How to build up a hash data structure

Question

I am having trouble figuring out how to create several %th2 structures (see below) each of which will be the values of $th1{0}, $th1{1}, and so on.

I am also trying to figure out how to traverse the keys in the second hash %th2. I am running into that error that is discussed frequently in SO,

Can't use string ("1") as a HASH ref while "strict refs" in use

Also, when I assign %th2 to each key in %th1, I am assuming this is copied into %th1 as an anonymous hash, and that I am not overriting those values as I re-use %th2.

use strict;

my %th1 = ();
my %th2 = ();
my $idx = 0;

$th2{"suffix"} = "A";
$th2{"status"} = 0;
$th2{"consumption"} = 42;

$th1{$idx} = %th2;

$idx++;

$th2{"suffix"} = "B";
$th2{"status"} = 0;
$th2{"consumption"} = 105;

$th1{$idx} = \%th2;

for my $key1 (keys %th1)
{
    print $key1."\n\n";
    for my $key2 (keys %$key1)
    {
      print $key2->{"status"};
    }

    #performing another for my $key2 won't work. I get the strict ref error.
}

The problem is that anyone looking at the question in the future will be confused as you've edited it to correct the error. — PP., Jul 31 '13 at 12:10
@ikegami Mea Culpa. Returned OP back to what it was and added code that would reproduce the error. — octopusgrabbus, Jul 31 '13 at 15:53
And I got votes to boot, but this post is a duplicate of stackoverflow.com/questions/18019278/using-same-hash-why-is-data-not-overwritten-after-hash-ref-assigned/18022667#18022667 and I've flagged it asking for moderator input. — octopusgrabbus, Aug 02 '13 at 17:42

score 4 · Accepted Answer · answered Jul 31 '13 at 12:02

4

Change:

$th1{$idx} = %th2;

to:

$th1{$idx} = \%th2;

Then you can create your loop as:

for my $key1 (keys %th1) {
    for my $key2 (keys %{$th1{$key1}} ) {
        print( "Key1=$key1, Key2=$key2, value=" . $th1{$key1}->{$key2} . "\n" );
    }
}

Or.. more explicitly:

for my $key1 (keys %th1) {
    my $inner_hash_ref = $th1{$key1};

    for my $key2 (keys %{$inner_hash_ref}) {
        print( "Key1=$key1, Key2=$key2, value=" . $inner_hash_ref->{$key2} . "\n" );
    }
}

answered Jul 31 '13 at 12:02

PP.

10,764
7
45
59

I had done that previously, but still cannot traverse the inner hash. – octopusgrabbus Jul 31 '13 at 12:04
Updated to provide a more explicit example of inner loop use. – PP. Jul 31 '13 at 12:05
Perfect. I knew how to make it, and I'll go edit the OP to reflect your reference correction. – octopusgrabbus Jul 31 '13 at 12:09
No need to edit the original question - it is easy to get confused with references at first with Perl, but with practice it will be become easy. – PP. Jul 31 '13 at 12:10

score 1 · Answer 2 · answered Jul 31 '13 at 16:17

```
$th1{$idx} = %th2;
```
should be
```
$th1{$idx} = \%th2;
```
Only scalars can be stored in a hash, so you want to store a reference to %th2. (%th2 in scalar context returns a weird string containing info about the hash's internals.)
```
keys %$key1
```
should be
```
keys %{ $th1{$key1} }
```
$key1 is a string, not a reference to a hash.
```
$key2->{"status"}
```
should be
```
$th1{$key1}{$key2}{"status"}
```
$key2 is a string, not a reference to a hash.

How to build up a hash data structure

2 Answers2