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How do I access an array for which I only have a reference?

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Contributed by Anonymous Monk on Mar 31, 2000 at 17:41 UTC
Q&A  > references


Answer: How do I access an array for which I only have a reference?
contributed by chromatic

You dereference the array reference.

For example, given

my @array = ( 'one', 'two', 'buckle my shoe', ); my $array_ref = \@array;
then you can access the array to which $array_ref refers by prefixing a @ to it, e.g.:
for ( @$array_ref ) { ... }
The @ symbol is essentially an array dereference operator. It can dereference any value which is an array reference, whether that value is in a variable (e.g. $array_ref) or is the result of some more complex expression. The general syntax is @{ ... }; but you can omit the curlies when the expression is a simple scalar variable holding an array ref.

To access elements of an array to which you have a reference, insert an arrow between the array ref and the square brackets used for indexing, e.g.

print "The third element is $array_ref->[2] \n";

Reference types are strictly checked by the interpreter; you can't dereference an array ref as a hash, for example.

print keys %{ $array_ref }; # fatal!
The error message I get is "Can't coerce array into hash at - line ..."

Answer: How do I access an array for which I only have a reference?
contributed by little_mistress

A really good book to get is O'Reilly's Advanced Perl Programming if you are going to get into this sort of thing. But here is my brief explanation.

Let's say you have an array reference...

my $r_array = [ 'inky', 'blinky', 'blu' ];
Now the question is, what exactly is in $r_array? It's sort of similar to the concept of pointers in C: a reference contains the address of where the data is actually located in memory. If you print $r_array directly, you get something funky looking like ARRAY(0xc70858), which is clearly not the data stored in the array. So think about your reference this way: it's just a sign saying where the data really is.

To get at the data where it really is, you have to dereference the reference, using the following syntax:

@{ $r_array }
You could use it in code, like this:
for ( @{ $r_array } ) { print "$_\n"; }
Now, as a shorthand, Perl lets you omit the curly braces (for reference variables only):
for ( @$r_array )

Answer: How do I access an array for which I only have a reference?
contributed by le

In places where you want the index of the last element (e.g. in foreach loops), you'd use the syntax

$#{ $arrayref }
It's just like the usual $#array, but with an array reference value in place of the array variable name. And you need the curly braces.

So, to iterate over the elements of an array to which you have a reference, using an index counter:

for ( my $i = 0; $i <= $#{ $arrayref }; $i++ ) { print "$arrayref->[$i] \n"; }

I think this is useful when you have arrays of arrays, or even more complex data structures.

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