looping through array references

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

Hi All,
I use nested data strcutures quite a lot, and can easily end up with arrays, inside hashes, inside arrays, etc. Now when I'm looping through I like to use foreach as it makes the notation shorter. For example:-

my $ref;
$ref = [
          {
           "key1" => 'a',
           "key2" => 'b',
          },
          {
           "key1" => 'c',
           "key2" => 'd',
          },
        ];

foreach $hashref (@{$ref}) {
    print $hashref->{'key1'};
}#foreach
[download]

Now I sometimes I want to know the array item number so I end up doing a for:-

for (my $item = 0; $item <= $#{$ref}; $item++) {
    print $item;
    print $ref->[$item]->{'key1'};
}#for
[download]

As you can see the print is much longer, when the data structure is more complex this can be a really big difference.
Is there a good way of getting the benefits of the notation in the first example while knowing the array item number. Is it safe to:-

my $item = 0;
foreach $hashref (@{$ref}) {
    print $hashref->{'key1'};
    $item++;
}#foreach
[download]

Most the time the data is loading from files and I do not know how the foreach loop with deal with missing array items, such as:-

my $ref;
$ref->[0]->{'key1'} = "a";
$ref->[5]->{'key1'} = "b";
my $item = 0;
foreach $hashref (@{$ref}) {
    print $item;
    print $hashref->{'key1'};
    $item++;
}#foreach
[download]

Having written this, I realized I could answer my own questions simply by testing the final code example, perl does indeed fill in any missing array items producing the result I expected. I thought I'd post this anyway encase others have similar questions, or I'm sure there might be some interesting comments.

Lyle

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Re: looping through array references by GrandFather (Saint) on Feb 03, 2008 at 18:20 UTC
Note that: `for (my $item = 0; $item <= $#{$ref}; $item++) {` [download] is much better written: `for my $item (0 .. $#{$ref}) {` [download] and you should always declare for loop variables in the loop header: `foreach my $hashref (@{$ref}) {` [download] to avoid any reader's confusion about the scope of the variable. Perl is environmentally friendly - it saves trees	[reply] [d/l] [select]
Re: looping through array references by bobf (Monsignor) on Feb 03, 2008 at 20:31 UTC
I echo GrandFather's reply. I'd also like to emphasize that Super Search is your friend. Searching for 'loop array index' finds a couple of recent threads that are quite relevant: array looping with foreach, which leads to Re: What's missing in Perl 5.10 A question of perlish elegance on array loop Those threads mention functions from two modules (in case you'd rather use one than roll your own method): `each` from Array::Each::Override and `natatime` from List::MoreUtils. After looking through the docs for the latter I think `each_array` is more appropriate here. `use warnings; use strict; use List::MoreUtils qw( each_array ); my @array = ( 'a' .. 'g' ); my $a_obj = each_array( @array ); while( my $var = $a_obj->() ) { my $i = $a_obj->( 'index' ); print "got $var at $i\n"; }` [download] `got a at 0 got b at 1 got c at 2 got d at 3 got e at 4 got f at 5 got g at 6` [download]	[reply] [d/l] [select]
Re: looping through array references by hipowls (Curate) on Feb 03, 2008 at 20:17 UTC
I use the following because it limits the scope of the two variables, $item and $hash_ref, to the loop where they are used. `for my $item ( 0 .. $#{$ref} ) { $hash_ref = $ref->[$item]; print "$hash_ref->{'key1'}; }` [download] It also continues to work when you add extra code that includes a next or redo statement. Update fixed code thanks to johngg. Teach me to post without running it first;)	[reply] [d/l]
Re^2: looping through array references by johngg (Canon) on Feb 04, 2008 at 00:08 UTC
I think there are a couple of problems with your code: `for my $item ( 0 .. @{$ref} )` is going to try to access an element beyond the end of the array. What you need here is `$#{$ref}`. `$hash_ref = $ref->{$item};` should be `$hash_ref = $ref->[$item];` as you are dereferencing an array, not a hash Cheers, JohnGG	[reply] [d/l] [select]