Greetings, Monks.

After witnessing the power of array and hash slices in responses to a previous question I had posed, I became determined that my next programming task would involve both understanding and utilizing them. What I needed was a simple challenge that would enable me to familiarize myself with them. I decided that coding a simple searching algorithm using array slices would suffice. With that goal in mind, I chose to implement a binary search algorithm, as its specification seemed the easiest to code. Here is my first novice attempt: 

#!/usr/bin/perl -w
use strict;

my @list = 1..10;

print binary_search(\@list, 3);

sub binary_search {
    my ($aref, $find) = @_;
    my  $mid;

    $mid = int $#$aref / 2;

    return 0 if $find == $aref->[$mid];
    return 1 if !$#$aref;

    if ($find > $aref->[$mid]) {
        @$aref = @$aref[++$mid..$#$aref];
        binary_search($aref, $find);
    }
    else {
        @$aref = @$aref[0..--$mid];
        binary_search($aref, $find);
    }
}

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This works as it should, though I'm positive the verbosity of the code can be greatly decreased. My question is this: why is it that when I alter this line, 
@$aref = @$aref[++$mid..$#$aref];

[download]
to this: 
@$aref = @$aref[++$mid..-1];

[download]

The script displays this error message: Use of uninitialized value in numeric qt (>) at BinarySort.pl line 21? I proceeded to run the script through the perl debugger and noticed that the array slice @$aref[++$mid..-1] returns nothing. Why is that? I was under the impression that -1 is the same as $#$aref in this context. Any help is appreciated.

Matt

In reply to Binary search algorithm by mjab

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