Re: [Fun With Perl] Re: How to swap $p and $q without $tmp

[Ariel Scolnicov <ariels@compugen.co.il>]

Bek Oberin <gossamer@tertius.net.au> writes:

> Roland Giersig wrote:
> > Easy. Just right to left. Equivalent multiline version:
> >  $q ^= $p;
> >  $p ^= $q;
> >  $q ^= $p;
> 
> I must be dumb.   I still don't get it.  Maybe its just too
> late at night?
> 

$oldp = $p;
$oldq = $q;
$q ^= $p;           # $q == $oldq ^ $oldp
$p ^= $q;           # $p == $oldp ^ $q == $oldp ^ $oldq ^ $oldp == $oldq
$q ^= $p;           # $q == $q ^ $p = $oldq ^ $oldp ^ $oldq == $oldp

All you need to know is that ^ is associative ((a^b)^c == a^(b^c)),
commutative (a^b == b^a), and that a^a == 0.

-- 
Ariel Scolnicov        |"GCAAGAATTGAACTGTAG"            |ariels@compugen.co.il
Compugen Ltd.          |Tel: +972-2-6795059 (Jerusalem)	\  NEW IMPROVED URL!
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