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Perl Data Structures Cookbook

The single feature most sorely lacking in the Perl programming language prior to its 5.0 
release was complex data structures. Even without direct language support,some valiant 
programmers did manage to emulate them,but it was hard work and not for the faint of heart. 
You could occasionally get away with the $m{$AoA,$b} notation borrowed from awk in which 
the keys are actually more like a single concatenated string "$AoA$b",but traversal and 
sorting were difficult. More desperate programmers even hacked Perl's internal symbol table 
directly,a strategy that proved hard to develop and maintain--to put it mildly.
在Perl 5.0之前,Perl最缺的功能是构建复杂的数据结构。
更疯狂的程序甚至直接hack Perl的内部符号表,当然这个更难搞。
The 5.0 release of Perl let us have complex data structures. You may now write something 
like this and all of a sudden,you'd have an array with three dimensions!
    for $x (1 .. 10) {
       for $y (1 .. 10) {
          for $z (1 .. 10) {
             $AoA[$x][$y][$z] =   $x ** $y + $z;
Alas,however simple this may appear,underneath it's a much more elaborate construct 
than meets the eye!
How do you print it out? Why can't you say just print @AoA ? How do you sort it? How can 
you pass it to a function or get one of these back from a function? Is it an object? Can 
you save it to disk to read back later? How do you access whole rows or columns of that 
matrix? Do all the values have to be numeric?
如何打印输出?为什么可用print @AoA?如何排序?
As you see,it's quite easy to become confused. While some small portion of the blame for 
this can be attributed to the reference-based implementation,it's really more due to a 
lack of existing documentation with examples designed for the beginner.
This document is meant to be a detailed but understandable treatment of the many different 
sorts of data structures you might want to develop. It should also serve as a cookbook of 
examples. That way,when you need to create one of these complex data structures,you can 
just pinch,pilfer,or purloin a drop-in example from here.
Let's look at each of these possible constructs in detail. There are separate sections on 
each of the following:
  arrays of arrays;           数组的数组
  hashes of arrays;           哈希表的元素为数组
  arrays of hashes;           数组的元素为哈希表
  hashes of hashes            哈希表的元素为哈希表
  more elaborate constructs   更复杂的结构
But for now,let's look at general issues common to all these types of data structures.
The most important thing to understand about all data structures in Perl--including 
multidimensional arrays--is that even though they might appear otherwise,Perl @ARRAY s 
and %HASH es are all internally one-dimensional. They can hold only scalar values 
(meaning a string,number,or a reference). They cannot directly contain other arrays 
or hashes,but instead contain references to other arrays or hashes.
  @ARRAY s 和 %HASH es 在内部的表示都是一维的;
You can't use a reference to an array or hash in quite the same way that you would 
a real array or hash. For C or C++ programmers unused to distinguishing between arrays 
and pointers to the same,this can be confusing. If so,just think of it as the difference 
between a structure and a pointer to a structure.
You can (and should) read more about references in perlref. Briefly,references are 
rather like pointers that know what they point to. (Objects are also a kind of reference, 
but we won't be needing them right away--if ever.) This means that when you have something 
which looks to you like an access to a two-or-more-dimensional array and/or hash,what's 
really going on is that the base type is merely a one-dimensional entity that contains 
references to the next level. It's just that you can use it as though it were a two-dimensional 
one. This is actually the way almost all C multidimensional arrays work as well.
    $array[7][12] # array of arrays
    $array[7]{string} # array of hashes 数组的元素为哈希
    $hash{string}[7] # hash of arrays  哈希的元素为数组
    $hash{string}{'another string'} # hash of hashes
Now,because the top level contains only references,if you try to print out your array 
in with a simple print() function,you'll get something that doesn't look very nice,like this:
    @AoA = ( [2,3],[4,5,7],[0] );
    print $AoA[1][2];
    print @AoA;
That's because Perl doesn't (ever) implicitly dereference your variables. If you want to 
get at the thing a reference is referring to,then you have to do this yourself using either 
prefix typing indicators,like ${$blah},@{$blah},@{$blah[$i]},or else postfix pointer 
arrows,like $a->[3],$h->{fred},or even $ob->method()->[3] .
The two most common mistakes made in constructing something like an array of arrays is 
either accidentally counting the number of elements or else taking a reference to the 
same memory location repeatedly. Here's the case where you just get the count instead 
of a nested array:
    for $i (1..10) {
       @array = somefunc($i);
       $AoA[$i] = @array; # WRONG!
That's just the simple case of assigning an array to a scalar and getting its element 
count. If that's what you really and truly want,then you might do well to consider 
being a tad more explicit about it,helvetica; font-size:14px">正确的方法应该是:
       $counts[$i] = scalar @array;
Here's the case of taking a reference to the same memory location again and again:
下面的错误用法是: 对同一个内存区进行重复引用.
        $AoA[$i] = \@array; # WRONG!
So,what's the big problem with that? It looks right,doesn't it? After all,I just 
told you that you need an array of references,so by golly,you've made me one!
Unfortunately,while this is true,it's still broken. All the references in @AoA 
refer to the very same place,and they will therefore all hold whatever was last 
in @array! It's similar to the problem demonstrated in the following C program:
    #include <pwd.h>
    main() {
        struct passwd *getpwnam(),*rp,*dp;
        rp = getpwnam("root");
        dp = getpwnam("daemon");
        printf("daemon name is %s\nroot name is %s\n",
Which will print
    daemon name is daemon
    root name is daemon
The problem is that both rp and dp are pointers to the same location in memory! 
In C,you'd have to remember to malloc() yourself some new memory. In Perl,you'll 
want to use the array constructor [] or the hash constructor {} instead. Here's 
the right way to do the preceding broken code fragments:
在Perl中,则要显式地使用数组构建符[]  或 哈希构建符{} 来分配新的内存。
$AoA[$i] = [ @array ];
The square brackets make a reference to a new array with a copy of what's in @array 
at the time of the assignment. This is what you want.
Note that this will produce something similar,but it's much harder to read:
       @array = 0 .. $i;
       @{$AoA[$i]} = @array;
Is it the same? Well,maybe so--and maybe not. The subtle difference is that when you 
assign something in square brackets,you know for sure it's always a brand new reference 
with a new copy of the data. Something else could be going on in this new case with 
the @{$AoA[$i]} dereference on the left-hand-side of the assignment. It all depends on 
whether $AoA[$i] had been undefined to start with,or whether it already contained a 
reference. If you had already populated @AoA with references,as in
   如果使用@{$AoA[$i]}方式,则当@AoA 包含有一个引用时,如下所示:
    $AoA[3] = \@another_array;
Then the assignment with the indirection on the left-hand-side would use the existing 
reference that was already there:
    @{$AoA[3]} = @array;
Of course,this would have the "interesting" effect of clobbering @another_array. (Have 
you ever noticed how when a programmer says something is "interesting",that rather than 
meaning "intriguing",they're disturbingly more apt to mean that it's "annoying","difficult",helvetica; font-size:14px">or both? :-)
So just remember always to use the array or hash constructors with [] or {},and you'll 
be fine,although it's not always optimally efficient.
Surprisingly,the following dangerous-looking construct will actually work out fine:
        my @array = somefunc($i);
        $AoA[$i] = \@array;
That's because my() is more of a run-time statement than it is a compile-time declaration 
per se. This means that the my() variable is remade afresh each time through the loop. 
So even though it looks as though you stored the same variable reference each time,you 
actually did not! This is a subtle distinction that can produce more efficient code at 
the risk of misleading all but the most experienced of programmers. So I usually advise 
against teaching it to beginners. In fact,except for passing arguments to functions,helvetica; font-size:14px">I seldom like to see the gimme-a-reference operator (backslash) used much at all in code. 
Instead,I advise beginners that they (and most of the rest of us) should try to use the 
much more easily understood constructors [] and {} instead of relying upon lexical (or 
dynamic) scoping and hidden reference-counting to do the right thing behind the scenes.
In summary:
    $AoA[$i] = [ @array ]; # usually best
    $AoA[$i] = \@array; # perilous; just how my() was that array?
    @{ $AoA[$i] } = @array; # way too tricky for most programmers
Speaking of things like @{$AoA[$i]},the following are actually the same thing:
    $aref->[2][2] # clear
    $$aref[2][2] # confusing
That's because Perl's precedence rules on its five prefix dereferencers (which look like 
someone swearing: $ @ * % & ) make them bind more tightly than the postfix subscripting 
brackets or braces! This will no doubt come as a great shock to the C or C++ programmer,helvetica; font-size:14px">who is quite accustomed to using *a[i] to mean what's pointed to by the i'th element of a . 
That is,they first take the subscript,and only then dereference the thing at that subscript. 
That's fine in C,but this isn't C.
The seemingly equivalent construct in Perl,$$aref[$i] first does the deref of $aref,helvetica; font-size:14px">making it take $aref as a reference to an array,and then dereference that,and finally
 tell you the i'th value of the array pointed to by $AoA. If you wanted the C notion,helvetica; font-size:14px">you'd have to write ${$AoA[$i]} to force the $AoA[$i] to get evaluated first before the 
leading $ dereferencer.
五、WHY YOU SHOULD ALWAYS【为什么总是】 use strict
If this is starting to sound scarier than it's worth,relax. Perl has some features to help 
you avoid its most common pitfalls. The best way to avoid getting confused is to start 
every program like this:
    #!/usr/bin/perl -w
    use strict;
This way,you'll be forced to declare all your variables with my() and also disallow 
accidental "symbolic dereferencing". Therefore if you'd done this:
    my $aref = [
                         [ "fred","barney","pebbles","bambam","dino",],helvetica; font-size:14px">                         [ "homer","bart","marge","maggie",helvetica; font-size:14px">                         [ "george","jane","elroy","judy",helvetica; font-size:14px">    ];
    print $aref[2][2];
The compiler would immediately flag that as an error at compile time,because you were 
accidentally accessing @aref,an undeclared variable,and it would thereby remind you 
to write instead:
    print $aref->[2][2]
Before version 5.002,the standard Perl debugger didn't do a very nice job of printing out 
complex data structures. With 5.002 or above,the debugger includes several new features,helvetica; font-size:14px">including command line editing as well as the x command to dump out complex data structures. 
For example,given the assignment to $AoA above,here's the debugger output:
    DB<1> x $AoA
    $AoA = ARRAY(0x13b5a0)
       0  ARRAY(0x1f0a24)
 0  'fred'
 1  'barney'
 2  'pebbles'
 3  'bambam'
 4  'dino'
       1  ARRAY(0x13b558)
 0  'homer'
 1  'bart'
 2  'marge'
 3  'maggie'
       2  ARRAY(0x13b540)
 0  'george'
 1  'jane'
 2  'elroy'
 3  'judy'
Presented with little comment (these will get their own manpages someday) here are short 
code examples illustrating access of various types of data structures.
1.1 Declaration of an ARRAY OF ARRAYS
 @AoA = (
        [ "fred","barney" ],helvetica; font-size:14px">        [ "george","elroy" ],helvetica; font-size:14px">        [ "homer","bart" ],helvetica; font-size:14px">      );
1.2 Generation of an ARRAY OF ARRAYS
 # reading from file
 while ( <> ) {
     push @AoA,[ split ];
 # calling a function
 for $i ( 1 .. 10 ) {
     $AoA[$i] = [ somefunc($i) ];
 # using temp vars
     @tmp = somefunc($i);
     $AoA[$i] = [ @tmp ];
 # add to an existing row
 push @{ $AoA[0] },"wilma","betty";
1.3 Access and Printing of an ARRAY OF ARRAYS
 # one element
 $AoA[0][0] = "Fred";
 # another element
 $AoA[1][1] =~ s/(\w)/\u$1/;
 # print the whole thing with refs
 for $aref ( @AoA ) {
     print "\t [ @$aref ],\n";
 # print the whole thing with indices
 for $i ( 0 .. $#AoA ) {
     print "\t [ @{$AoA[$i]} ],helvetica; font-size:14px"> # print the whole thing one at a time
     for $j ( 0 .. $#{ $AoA[$i] } ) {
         print "elt $i $j is $AoA[$i][$j]\n";
2. HASHES OF ARRAYS【哈希表的元素为数组】
2.1 Declaration of a HASH OF ARRAYS
 %HoA = (
        flintstones        => [ "fred",helvetica; font-size:14px">        jetsons            => [ "george",helvetica; font-size:14px">        simpsons           => [ "homer",helvetica; font-size:14px">2.2 Generation of a HASH OF ARRAYS
 # flintstones: fred barney wilma dino
     next unless s/^(.*?):\s*//;
     $HoA{$1} = [ split ];
 # reading from file; more temps
 while ( $line = <> ) {
     ($who,$rest) = split /:\s*/,$line,2;
     @fields = split ' ',$rest;
     $HoA{$who} = [ @fields ];
 # calling a function that returns a list
 for $group ( "simpsons","jetsons","flintstones" ) {
     $HoA{$group} = [ get_family($group) ];
 # likewise,but using temps
     @members = get_family($group);
     $HoA{$group} = [ @members ];
 # append new members to an existing family
 push @{ $HoA{"flintstones"} },helvetica; font-size:14px">2.3 Access and Printing of a HASH OF ARRAYS
 $HoA{flintstones}[0] = "Fred";
 $HoA{simpsons}[1] =~ s/(\w)/\u$1/;
 # print the whole thing
 foreach $family ( keys %HoA ) {
     print "$family: @{ $HoA{$family} }\n"
     print "family: ";
     foreach $i ( 0 .. $#{ $HoA{$family} } ) {
         print " $i = $HoA{$family}[$i]";
     print "\n";
 # print the whole thing sorted by number of members
 foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
 # print the whole thing sorted by number of members and name
 foreach $family ( sort {
   @{$HoA{$b}} <=> @{$HoA{$a}}
   $a cmp $b
   } keys %HoA )
     print "$family: ",join(",",sort @{ $HoA{$family} }),"\n";
3. ARRAYS OF HASHES【数组的元素为哈希】
3.1 Declaration of an ARRAY OF HASHES
 @AoH = (
            Lead     => "fred",helvetica; font-size:14px">            Friend   => "barney",helvetica; font-size:14px">        },helvetica; font-size:14px">            Lead     => "george",helvetica; font-size:14px">            Wife     => "jane",helvetica; font-size:14px">            Son      => "elroy",helvetica; font-size:14px">            Lead     => "homer",helvetica; font-size:14px">            Wife     => "marge",helvetica; font-size:14px">            Son      => "bart",helvetica; font-size:14px">        }
3.2 Generation of an ARRAY OF HASHES
 # format: LEAD=fred FRIEND=barney
     $rec = {};
     for $field ( split ) {
         ($key,$value) = split /=/,$field;
         $rec->{$key} = $value;
     push @AoH,$rec;
 # no temp
 # calling a function  that returns a key/value pair list,like
 # "lead","fred","daughter","pebbles"
 while ( %fields = getnextpairset() ) {

 while (<>) {
 # add key/value to an element
 $AoH[0]{pet} = "dino";
 $AoH[2]{pet} = "santa's little helper";
3.3 Access and Printing of an ARRAY OF HASHES
 $AoH[0]{lead} = "fred";
 $AoH[1]{lead} =~ s/(\w)/\u$1/;
 for $href ( @AoH ) {
     print "{ ";
     for $role ( keys %$href ) {
         print "$role=$href->{$role} ";
     print "}\n";
 for $i ( 0 .. $#AoH ) {
     print "$i is { ";
     for $role ( keys %{ $AoH[$i] } ) {
         print "$role=$AoH[$i]{$role} ";
         print "elt $i $role is $AoH[$i]{$role}\n";
4.1 Declaration of a HASH OF HASHES
 %HoH = (
        flintstones => {
lead      => "fred",helvetica; font-size:14px">pal       => "barney",helvetica; font-size:14px">        jetsons     => {
lead      => "george",helvetica; font-size:14px">wife      => "jane",helvetica; font-size:14px">"his boy" => "elroy",helvetica; font-size:14px">        simpsons    => {
lead      => "homer",helvetica; font-size:14px">wife      => "marge",helvetica; font-size:14px">kid       => "bart",helvetica; font-size:14px">},helvetica; font-size:14px"> );
4.2 Generation of a HASH OF HASHES
 # flintstones: lead=fred pal=barney wife=wilma pet=dino
     $who = $1;
         $HoH{$who}{$key} = $value;
     $HoH{$who} = $rec;
 # calling a function  that returns a key,value hash
     $HoH{$group} = { get_family($group) };
     %members = get_family($group);
     $HoH{$group} = { %members };
 %new_folks = (
     wife => "wilma",helvetica; font-size:14px">     pet  => "dino",helvetica; font-size:14px"> for $what (keys %new_folks) {
     $HoH{flintstones}{$what} = $new_folks{$what};
4.3 Access and Printing of a HASH OF HASHES
 $HoH{flintstones}{wife} = "wilma";
 $HoH{simpsons}{lead} =~ s/(\w)/\u$1/;
 foreach $family ( keys %HoH ) {
     print "$family: { ";
     for $role ( keys %{ $HoH{$family} } ) {
         print "$role=$HoH{$family}{$role} ";
 # print the whole thing  somewhat sorted
 foreach $family ( sort keys %HoH ) {
     for $role ( sort keys %{ $HoH{$family} } ) {
 foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
 # 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 ) {
     # and print these according to rank order
     for $role ( sort { $rank{$a} <=> $rank{$b} }  keys %{ $HoH{$family} } ) {
Here's a sample showing how to create and use a record whose fields are of many different sorts:
     $rec = {
TEXT      => $string,helvetica; font-size:14px">SEQUENCE  => [ @old_values ],helvetica; font-size:14px">LOOKUP    => { %some_table },helvetica; font-size:14px">THATCODE  => \&some_function,helvetica; font-size:14px">THISCODE  => sub { $_[0] ** $_[1] },helvetica; font-size:14px">HANDLE    => \*STDOUT,helvetica; font-size:14px">     };
     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}->print(" a string\n");
5.2 Declaration of a HASH OF COMPLEX RECORDS
     %TV = (
            series   => "flintstones",helvetica; font-size:14px">            nights   => [ qw(monday thursday friday) ],helvetica; font-size:14px">            members  => [
                { name => "fred",   role => "lead",age  => 36,},helvetica; font-size:14px">                { name => "wilma",  role => "wife",age  => 31,helvetica; font-size:14px">                { name => "pebbles",role => "kid", age  =>  4,helvetica; font-size:14px">            ],helvetica; font-size:14px">            series   => "jetsons",helvetica; font-size:14px">            nights   => [ qw(wednesday saturday) ],helvetica; font-size:14px">                { name => "george", role => "lead",age  => 41,helvetica; font-size:14px">                { name => "jane",   role => "wife",age  => 39,helvetica; font-size:14px">                { name => "elroy",  role => "kid", age  =>  9,helvetica; font-size:14px">         },helvetica; font-size:14px">            series   => "simpsons",helvetica; font-size:14px">            nights   => [ qw(monday) ],helvetica; font-size:14px">                { name => "homer",role => "lead",age  => 34,helvetica; font-size:14px">                { name => "marge",role => "wife",age => 37,helvetica; font-size:14px">                { name => "bart", role => "kid", age  =>  11,helvetica; font-size:14px">5.3 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->{series} = "flintstones";
     $rec->{nights} = [ find_days() ];
     @members = ();
     # assume this file in field=value syntax
     while (<>) {
         %fields = split /[\s=]+/;
         push @members,helvetica; font-size:14px">     $rec->{members} = [ @members ];
     # now remember the whole thing
     $TV{ $rec->{series} } = $rec;
     # now,you might want to make interesting extra fields that
     # include pointers back into the same data structure so if
     # change one piece,it changes everywhere,like for example
     # if you wanted a {kids} field that was a reference
     # to an array of the kids' records without having duplicate
     # records and thus update problems.
     foreach $family (keys %TV) {
         $rec = $TV{$family}; # temp pointer
         @kids = ();
         for $person ( @{ $rec->{members} } ) {
             if ($person->{role} =~ /kid|son|daughter/) {
                 push @kids,$person;
         # REMEMBER: $rec and $TV{$family} point to same data!!
         $rec->{kids} = [ @kids ];
     # you copied the array,but the array itself contains pointers
     # to uncopied objects. this means that if you make bart get
     # older via
     # 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";
八、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,helvetica; font-size:14px">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 perlmodlib for source code to MLDBM.





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本文链接:Perl基础教程:复杂数据结构 -