How to use hash of arrays

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

Hello, I have some data that I would like to store in a hash of arrays, however this has proved to be a little complicated for me. I have written some test code below to investigate the behavior of this particular data type, and would like some help trying to understand it.

%Hash = (
                ArrayInHash => [1,2,3,4],
                );
                


print "Array in hash: @$Hash{ArrayInHash}";

@Array = @$Hash{ArrayInHash};
print "\nArray: @Array";

$ArrayRef = $Hash{ArrayInHash};
@Array2 = @$ArrayRef;
print "\nArray from ordinary scalar reference: @Array2";

print "\n\nTrying a foreach loop with the hash";
foreach(@$Hash{ArrayInHash})
{
    print "\n$_";
}

print "\n\nTrying a foreach loop with the scalar array reference";
foreach(@$ArrayRef)
{
    print "\n$_";
}
[download]

This produces:

Array in hash:
Array:
Array from ordinary scalar reference: 1 2 3 4

Trying a foreach loop with the hash


Trying a foreach loop with the scalar array reference
1
2
3
4
[download]

Why didn't the first two print statement or foreach loop act on the array? There's got to be a better way to access the array besides setting the reference to an ordinary scalar first. Thanks

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Re: How to use hash of arrays by saskaqueer (Friar) on Feb 15, 2005 at 21:24 UTC
You are close, but off by a little. The code `@$Hash{ArrayInHash}` translates into `@{$Hash}{ArrayInHash}`, which is definitely not what you want; what you need is `@{ $Hash{ArrayInHash} }`. So you'd update your code as follows: `%Hash = ( ArrayInHash => [1,2,3,4], ); print "Array in hash: @{$Hash{ArrayInHash}}"; @Array = @{$Hash{ArrayInHash}}; print "\nArray: @Array"; $ArrayRef = $Hash{ArrayInHash}; @Array2 = @$ArrayRef; print "\nArray from ordinary scalar reference: @Array2"; print "\n\nTrying a foreach loop with the hash"; foreach(@{$Hash{ArrayInHash}}) { print "\n$_"; } print "\n\nTrying a foreach loop with the scalar array reference"; foreach(@$ArrayRef) { print "\n$_"; }` [download]	[reply] [d/l] [select]
Re^2: How to use hash of arrays by iKnowNothing (Scribe) on Feb 16, 2005 at 01:32 UTC
Awesome! Thanks for the help!	[reply]
Re: How to use hash of arrays by Animator (Hermit) on Feb 15, 2005 at 21:17 UTC
I suggest you take a look at `perldoc perlreftut`, the example they use is exactly the same as your (except the data ofc). What goes wrong with `@$Hash{ArrayInHash}` is that Perl evalutes it as being: `@{$Hash}{ArrayInHash}` which means that it will try to dereference `$Hash to` an array, and then access the key 'ArrayInHash' in that array (which does not make sense as you can see). What you should use: `@{ $Hash{ArrayInHash} }` (the whitespace is not required)	[reply] [d/l] [select]
Re: How to use hash of arrays by sh1tn (Priest) on Feb 15, 2005 at 21:18 UTC
`%Hash = ( ArrayInHash => [1,2,3,4] ); print "Array in hash: @{$Hash{ArrayInHash}}"; @Array = @{$Hash{ArrayInHash}}; print "\nArray: @Array"; $ArrayRef = $Hash{ArrayInHash}; @Array2 = @$ArrayRef; print "\nArray from ordinary scalar reference: @Array2";` [download] perldoc perlref	[reply] [d/l]
Re: How to use hash of arrays by tphyahoo (Vicar) on Feb 16, 2005 at 14:01 UTC
Actually, contra my earlier post, `use strict` [download] would have diagnosed the problem after all. `#from http://www.perlmonks.org/?node_id=431329 #use strict; #Uncomment me to get to the root of the problem. use warnings; my %Hash = ( ArrayInHash => [1,2,3,4], ); print "Array in hash: $Hash{ArrayInHash}"; #Only get error if you use strict; print "WRONG Dereffed Array in hash: @$Hash{ArrayInHash}"; print "RIGHT Dereffed Array in hash with brackets: @{$Hash{ArrayInHash +}}";` [download] Use the strictures!	[reply] [d/l] [select]
Re: How to use hash of arrays by tphyahoo (Vicar) on Feb 16, 2005 at 09:09 UTC
Don't forget to `use strict; use warnings;` [download] Though that wouldn't have helped you here, it's a good habit.	[reply] [d/l]