Accessing arrays within arrays

Question

I have two bits of code that produce different output, and I am having great difficulty understanding why.

Code snippet 1:

my @args = $bighash{'arguments'}{'allocations'};
print "$args[0][1]";

Code snippet 2:

my @args = $bighash{'arguments'}{'allocations'}[0];
print "$args[1]";

In the first case, it is printing the value I expect. In the second case, it's not printing anything at all. Can anyone explain this?

this code seems wrong: As Eric pointed out, the thing in the hash would be a array reference, not an array. So it should be my $args = $bighash... ; print $args->[0]->[1] ; Try using Data::Dumper to print the whole thing, and take a look at perldoc.perl.org/perlreftut.html — Øyvind Skaar
– Øyvind Skaar, Commented Oct 28, 2011 at 8:42

Eric Strom · Accepted Answer · 2011-10-28 00:35:33Z

2

The values stored in your hash are array references. You can work with the reference directly:

my $args = $bighash{'arguments'}{'allocations'};
print $$args[1];  # or $args->[1]

Or you can unpack the array into a new one:

my @args = @{ $bighash{'arguments'}{'allocations'} };
print $args[1];

More detail on the perlref and perldsc man pages.

edited Oct 28, 2011 at 0:35

answered Oct 27, 2011 at 22:53

Eric Strom

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4 Comments

Eric Strom Over a year ago

what does print $bighash{arguments}{allocations}[0][1]; print out?

Adam S Over a year ago

Actually, I think I got it figured out. I am using your second solution without the [0].

Eric Strom Over a year ago

Yep, sorry, I misread your question. The answer should be correct now.

naught101 Over a year ago

What is the meaning of the double dollar sign?

Jimi WIlls · Accepted Answer · 2011-10-28 12:40:55Z

2

Eric Strom's answer basically set's you right... but I'm surprised nobody has suggested using Data::Dumper to examine the data-structures you are working with. You can really see the difference between what you've done and Eric's correction. This might help set things straight for you.

Re: use warnings... you must get a warning when you try to print the non-existent element of the newly created array? I once got told never to ask anything online until I've used strict and warnings. That's maybe a bit extreme, but -w and Data::Dumper definitely help me :-)

answered Oct 28, 2011 at 12:40

Jimi WIlls

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1 Comment

Brad Gilbert Over a year ago

Someone has suggested using Data::Dumper. They posted it as a comment.

ath88 · Accepted Answer · 2011-10-27 22:48:10Z

0

In the code snippet 2, youre effectively taking the first element in your bighash ([0] is the first element) and putting that as the only element in an array. Then you ask for the second element, which does not exist.

If you remember to 'use warnings', this should yield one, since you're assigning a scalar as an array. I have not tested this myself, though.

answered Oct 27, 2011 at 22:48

ath88

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3 Comments

Adam S Over a year ago

Wow...that was not my intention at all. Does the assignment operator effectively act as an "insert" or "append" into a new array? How can I create a new array which is a reference to $bighash{'arguments'}{'allocations'}?

Eric Strom Over a year ago

No warnings here, assigning a scalar to an array is fine, it just creates an array with one element.

Øyvind Skaar Over a year ago

Adam: You don't create an array which is a reference to.., you create a reference to an array. The reference itself is not an array.

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