convenientstore has asked for the wisdom of the Perl Monks concerning the following question:

From perldsc tutorial, I don't understand why below \@array is refering to the samething and code is wrong.. It says "all the ferences in @AoA refer to the very same place, and they will therefore all hold whatever was last in @array "
Trying to understand thist but if someone can explain... 
@AoA = ( [2, 3], [4, 5, 7], [0] );
    print $AoA[1][2];
  7
    print @AoA;
  ARRAY(0x83c38)ARRAY(0x8b194)ARRAY(0x8b1d0)

  for $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = \@array;    # WRONG!
    }

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Re: while going through perldsc tutorial...
by philcrow (Priest) on Oct 16, 2007 at 19:41 UTC
    In the line marked #WRONG, the reference is always taken to @array which is only allocated once in the program (probably implicitly). Therefore, on each iteration, @array gets new values, and its location gets stored and all those references refer to the last set of values. To fix it, declare @array on each iteration: 
      my @array = somefunc($i);

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    Then, new memory is allocated each time and the references are to those different memory areas.

    Phil

    The Gantry Web Framework Book is now available.
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[d/l]
      i understand now.. thank you
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Re: while going through perldsc tutorial...
by naikonta (Curate) on Oct 17, 2007 at 08:31 UTC
    You keep getting the same memory address of @array. No matter how many times you iterate, since all elements of @AoA refer to the same memory address, they all share the same value of @array, whatever its last content. Other than making array @array lexical, you can also assign anonymous array reference. Consider this, 
    #!/usr/bin/perl

use strict;
use warnings;

## first sample
print "---- 1. global \@array ----\n";
my(@aoa, @array);
for my $i (0..5) {
    @array = map { "#" x $_ } (0 .. $i);
    $aoa[$i] = \@array;
    print "$aoa[$i]\n"; # same memory address
}
print "@$_\n" for @aoa;

## second sample
print "\n---- 2. lexical \@array ----\n";
my(@AoA);
for my $i (0..5) {
    my @array = map { "#" x $_ } (0 .. $i);
    $AoA[$i] = \@array;
    print $AoA[$i], "\n"; # different memory address
}
print "@$_\n" for @AoA;

## third  sample
print "\n---- 3. anonymous array reference ----\n";
for my $i (0..5) {
    my @array =
    $AoA[$i] = [ map { "#" x $_ } (0 .. $i) ];
    print $AoA[$i], "\n"; # different memory address
}
print "@$_\n" for @AoA;

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    Output, 
    ---- 1. global @array ----
ARRAY(0x814f618)
ARRAY(0x814f618)
ARRAY(0x814f618)
ARRAY(0x814f618)
ARRAY(0x814f618)
ARRAY(0x814f618)
 # ## ### #### #####
 # ## ### #### #####
 # ## ### #### #####
 # ## ### #### #####
 # ## ### #### #####
 # ## ### #### #####

---- 2. lexical @array ----
ARRAY(0x818b03c)
ARRAY(0x814ed48)
ARRAY(0x8185ab8)
ARRAY(0x8185aac)
ARRAY(0x8185a58)
ARRAY(0x8185a40)

 #
 # ##
 # ## ###
 # ## ### ####
 # ## ### #### #####

---- 3. anonymous array reference ----
ARRAY(0x8183e38)
ARRAY(0x818b03c)
ARRAY(0x814ed48)
ARRAY(0x8185ab8)
ARRAY(0x8185aac)
ARRAY(0x8185a58)

 #
 # ##
 # ## ###
 # ## ### ####
 # ## ### #### #####

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