Let's try that again. I was on the right track in my first post, but I took a wrong turn.

The problem is with
$monthhash{ @{$months->[$x]} }

$months->[$x]
returns a reference to a array of values (one for each field).

When evaluated in scalar context,
@{$months->[$x]}
returns the number of elements in the dereferenced array. In this case, that means it returns the number of fields (1).

$monthhash{ @{$months->[$x]} }
is therefore the same as
$monthhash{1}

You wanted
$monthhash{ $months->[$x][0] }
Remember, you are dealing with an Array (records) of Array (fields).

Thanks for the reply and the clarification, I understand what you're saying now. :)

I remembered something I had problems with before and tried that and now I'm even more puzzled...(I've had problems with things not working due to whitespace issues so I fairly routinely use a trim function I got from the cookbook), I tried it with this problem and lo and behold it works.

my $months = $dbh->selectall_arrayref("select to_char(last_day(add_mon
+ths('14-May-2004', (rownum-1))),'MON-RRRR')  dt
from all_objects
where rownum <= months_between(sysdate,'31-May-2004')+1");

my $month;
my $monthhash;
my $i = 0;
# populate month hash with zero's for now (this may need to be done la
+ter)
foreach $month (@$months) {
    $monthhash{@$month[0]} = $i;
#    print "pop @$month[0] $i\n";
    $i++;
}

for($i=0; $i<$#$months; $i++) {
    print "hope 1 @{$months->[$i]}\n";
    print "hope 2 $monthhash{trim(@{$months->[$i]})}\n";
    print "hope 2A $monthhash{@{$months->[$i]}}\n";
}

sub trim {
    my @out = @_;
    for (@out) {
        s/^\s+//;        #trim left
        s/\s+$//;        #trim right
        }
    return @out == 1
        ? $out[0]        #only one to return
        : @out;            #or many
}
[download]

Results. I added an incrementing value to the hash when I created it to make sure that I was indeed getting the correct value - As far as I can see the only difference between hope 2 & hope 2A is the trim call, yet one works and the other doesn't. I don't know why this should make any difference but it is now working.

I was wondering whether it had anything to do with the array only having one element. I started with the simple query so I'll be using your notation later on. If thats not it, then I have no idea why the trim should make any difference.

hope 1 MAY-2004
hope 2 0
hope 2A
hope 1 JUN-2004
hope 2 1
hope 2A
hope 1 JUL-2004
hope 2 2
hope 2A
hope 1 AUG-2004
hope 2 3
hope 2A
hope 1 SEP-2004
hope 2 4
hope 2A
[download]

$key = @...[...];    <--- Wrong
^      ^                  It may work because Perl is "smart",
|      |                  but Perl doesn't always guess correctly.
+------+

$key = $...[...];    <--- Right
^      ^
|      |
+------+
[download]

A hash key is a scalar, so you need a "$ expression".

# Wrong:
print "hope 1 @{$months->[$i]}\n";
print "hope 2 $monthhash{trim(@{$months->[$i]})}\n";
print "hope 2A $monthhash{@{$months->[$i]}}\n";

# Right:
print "hope 1 $months->[$i][0]\n";
print "hope 2 $monthhash{trim($months->[$i][0])}\n";
print "hope 2A $monthhash{$months->[$i][0]}\n";
[download]

You're assuming an array of one element (@array) is interchangeable with the element ($array[0]). That's not always the case. In fact, you should assume it's not the case.

By the way, @out == 1 is a bug. You should replace @out == 1 with wantarray. You'll get some interesting results when you do.

Hash keys are evaluated in scalar context. @something (including @{expr}) returns the number of elements in @something when evaluated in scalar context.

$monthhash{@$month[0]}
should be
$monthhash{$month->[0]}
if you want the first element of @$month to be used as the key.

By the way,
$month->[0]
can also be written as
${$month}[0]
and
$$month[0]
but they're less readable.

Update: Oops! Array slices, such as @something[0] and @{expr}[0] do *not* return a number of elements in scalar context. You're needlessly using array slices, but that's not your problem. While $monthhash{$month->[0]} would be better, it won't fix anything.

I can access the data in the arrayref returned by the query by using @{$months->$x}, but when I try to use @{$months->$x} as the key for the hash, I get nothing - $monthhash{ @{$months->$x} } which I dont understand.

It has been extensively explained to you, but just to stress it: hash keys are strings. Thus you can't use an array as a key, whether it's an actual array or a dereference of an arrayref. You could use "@array" as a key, but generally there's no need to do that and I don't think it's what you have in mind. OTOH you can take a slice of an hash, but in that case that's

@monthhash{ (@{$months->[$x]}) }
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not

$monthhash{ (@{$months->[$x]}) }
[download]

I'm more and more convinced of what I wrote!