From:

Date:

June 23, 2006 16:42Subject:

RE: SPUG: swapping two numbersMessage ID:

FE10499CE029B841A9DAAF12E5A52A1B0296943A@XCH-NW-3V1.nw.nos.boeing.comM> I was digging through some old code and came across a proof that I had done M> that you can swap two numbers in place (i.e. without using an external M> temporary value). M> Of course, in Perl, we can just do this: M> ($x, $y) = ($y, $x) M> But I was challenged (at a GSLUG meeting) to do it in a language-independent way. M> Since the guy mentioned he had known the solution for 20+ years, I descended into M> bit-flipping land and came up with a solution. M> (BTW: I'm not sure why my first gut reaction was right. I guess I've been in the M> business long enough. There's no way my conscious mind came up with this solution!!! M> Nevertheless, my subconscious came up with this solution within about 3 seconds. M> Brains are amazing!!!) M> I present it here as an interesting piece of code: M> #! /usr/bin/perl -w M> use warnings; M> use strict; M> my $iterations = shift @ARGV || 10; M> for (my $i = 0; $i < $iterations; $i++) { M> my ($orig_a, $a) = (int(rand 1000)) x 2; M> my ($orig_b, $b) = (int(rand 1000)) x 2; M> # Original solution: M> # $a = $a ^ $b; M> # $b = $b ^ $a; M> # $a = $a ^ $b; M> # Refined to use op= M> # $a ^= $b; M> # $b ^= $a; M> # $a ^= $b; M> # Utilizing op= value, factored onto one line M> # $a ^= ($b ^= ($a ^= $b)); M> # Refined to remove unnecessary parens M> $a ^= $b ^= $a ^= $b; 'Algorithms with Perl' has your original, refined solution in the Crytography chapter (pg. 539 'Swapping values with XOR') but not the nice, de-cluttering refinement. -- Charles DeRykus

**RE: SPUG: swapping two numbers**by DeRykus, Charles E

nntp.perl.org: Perl Programming lists via nntp and http.

Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About