 240 | passing arguments to functions, I seldom like to see the gimme-a-reference
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| 241 | operator (backslash) used much at all in code. Instead, I advise
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| 242 | beginners that they (and most of the rest of us) should try to use the
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| 243 | much more easily understood constructors C<[]> and C<{}> instead of
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| 244 | relying upon lexical (or dynamic) scoping and hidden reference-counting to
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| 245 | do the right thing behind the scenes.
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| 246 |
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| 247 | In summary:
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| 248 |
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| 249 | $AoA[$i] = [ @array ]; # usually best
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| 250 | $AoA[$i] = \@array; # perilous; just how my() was that array?
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| 251 | @{ $AoA[$i] } = @array; # way too tricky for most programmers
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| 252 |
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| 253 |
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| 254 | =head1 CAVEAT ON PRECEDENCE
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| 255 | X<dereference, precedence> X<dereferencing, precedence>
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| 256 |
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| 257 | Speaking of things like C<@{$AoA[$i]}>, the following are actually the
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| 258 | same thing:
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| 259 | X<< -> >>
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| 260 |
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| 261 | $aref->[2][2] # clear
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| 262 | $$aref[2][2] # confusing
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| 263 |
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| 264 | That's because Perl's precedence rules on its five prefix dereferencers
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| 265 | (which look like someone swearing: C<$ @ * % &>) make them bind more
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| 266 | tightly than the postfix subscripting brackets or braces! This will no
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| 267 | doubt come as a great shock to the C or C++ programmer, who is quite
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| 268 | accustomed to using C<*a[i]> to mean what's pointed to by the I<i'th>
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| 269 | element of C<a>. That is, they first take the subscript, and only then
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| 270 | dereference the thing at that subscript. That's fine in C, but this isn't C.
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| 271 |
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| 272 | The seemingly equivalent construct in Perl, C<$$aref[$i]> first does
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| 273 | the deref of $aref, making it take $aref as a reference to an
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| 274 | array, and then dereference that, and finally tell you the I<i'th> value
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| 275 | of the array pointed to by $AoA. If you wanted the C notion, you'd have to
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| 276 | write C<${$AoA[$i]}> to force the C<$AoA[$i]> to get evaluated first
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| 277 | before the leading C<$> dereferencer.
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| 278 |
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| 279 | =head1 WHY YOU SHOULD ALWAYS C<use strict>
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| 280 |
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| 281 | If this is starting to sound scarier than it's worth, relax. Perl has
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| 282 | some features to help you avoid its most common pitfalls. The best
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| 283 | way to avoid getting confused is to start every program like this:
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| 284 |
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| 285 | #!/usr/bin/perl -w
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| 286 | use strict;
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| 287 |
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| 288 | This way, you'll be forced to declare all your variables with my() and
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| 289 | also disallow accidental "symbolic dereferencing". Therefore if you'd done
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| 290 | this:
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| 291 |
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| 292 | my $aref = [
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| 293 | [ "fred", "barney", "pebbles", "bambam", "dino", ],
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| 294 | [ "homer", "bart", "marge", "maggie", ],
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| 295 | [ "george", "jane", "elroy", "judy", ],
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| 296 | ];
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| 297 |
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| 298 | print $aref[2][2];
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| 299 |
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| 300 | The compiler would immediately flag that as an error I<at compile time>,
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| 301 | because you were accidentally accessing C<@aref>, an undeclared
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| 302 | variable, and it would thereby remind you to write instead:
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| 303 |
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| 304 | print $aref->[2][2]
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| 305 |
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| 306 | =head1 DEBUGGING
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| 307 | X<data structure, debugging> X<complex data structure, debugging>
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| 308 | X<AoA, debugging> X<HoA, debugging> X<AoH, debugging> X<HoH, debugging>
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| 309 | X<array of arrays, debugging> X<hash of arrays, debugging>
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| 310 | X<array of hashes, debugging> X<hash of hashes, debugging>
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| 311 |
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| 312 | Before version 5.002, the standard Perl debugger didn't do a very nice job of
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| 313 | printing out complex data structures. With 5.002 or above, the
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| 314 | debugger includes several new features, including command line editing as
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| 315 | well as the C<x> command to dump out complex data structures. For
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| 316 | example, given the assignment to $AoA above, here's the debugger output:
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| 317 |
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| 318 | DB<1> x $AoA
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| 319 | $AoA = ARRAY(0x13b5a0)
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| 320 | 0 ARRAY(0x1f0a24)
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| 321 | 0 'fred'
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| 322 | 1 'barney'
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| 323 | 2 'pebbles'
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| 324 | 3 'bambam'
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| 325 | 4 'dino'
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| 326 | 1 ARRAY(0x13b558)
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| 327 | 0 'homer'
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| 328 | 1 'bart'
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| 329 | 2 'marge'
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| 330 | 3 'maggie'
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| 331 | 2 ARRAY(0x13b540)
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| 332 | 0 'george'
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| 333 | 1 'jane'
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| 334 | 2 'elroy'
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| 335 | 3 'judy'
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| 336 |
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| 337 | =head1 CODE EXAMPLES
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| 338 |
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| 339 | Presented with little comment (these will get their own manpages someday)
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| 340 | here are short code examples illustrating access of various
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| 341 | types of data structures.
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| 342 |
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| 343 | =head1 ARRAYS OF ARRAYS
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| 344 | X<array of arrays> X<AoA>
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| 345 |
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| 346 | =head2 Declaration of an ARRAY OF ARRAYS
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| 347 |
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| 348 | @AoA = (
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| 349 | [ "fred", "barney" ],
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| 350 | [ "george", "jane", "elroy" ],
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| 351 | [ "homer", "marge", "bart" ],
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| 352 | );
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| 353 |
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| 354 | =head2 Generation of an ARRAY OF ARRAYS
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| 355 |
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| 356 | # reading from file
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| 357 | while ( <> ) {
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| 358 | push @AoA, [ split ];
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| 359 | }
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| 360 |
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| 361 | # calling a function
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| 362 | for $i ( 1 .. 10 ) {
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| 363 | $AoA[$i] = [ somefunc($i) ];
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| 364 | }
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| 365 |
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| 366 | # using temp vars
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| 367 | for $i ( 1 .. 10 ) {
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| 368 | @tmp = somefunc($i);
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| 369 | $AoA[$i] = [ @tmp ];
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| 370 | }
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| 371 |
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| 372 | # add to an existing row
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| 373 | push @{ $AoA[0] }, "wilma", "betty";
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| 374 |
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| 375 | =head2 Access and Printing of an ARRAY OF ARRAYS
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| 376 |
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| 377 | # one element
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| 378 | $AoA[0][0] = "Fred";
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| 379 |
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| 380 | # another element
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| 381 | $AoA[1][1] =~ s/(\w)/\u$1/;
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| 382 |
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| 383 | # print the whole thing with refs
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| 384 | for $aref ( @AoA ) {
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| 385 | print "\t [ @$aref ],\n";
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| 386 | }
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| 387 |
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| 388 | # print the whole thing with indices
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| 389 | for $i ( 0 .. $#AoA ) {
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| 390 | print "\t [ @{$AoA[$i]} ],\n";
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| 391 | }
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| 392 |
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| 393 | # print the whole thing one at a time
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| 394 | for $i ( 0 .. $#AoA ) {
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| 395 | for $j ( 0 .. $#{ $AoA[$i] } ) {
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| 396 | print "elt $i $j is $AoA[$i][$j]\n";
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| 397 | }
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| 398 | }
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| 399 |
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| 400 | =head1 HASHES OF ARRAYS
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| 401 | X<hash of arrays> X<HoA>
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| 402 |
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| 403 | =head2 Declaration of a HASH OF ARRAYS
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| 404 |
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| 405 | %HoA = (
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| 406 | flintstones => [ "fred", "barney" ],
|
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| 407 | jetsons => [ "george", "jane", "elroy" ],
|
---|
| 408 | simpsons => [ "homer", "marge", "bart" ],
|
---|
| 409 | );
|
---|
| 410 |
|
---|
| 411 | =head2 Generation of a HASH OF ARRAYS
|
---|
| 412 |
|
---|
| 413 | # reading from file
|
---|
| 414 | # flintstones: fred barney wilma dino
|
---|
| 415 | while ( <> ) {
|
---|
| 416 | next unless s/^(.*?):\s*//;
|
---|
| 417 | $HoA{$1} = [ split ];
|
---|
| 418 | }
|
---|
| 419 |
|
---|
| 420 | # reading from file; more temps
|
---|
| 421 | # flintstones: fred barney wilma dino
|
---|
| 422 | while ( $line = <> ) {
|
---|
| 423 | ($who, $rest) = split /:\s*/, $line, 2;
|
---|
| 424 | @fields = split ' ', $rest;
|
---|
| 425 | $HoA{$who} = [ @fields ];
|
---|
| 426 | }
|
---|
| 427 |
|
---|
| 428 | # calling a function that returns a list
|
---|
| 429 | for $group ( "simpsons", "jetsons", "flintstones" ) {
|
---|
| 430 | $HoA{$group} = [ get_family($group) ];
|
---|
| 431 | }
|
---|
| 432 |
|
---|
| 433 | # likewise, but using temps
|
---|
| 434 | for $group ( "simpsons", "jetsons", "flintstones" ) {
|
---|
| 435 | @members = get_family($group);
|
---|
| 436 | $HoA{$group} = [ @members ];
|
---|
| 437 | }
|
---|
| 438 |
|
---|
| 439 | # append new members to an existing family
|
---|
| 440 | push @{ $HoA{"flintstones"} }, "wilma", "betty";
|
---|
| 441 |
|
---|
| 442 | =head2 Access and Printing of a HASH OF ARRAYS
|
---|
| 443 |
|
---|
| 444 | # one element
|
---|
| 445 | $HoA{flintstones}[0] = "Fred";
|
---|
| 446 |
|
---|
| 447 | # another element
|
---|
| 448 | $HoA{simpsons}[1] =~ s/(\w)/\u$1/;
|
---|
| 449 |
|
---|
| 450 | # print the whole thing
|
---|
| 451 | foreach $family ( keys %HoA ) {
|
---|
| 452 | print "$family: @{ $HoA{$family} }\n"
|
---|
| 453 | }
|
---|
| 454 |
|
---|
| 455 | # print the whole thing with indices
|
---|
| 456 | foreach $family ( keys %HoA ) {
|
---|
| 457 | print "family: ";
|
---|
| 458 | foreach $i ( 0 .. $#{ $HoA{$family} } ) {
|
---|
| 459 | print " $i = $HoA{$family}[$i]";
|
---|
| 460 | }
|
---|
| 461 | print "\n";
|
---|
| 462 | }
|
---|
| 463 |
|
---|
| 464 | # print the whole thing sorted by number of members
|
---|
| 465 | foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
|
---|
| 466 | print "$family: @{ $HoA{$family} }\n"
|
---|
| 467 | }
|
---|
| 468 |
|
---|
| 469 | # print the whole thing sorted by number of members and name
|
---|
| 470 | foreach $family ( sort {
|
---|
| 471 | @{$HoA{$b}} <=> @{$HoA{$a}}
|
---|
| 472 | ||
|
---|
| 473 | $a cmp $b
|
---|
| 474 | } keys %HoA )
|
---|
| 475 | {
|
---|
| 476 | print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
|
---|
| 477 | }
|
---|
| 478 |
|
---|
| 479 | =head1 ARRAYS OF HASHES
|
---|
| 480 | X<array of hashes> X<AoH>
|
---|
| 481 |
|
---|
| 482 | =head2 Declaration of an ARRAY OF HASHES
|
---|
| 483 |
|
---|
| 484 | @AoH = (
|
---|
| 485 | {
|
---|
| 486 | Lead => "fred",
|
---|
| 487 | Friend => "barney",
|
---|
| 488 | },
|
---|
| 489 | {
|
---|
| 490 | Lead => "george",
|
---|
| 491 | Wife => "jane",
|
---|
| 492 | Son => "elroy",
|
---|
| 493 | },
|
---|
| 494 | {
|
---|
| 495 | Lead => "homer",
|
---|
| 496 | Wife => "marge",
|
---|
| 497 | Son => "bart",
|
---|
| 498 | }
|
---|
| 499 | );
|
---|
| 500 |
|
---|
| 501 | =head2 Generation of an ARRAY OF HASHES
|
---|
| 502 |
|
---|
| 503 | # reading from file
|
---|
| 504 | # format: LEAD=fred FRIEND=barney
|
---|
| 505 | while ( <> ) {
|
---|
| 506 | $rec = {};
|
---|
| 507 | for $field ( split ) {
|
---|
| 508 | ($key, $value) = split /=/, $field;
|
---|
| 509 | $rec->{$key} = $value;
|
---|
| 510 | }
|
---|
| 511 | push @AoH, $rec;
|
---|
| 512 | }
|
---|
| 513 |
|
---|
| 514 |
|
---|
| 515 | # reading from file
|
---|
| 516 | # format: LEAD=fred FRIEND=barney
|
---|
| 517 | # no temp
|
---|
| 518 | while ( <> ) {
|
---|
| 519 | push @AoH, { split /[\s+=]/ };
|
---|
| 520 | }
|
---|
| 521 |
|
---|
| 522 | # calling a function that returns a key/value pair list, like
|
---|
| 523 | # "lead","fred","daughter","pebbles"
|
---|
| 524 | while ( %fields = getnextpairset() ) {
|
---|
| 525 | push @AoH, { %fields };
|
---|
| 526 | }
|
---|
| 527 |
|
---|
| 528 | # likewise, but using no temp vars
|
---|
| 529 | while (<>) {
|
---|
| 530 | push @AoH, { parsepairs($_) };
|
---|
| 531 | }
|
---|
| 532 |
|
---|
| 533 | # add key/value to an element
|
---|
| 534 | $AoH[0]{pet} = "dino";
|
---|
| 535 | $AoH[2]{pet} = "santa's little helper";
|
---|
| 536 |
|
---|
| 537 | =head2 Access and Printing of an ARRAY OF HASHES
|
---|
| 538 |
|
---|
| 539 | # one element
|
---|
| 540 | $AoH[0]{lead} = "fred";
|
---|
| 541 |
|
---|
| 542 | # another element
|
---|
| 543 | $AoH[1]{lead} =~ s/(\w)/\u$1/;
|
---|
| 544 |
|
---|
| 545 | # print the whole thing with refs
|
---|
| 546 | for $href ( @AoH ) {
|
---|
| 547 | print "{ ";
|
---|
| 548 | for $role ( keys %$href ) {
|
---|
| 549 | print "$role=$href->{$role} ";
|
---|
| 550 | }
|
---|
| 551 | print "}\n";
|
---|
| 552 | }
|
---|
| 553 |
|
---|
| 554 | # print the whole thing with indices
|
---|
| 555 | for $i ( 0 .. $#AoH ) {
|
---|
| 556 | print "$i is { ";
|
---|
| 557 | for $role ( keys %{ $AoH[$i] } ) {
|
---|
| 558 | print "$role=$AoH[$i]{$role} ";
|
---|
| 559 | }
|
---|
| 560 | print "}\n";
|
---|
| 561 | }
|
---|
| 562 |
|
---|
| 563 | # print the whole thing one at a time
|
---|
| 564 | for $i ( 0 .. $#AoH ) {
|
---|
| 565 | for $role ( keys %{ $AoH[$i] } ) {
|
---|
| 566 | print "elt $i $role is $AoH[$i]{$role}\n";
|
---|
| 567 | }
|
---|
| 568 | }
|
---|
| 569 |
|
---|
| 570 | =head1 HASHES OF HASHES
|
---|
| 571 | X<hass of hashes> X<HoH>
|
---|
| 572 |
|
---|
| 573 | =head2 Declaration of a HASH OF HASHES
|
---|
| 574 |
|
---|
| 575 | %HoH = (
|
---|
| 576 | flintstones => {
|
---|
| 577 | lead => "fred",
|
---|
| 578 | pal => "barney",
|
---|
| 579 | },
|
---|
| 580 | jetsons => {
|
---|
| 581 | lead => "george",
|
---|
| 582 | wife => "jane",
|
---|
| 583 | "his boy" => "elroy",
|
---|
| 584 | },
|
---|
| 585 | simpsons => {
|
---|
| 586 | lead => "homer",
|
---|
| 587 | wife => "marge",
|
---|
| 588 | kid => "bart",
|
---|
| 589 | },
|
---|
| 590 | );
|
---|
| 591 |
|
---|
| 592 | =head2 Generation of a HASH OF HASHES
|
---|
| 593 |
|
---|
| 594 | # reading from file
|
---|
| 595 | # flintstones: lead=fred pal=barney wife=wilma pet=dino
|
---|
| 596 | while ( <> ) {
|
---|
| 597 | next unless s/^(.*?):\s*//;
|
---|
| 598 | $who = $1;
|
---|
| 599 | for $field ( split ) {
|
---|
| 600 | ($key, $value) = split /=/, $field;
|
---|
| 601 | $HoH{$who}{$key} = $value;
|
---|
| 602 | }
|
---|
| 603 |
|
---|
| 604 |
|
---|
| 605 | # reading from file; more temps
|
---|
| 606 | while ( <> ) {
|
---|
| 607 | next unless s/^(.*?):\s*//;
|
---|
| 608 | $who = $1;
|
---|
| 609 | $rec = {};
|
---|
| 610 | $HoH{$who} = $rec;
|
---|
| 611 | for $field ( split ) {
|
---|
| 612 | ($key, $value) = split /=/, $field;
|
---|
| 613 | $rec->{$key} = $value;
|
---|
| 614 | }
|
---|
| 615 | }
|
---|
| 616 |
|
---|
| 617 | # calling a function that returns a key,value hash
|
---|
| 618 | for $group ( "simpsons", "jetsons", "flintstones" ) {
|
---|
| 619 | $HoH{$group} = { get_family($group) };
|
---|
| 620 | }
|
---|
| 621 |
|
---|
| 622 | # likewise, but using temps
|
---|
| 623 | for $group ( "simpsons", "jetsons", "flintstones" ) {
|
---|
| 624 | %members = get_family($group);
|
---|
| 625 | $HoH{$group} = { %members };
|
---|
| 626 | }
|
---|
| 627 |
|
---|
| 628 | # append new members to an existing family
|
---|
| 629 | %new_folks = (
|
---|
| 630 | wife => "wilma",
|
---|
| 631 | pet => "dino",
|
---|
| 632 | );
|
---|
| 633 |
|
---|
| 634 | for $what (keys %new_folks) {
|
---|
| 635 | $HoH{flintstones}{$what} = $new_folks{$what};
|
---|
| 636 | }
|
---|
| 637 |
|
---|
| 638 | =head2 Access and Printing of a HASH OF HASHES
|
---|
| 639 |
|
---|
| 640 | # one element
|
---|
| 641 | $HoH{flintstones}{wife} = "wilma";
|
---|
| 642 |
|
---|
| 643 | # another element
|
---|
| 644 | $HoH{simpsons}{lead} =~ s/(\w)/\u$1/;
|
---|
| 645 |
|
---|
| 646 | # print the whole thing
|
---|
| 647 | foreach $family ( keys %HoH ) {
|
---|
| 648 | print "$family: { ";
|
---|
| 649 | for $role ( keys %{ $HoH{$family} } ) {
|
---|
| 650 | print "$role=$HoH{$family}{$role} ";
|
---|
| 651 | }
|
---|
| 652 | print "}\n";
|
---|
| 653 | }
|
---|
| 654 |
|
---|
| 655 | # print the whole thing somewhat sorted
|
---|
| 656 | foreach $family ( sort keys %HoH ) {
|
---|
| 657 | print "$family: { ";
|
---|
| 658 | for $role ( sort keys %{ $HoH{$family} } ) {
|
---|
| 659 | print "$role=$HoH{$family}{$role} ";
|
---|
| 660 | }
|
---|
| 661 | print "}\n";
|
---|
| 662 | }
|
---|
| 663 |
|
---|
| 664 |
|
---|
| 665 | # print the whole thing sorted by number of members
|
---|
| 666 | foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
|
---|
| 667 | print "$family: { ";
|
---|
| 668 | for $role ( sort keys %{ $HoH{$family} } ) {
|
---|
| 669 | print "$role=$HoH{$family}{$role} ";
|
---|
| 670 | }
|
---|
| 671 | print "}\n";
|
---|
| 672 | }
|
---|
| 673 |
|
---|
| 674 | # establish a sort order (rank) for each role
|
---|
| 675 | $i = 0;
|
---|
| 676 | for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }
|
---|
| 677 |
|
---|
| 678 | # now print the whole thing sorted by number of members
|
---|
| 679 | foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) {
|
---|
| 680 | print "$family: { ";
|
---|
| 681 | # and print these according to rank order
|
---|
| 682 | for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) {
|
---|
| 683 | print "$role=$HoH{$family}{$role} ";
|
---|
| 684 | }
|
---|
| 685 | print "}\n";
|
---|
| 686 | }
|
---|
| 687 |
|
---|
| 688 |
|
---|
| 689 | =head1 MORE ELABORATE RECORDS
|
---|
| 690 | X<record> X<structure> X<struct>
|
---|
| 691 |
|
---|
| 692 | =head2 Declaration of MORE ELABORATE RECORDS
|
---|
| 693 |
|
---|
| 694 | Here's a sample showing how to create and use a record whose fields are of
|
---|
| 695 | many different sorts:
|
---|
| 696 |
|
---|
| 697 | $rec = {
|
---|
| 698 | TEXT => $string,
|
---|
| 699 | SEQUENCE => [ @old_values ],
|
---|
| 700 | LOOKUP => { %some_table },
|
---|
| 701 | THATCODE => \&some_function,
|
---|
| 702 | THISCODE => sub { $_[0] ** $_[1] },
|
---|
| 703 | HANDLE => \*STDOUT,
|
---|
| 704 | };
|
---|
| 705 |
|
---|
| 706 | print $rec->{TEXT};
|
---|
| 707 |
|
---|
| 708 | print $rec->{SEQUENCE}[0];
|
---|
| 709 | $last = pop @ { $rec->{SEQUENCE} };
|
---|
| 710 |
|
---|
| 711 | print $rec->{LOOKUP}{"key"};
|
---|
| 712 | ($first_k, $first_v) = each %{ $rec->{LOOKUP} };
|
---|
| 713 |
|
---|
| 714 | $answer = $rec->{THATCODE}->($arg);
|
---|
| 715 | $answer = $rec->{THISCODE}->($arg1, $arg2);
|
---|
| 716 |
|
---|
| 717 | # careful of extra block braces on fh ref
|
---|
| 718 | print { $rec->{HANDLE} } "a string\n";
|
---|
| 719 |
|
---|
| 720 | use FileHandle;
|
---|
| 721 | $rec->{HANDLE}->autoflush(1);
|
---|
| 722 | $rec->{HANDLE}->print(" a string\n");
|
---|
| 723 |
|
---|
| 724 | =head2 Declaration of a HASH OF COMPLEX RECORDS
|
---|
| 725 |
|
---|
| 726 | %TV = (
|
---|
| 727 | flintstones => {
|
---|
| 728 | series => "flintstones",
|
---|
| 729 | nights => [ qw(monday thursday friday) ],
|
---|
| 730 | members => [
|
---|
| 731 | { name => "fred", role => "lead", age => 36, },
|
---|
| 732 | { name => "wilma", role => "wife", age => 31, },
|
---|
| 733 | { name => "pebbles", role => "kid", age => 4, },
|
---|
| 734 | ],
|
---|
| 735 | },
|
---|
| 736 |
|
---|
| 737 | jetsons => {
|
---|
| 738 | series => "jetsons",
|
---|
| 739 | nights => [ qw(wednesday saturday) ],
|
---|
| 740 | members => [
|
---|
| 741 | { name => "george", role => "lead", age => 41, },
|
---|
| 742 | { name => "jane", role => "wife", age => 39, },
|
---|
| 743 | { name => "elroy", role => "kid", age => 9, },
|
---|
| 744 | ],
|
---|
| 745 | },
|
---|
| 746 |
|
---|
| 747 | simpsons => {
|
---|
| 748 | series => "simpsons",
|
---|
| 749 | nights => [ qw(monday) ],
|
---|
| 750 | members => [
|
---|
| 751 | { name => "homer", role => "lead", age => 34, },
|
---|
| 752 | { name => "marge", role => "wife", age => 37, },
|
---|
| 753 | { name => "bart", role => "kid", age => 11, },
|
---|
| 754 | ],
|
---|
| 755 | },
|
---|
| 756 | );
|
---|
| 757 |
|
---|
| 758 | =head2 Generation of a HASH OF COMPLEX RECORDS
|
---|
| 759 |
|
---|
| 760 | # reading from file
|
---|
| 761 | # this is most easily done by having the file itself be
|
---|
| 762 | # in the raw data format as shown above. perl is happy
|
---|
| 763 | # to parse complex data structures if declared as data, so
|
---|
| 764 | # sometimes it's easiest to do that
|
---|
| 765 |
|
---|
| 766 | # here's a piece by piece build up
|
---|
| 767 | $rec = {};
|
---|
| 768 | $rec->{series} = "flintstones";
|
---|
| 769 | $rec->{nights} = [ find_days() ];
|
---|
| 770 |
|
---|
| 771 | @members = ();
|
---|
| 772 | # assume this file in field=value syntax
|
---|
| 773 | while (<>) {
|
---|
| 774 | %fields = split /[\s=]+/;
|
---|
| 775 | push @members, { %fields };
|
---|
| 776 | }
|
---|
| 777 | $rec->{members} = [ @members ];
|
---|
| 778 |
|
---|
| 779 | # now remember the whole thing
|
---|
| 780 | $TV{ $rec->{series} } = $rec;
|
---|
| 781 |
|
---|
| 782 | ###########################################################
|
---|
| 783 | # now, you might want to make interesting extra fields that
|
---|
| 784 | # include pointers back into the same data structure so if
|
---|
| 785 | # change one piece, it changes everywhere, like for example
|
---|
| 786 | # if you wanted a {kids} field that was a reference
|
---|
| 787 | # to an array of the kids' records without having duplicate
|
---|
| 788 | # records and thus update problems.
|
---|
| 789 | ###########################################################
|
---|
| 790 | foreach $family (keys %TV) {
|
---|
| 791 | $rec = $TV{$family}; # temp pointer
|
---|
| 792 | @kids = ();
|
---|
| 793 | for $person ( @{ $rec->{members} } ) {
|
---|
| 794 | if ($person->{role} =~ /kid|son|daughter/) {
|
---|
| 795 | push @kids, $person;
|
---|
| 796 | }
|
---|
| 797 | }
|
---|
| 798 | # REMEMBER: $rec and $TV{$family} point to same data!!
|
---|
| 799 | $rec->{kids} = [ @kids ];
|
---|
| 800 | }
|
---|
| 801 |
|
---|
| 802 | # you copied the array, but the array itself contains pointers
|
---|
| 803 | # to uncopied objects. this means that if you make bart get
|
---|
| 804 | # older via
|
---|
| 805 |
|
---|
| 806 | $TV{simpsons}{kids}[0]{age}++;
|
---|
| 807 |
|
---|
| 808 | # then this would also change in
|
---|
| 809 | print $TV{simpsons}{members}[2]{age};
|
---|
| 810 |
|
---|
| 811 | # because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
|
---|
| 812 | # both point to the same underlying anonymous hash table
|
---|
| 813 |
|
---|
| 814 | # print the whole thing
|
---|
| 815 | foreach $family ( keys %TV ) {
|
---|
| 816 | print "the $family";
|
---|
| 817 | print " is on during @{ $TV{$family}{nights} }\n";
|
---|
| 818 | print "its members are:\n";
|
---|
| 819 | for $who ( @{ $TV{$family}{members} } ) {
|
---|
| 820 | print " $who->{name} ($who->{role}), age $who->{age}\n";
|
---|
| 821 | }
|
---|
| 822 | print "it turns out that $TV{$family}{lead} has ";
|
---|
| 823 | print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
|
---|
| 824 | print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
|
---|
| 825 | print "\n";
|
---|
| 826 | }
|
---|
| 827 |
|
---|
| 828 | =head1 Database Ties
|
---|
| 829 |
|
---|
| 830 | You cannot easily tie a multilevel data structure (such as a hash of
|
---|
| 831 | hashes) to a dbm file. The first problem is that all but GDBM and
|
---|
| 832 | Berkeley DB have size limitations, but beyond that, you also have problems
|
---|
| 833 | with how references are to be represented on disk. One experimental
|
---|
| 834 | module that does partially attempt to address this need is the MLDBM
|
---|
| 835 | module. Check your nearest CPAN site as described in L<perlmodlib> for
|
---|
| 836 | source code to MLDBM.
|
---|
| 837 |
|
---|
| 838 | =head1 SEE ALSO
|
---|
| 839 |
|
---|
| 840 | perlref(1), perllol(1), perldata(1), perlobj(1)
|
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| 841 |
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| 842 | =head1 AUTHOR
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| 843 |
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| 844 | Tom Christiansen <F<[email protected]>>
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| 845 |
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| 846 | Last update:
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| 847 | Wed Oct 23 04:57:50 MET DST 1996
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