SPUG: scalar swap challenge

[Michael R. Wolf]

I sent the following to MJD for use as a possible QOTW, then decided
to share it wit SPUG first.

Enjoy,
Michael

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swap challenge> In most languages, values cannot be directly swapped without a
swap challenge> temporary variable. It's a classic problem shared by passing trains,
swap challenge> elevator passengers, and virtual memory systems.
swap challenge> 
swap challenge> A swap code fragment typically looks like this.
swap challenge> 
swap challenge>     $temp = $left;
swap challenge>     $right = $left;
swap challenge>     $left = $temp;
swap challenge> 
swap challenge> Perl's list assignment can do this directly.
swap challenge> 
swap challenge>     ($left, $right) = ($right, $left);
swap challenge> 
swap challenge> Recently, I got challenged to do a swap without a temporary variable
swap challenge> in C by someone who claimed it's always possible. Actually, it's
swap challenge> always possible for *integral* types. Because I no longer enjoy coding
swap challenge> in C, I reworded it to be a Perl challenge:
swap challenge> 
swap challenge>    swap two integral scalars without using a temporary scalar and
swap challenge>    without using the list assignment swapping idiom.

Here's a little test script I put together to test my solution *after*
I'd proven it on paper.

#! /usr/bin/perl -w

use warnings;
use strict;

my $iterations = shift @ARGV || 10;

for (my $i = 0; $i < $iterations; $i++) {
    my ($orig_a, $a) = (int(rand 1000)) x 2;
    my ($orig_b, $b) = (int(rand 1000)) x 2;

    # Your $a/$b swap code goes here.
    $a = "something";
    $b = "something";

    printf("%4d %4d %s\n" =>
	   $a, $b,
	   $a == $orig_b && $b == $orig_a ? "passed" : "failed: a was $orig_a, b was $orig_b");
}


-- 
Michael R. Wolf
    All mammals learn by playing!
        MichaelRWolf at att.net