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

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (references)

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Re: How do I access an array for which I only have a reference?⭐ by chromatic (Archbishop) on Mar 31, 2000 at 21:39 UTC
You dereference the array reference. For example, given `my @array = ( 'one', 'two', 'buckle my shoe', ); my $array_ref = \@array;` [download] then you can access the array to which `$array_ref` refers by prefixing a `@` to it, e.g.: `for ( @$array_ref ) { ... }` [download] The `@` symbol is essentially an array dereference operator. It can dereference any value which is an array reference, whether that value is in a variable (e.g. `$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";` [download] 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!` [download] The error message I get is "`Can't coerce array into hash at - line ...`"	[reply] [d/l] [select]
Re: How do I access an array for which I only have a reference?⭐ by little_mistress (Monk) on Apr 01, 2000 at 02:22 UTC
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 $r_array = [ 'inky', 'blinky', 'blu' ];` [download] Now the question is, what exactly is in `$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 }` [download] You could use it in code, like this: `for ( @{ $r_array } ) { print "$_\n"; }` [download] Now, as a shorthand, Perl lets you omit the curly braces (for reference variables only): `for ( @$r_array )` [download]	[reply] [d/l] [select]
Re: How do I access an array for which I only have a reference?⭐ by le (Friar) on Jun 07, 2000 at 00:59 UTC
In places where you want the index of the last element (e.g. in `foreach` loops), you'd use the syntax `$#{ $arrayref }` [download] It's just like the usual `$#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->[$i] \n"; }` [download] I think this is useful when you have arrays of arrays, or even more complex data structures.	[reply] [d/l] [select]
Re: How do I loop over a reference to an array? by bageler (Hermit) on Mar 08, 2004 at 23:18 UTC
You can also use map as the looping mechanism...some may call it map abuse ;) Dereference in the usual fashion. `$arrayref = [0..10]; map {print "$_\n"} @$arrayref;` [download]	[reply] [d/l]
Re: Answer: How do I loop over a reference to an array? by Wonko the sane (Deacon) on Mar 09, 2004 at 19:27 UTC
Map Abuse Map Abuse!! :) IMHO,a map in void context is a sign you should be using a for loop instead. `print qq{$_\n} for ( @$arrayref );` [download] Update: Oops, read rest of thread. Apologize for stating the obvious. :P Wonko.	[reply] [d/l]
Re: How do I loop over a reference to an array? by sdaniels (Initiate) on Feb 02, 2001 at 01:47 UTC
Or you could try: `foreach my $i ( 0 .. $#$array ) { # dereference the element print $array->[$i], "\n"; }` [download] Originally posted as a Categorized Answer.	[reply] [d/l]
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