Changling here?

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

I can't figure this one out.

I would appreciate any help anyone can give.

This snippet below has a scalar reference pointing to an array. The second member of the array is a reference pointing to another array.

The code in question is:

$aray_size = scalar (@$states[ 1 ]);
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After this line executes, my debugger says that $array_size is now an array, not a scalar holding the length of the array. Why???

my $states = [ ];            # Annonymous array constructed
push @$states, 'Oregon';
print "@$states [ 0 ]\n";

my $city;
my $cities = [ ];            # Inner annonymous array constructed 
push @$cities, "Portland";
push @$cities, "Salem";
push @$cities, "Bend";
push @$states, $cities;

my $counter;
my $array_size = scalar (@$states [ 1 ]);  # here

for ($counter = 0; $counter < $array_size; $counter++) {
print "\t@$states [  1 ]->\[$counter\]\n";
}
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Re: Changling here? by GrandFather (Saint) on May 30, 2008 at 02:53 UTC
`@$states [ 1 ]` is an array slice (see Slices). The syntax you require is `my $array_size = @{$states->[1]}`. Note that $array_size is a scalar so you don't need the scalar operator. Perl is environmentally friendly - it saves trees	[reply] [d/l] [select]
Re: Changling here? by jdporter (Paladin) on May 30, 2008 at 02:58 UTC
Quite simply, because array slices are not arrays. Consider the equivalent using a regular array instead of a ref: `my @states = ( [], [] ); my $aray_size = @states[1];` [download] Even though we know that the element at `$states[1]` is an array, it doesn't matter; the slice is returning the single-element list `([])`, and thus the scalar `$aray_size` receives the last element thereof - the (second, inner) arrayref. What you need to do is make sure you've actually got the array-ref which is the second element of the (outer) array, dereference that, and subject that to the `scalar()` operator. Something like `my $aray_size = @{ ${$states}[ 1 ] }; # or my $aray_size = @{ $states->[ 1 ] };` [download] (Note that the `scalar` op is implicit, due to the context transmitted by the assignment operator from the scalar variable on the LHS.) A word spoken in Mind will reach its own level, in the objective world, by its own weight	[reply] [d/l] [select]
Re: Changling here? by hangon (Deacon) on May 30, 2008 at 03:33 UTC
The for loop at the end was a bit of a puzzler because you didn't put your code between <code> </code> tags. See this node. `# original for ($counter = 0; $counter < $array_size; $counter++) { print "\t@$states [ 1 ]->\$counter\\n"; } # did you really mean this? for ($counter = 0; $counter < $array_size; $counter++) { print "\t@$states [ 1 ]->[$counter]\n"; }` [download] If so, once you've got slices and dereferencing under control, this is a clearer and more perlish way to iterate over an array. `for my $city (@{ $states->[1] }) { print "\t$city\n"; } # or for (@{ $states->[1] }) { print "\t$_\n"; }` [download]	[reply] [d/l] [select]
Re^2: Changling here? by GrandFather (Saint) on May 30, 2008 at 03:41 UTC
or: `print "\t$_\n" for @{$states->[1]};` [download] Perl is environmentally friendly - it saves trees	[reply] [d/l]