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From: Rainer Weikusat <rweikusat@talktalk.net>
Newsgroups: comp.lang.perl.misc
Subject: Re: compare two sorted array, item by item, which one is bigger
Date: Wed, 28 Feb 2024 16:34:35 +0000
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hymie! <hymie@nasalinux.net> writes:
> In our last episode, the evil Dr. Lacto had captured our hero,
>   Rainer Weikusat <rweikusat@talktalk.net>, who said:
>> hymie! <hymie@nasalinux.net> writes:
>>> The question is -- how can I (or can I) programatically keep checking
>>> entries in the arrays of the %scores hash until I find a pair of
>>> entries that are not equal?
>>
>> If your arrays are always of equal length, you could use
>
> I don't think I can depend on that :(
>
>> otherwise, it's a bit more difficult.
>>
>> sub ary_cmp
>> {
>>     my ($a0, $a1) = @_;
>>     my ($last, $rc);
>>
>>     # we need the length of the shorter array
>>     # start with the length of array a0
>>     # and see if the length of array a1 is less
>>     $last = $#$a0;
>>     $_ < $last and $last = $_ for $#$a1;
>>
>>     # for each entry in the shorter array
>>     # compare that numbered entry in the two arrays
>>     # return <=> if the result is not 0
>>     for (0 .. $last) {
>>         $_ and return $_ for $$a0[$_] <=> $$a1[$_];
>>     }
>>
>>     # all of the elements are equal, so return the longer array
>>     return @$a0 <=> @$a1;
>> }
>
> I took the liberty of adding your improvement to this function.
>
> I'll definitely try this out and see how well it work.
>
> I have a few followup questions...
>
> (*) I added some comments.  Can you tell me if I'm correct?

Yes.

>
> (*) Could I have set $last this way?
>
> $last = $#$a0 < $#$a1 ? $#$a0 : $#$a1 ;
>
> or
>
> $last = @$a0 < @$a1 ? @$a0 : @$a1 ;

The first yes, second no as $#$a0 == @$a0 - 1. It's just syntactically a
bit more repetitive.

>
> ?
>
> (*) In this construct
>
>>     for (0 .. $last) {
>>         $_ and return $_ for $$a0[$_] <=> $$a1[$_];
>
> is it safe to reuse $_ like that?

Yes. The foreach-for aliases a localized $_ to the first element and
then executes the loop body, ie, either the block in case of for (...) {
} or the statement for the statement modifier. Then, it does the same
with the second element and so forth, until the body has been run for
all list elements. This means $_ reverts back to the current index after
the

$_ and return $_

has been executed.