in reply to interchanging variables the tough way

Ok, so as an update in benchmarking, I have written a quick benchmark of the 3 main swapping mechanisms I see written here. I also wanted to say, that the xor technique really just blew me away... never seen that before... anyways, here are some benchmarks: 
#!/usr/bin/perl -w

use vars qw/$a $b/;
use strict;
use Benchmark;

$a = 0;
$b = 1;

sub traditional {
        my $c = $a;
        $a = $b;
        $b = $c;
}

sub arrays {
        ($b, $a) = ($a, $b);
}

sub xor {
        $a = $a ^ $b;
        $b = $a ^ $b;
        $a = $a ^ $b;
}

timethese(-5, {
        trad            => \&traditional,
        arrays  => \&arrays,
        xor             => \&xor,
        });

[ed@ci478467-a ed]$ perl swap.pl
Benchmark: running arrays, trad, xor, each for at least 5 CPU seconds.
+..
    arrays:  5 wallclock secs ( 5.12 usr +  0.00 sys =  5.12 CPU) @ 24
+242.19/s (n=124120)
      trad:  6 wallclock secs ( 5.00 usr +  0.01 sys =  5.01 CPU) @ 40
+630.74/s (n=203560)
       xor:  5 wallclock secs ( 5.11 usr +  0.00 sys =  5.11 CPU) @ 30
+829.16/s (n=157537)

[download]
so unfortunatelly, as amazingly cool as the XOR swap is, it seem slike it is slighly slower in perl than just using 3 variables... curses :)

Replies are listed 'Best First'.
RE: Re: interchanging variables the tough way (BENCHMARK)
by ncw (Friar) on Aug 30, 2000 at 20:09 UTC
    In ARM assembler the XOR method is actually equal in speed to the temporary variable method, and saves a register, and hence is a standard trick in any ARM programmers macro library. Eg :- 
    ; Swap r0,r1 using r2 as temporary
; Takes 3 cycles

traditional
    MOV  r2, r0 ; r2 <- r0
    MOV  r0, r1 ; r0 <- r1
    MOV  r1, r2 ; r1 <- r2

; Swap r0, r1 with no extra register
; Takes 3 cycles

xor
    EOR  r0, r0, r1 ; r0 <- r0 ^ r1
    EOR  r1, r0, r1 ; r1 <- r0 ^ r1
    EOR  r0, r0, r1 ; r0 <- r0 ^ r1

    [download]
    Apologies for non perl post, but perspective can be useful sometimes ;-)
[reply]
[d/l]
RE: Re: interchanging variables the tough way (BENCHMARK)
by turnstep (Parson) on Aug 30, 2000 at 20:18 UTC

    Note that XOR will not work properly in at least three cases: if the variable is a large value, a reference, or is undefined. The other two methods handle all three without a problem. (although -w will complain about the undef). Just something to watch out for.

    sub xor { $a = $a ^ $b; $b = $a ^ $b; $a = $a ^ $b; }
@foo= qw(bar none at all);

$a=999999999999;
$b=8888888;
print "BEFORE: A=$a B=$b\n";
&xor();
print "AFTER:  A=$a B=$b\n\n";

$a=\@foo; $b=8888888;
print "BEFORE: A=$a ($a->[0]) B=$b\n";
&xor();
print "AFTER:  A=$a ($a->[0]) B=$b\n\n";

$a=undef; $b=8888888;
print "BEFORE: A=$a B=$b\n";
&xor();
print "AFTER:  A=$a B=$b\n\n";

## Running the above produces:
BEFORE: A=999999999999 B=8888888
AFTER:  A=8888888 B=4294967295

BEFORE: A=ARRAY(0x87650a4) (bar) B=8888888
AFTER:  A=8888888 () B=141971620

BEFORE: A= B=8888888
AFTER:  A=8888888 B=0

    [download]
[reply]
[d/l]
      You can get the xor method to work on large numeric values by changing the swap function to stringify the values first: 
      sub xor { $a = "$a" ^ "$b"; $b = "$a" ^ "$b"; $a = "$a" ^ "$b"; }

$a=999999999999;
$b=8888888;
print "BEFORE: A=$a B=$b\n";
&xor();
print "AFTER:  A=$a B=$b\n\n";

      [download]
      output 
      
BEFORE: A=999999999999 B=8888888
AFTER:  A=8888888 B=999999999999
[reply]
[d/l]

        Though this adds trialing nulls ("\0") to whichever string is shorter. ):

                - tye (but my friends call me "Tye")
[reply]

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