Re: interchanging variables the tough way (BENCHMARK)

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 :)

Comment on Re: interchanging variables the tough way (BENCHMARK) Download Code

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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]
RE: RE: Re: interchanging variables the tough way (BENCHMARK) by lhoward (Vicar) on Aug 30, 2000 at 20:51 UTC
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]
RE: RE: RE: Re: interchanging variables the tough way (BENCHMARK) by tye (Sage) on Aug 30, 2000 at 21:09 UTC
Though this adds trialing nulls ("\0") to whichever string is shorter. ): - tye (but my friends call me "Tye")	[reply]


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