Array of hashes misbehaving?

Dr. Mu has asked for the wisdom of the Perl Monks concerning the following question:

I thought I understood hash arrays until I encountered this:

use strict;

my @hashes = ({}) x 10;
$hashes[$_]{$_} = 1 foreach (0 .. 9);

foreach (0 .. 9) {
  print join(':', keys %{$hashes[$_]}), "\n"
}
[download]

which prints:

6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
6:3:7:9:2:8:1:4:0:5
[download]

What gives? I was expecting it to output:

0
1
2
3
4
5
6
7
8
9
[download]

('Using ActivePerl 5.8.4 on Win XP.)

Thanks,
-Phil

Edit: Oh nevermind. I know what the problem is: every element of the array contains a reference to the same hash. D'oh!

This fixes the problem:

my @hashes = map{{}} (0 .. 9);
[download]

-Phil

Comment on Array of hashes misbehaving? Select or Download Code

Replies are listed 'Best First'.
Re: Array of hashes misbehaving? by Eliya (Vicar) on May 23, 2012 at 22:22 UTC
This fixes the problem: ... Actually, in this particular case, you could also just not initialize the array, i.e. simply say `my @hashes; $hashes[$_]{$_} = 1 foreach (0 .. 9);` [download] and let autovivification do the job.	[reply] [d/l]
Re^2: Array of hashes misbehaving? by Dr. Mu (Hermit) on May 23, 2012 at 22:58 UTC
Good point. Thanks for the reminder! -Phil	[reply]
Re: Array of hashes misbehaving? by ikegami (Patriarch) on May 23, 2012 at 22:33 UTC
If I were to use `map`, I wouldn't use `my @hashes = map { {} } 0..9; $hashes[$_]{$_} = 1 for 0..9;` [download] I would use `my @hashes = map { { $_ => 1 } } 0..9;` [download]	[reply] [d/l] [select]
Re^2: Array of hashes misbehaving? by Dr. Mu (Hermit) on May 23, 2012 at 22:57 UTC
In my minimalistic example, used to illustrate the problem I was having, that would certainly work. But in the real program which spurred my example, it would not. But thanks just the same! -Phil	[reply]

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