Perl: the Markov chain saw PerlMonks

### <strike>6 times faster?</strike> (Re: Help w/ Code Optimization)

by arhuman (Vicar)
 on Dec 26, 2001 at 22:27 UTC ( #134454=note: print w/replies, xml ) Need Help??

in reply to Help w/ Code Optimization

You might also try this :
```sub Root2 {
my \$num        = shift;
my \$root       = shift;
my \$iterations = shift || 10;
if ( \$root == 0 ) { return 1 }
if ( \$num < 0 ) { return undef }
my \$current = Math::BigFloat->new();
my \$guess   = Math::BigFloat->new( \$num / \$root );
my \$t=Math::BigFloat->new(\$guess**(\$root-1)); 1st version : WRONG ! sh
+ould be in the loop.
for ( 1 .. \$iterations ) {
\$current = \$guess - ( \$guess * \$t - \$num ) / ( \$root * \$t  );
if ( \$guess eq \$current ) { last }
\$t=Math::BigFloat->new(\$current**(\$root-1));
\$guess = \$current;
}
return \$current;
}
The idea behind, is that '**' is MUCH slower than '*', so I exchange 2 '**' for one '**' and 2 '*'
It seems indeed to speed things alot :
(I'm sure your numerous tests will make it sure ;-)
```timethese(10,{
'Root'=> sub {Root(1000,60)},
'Root2'=> sub {Root2(1000,60)}});

gives as result :

Benchmark: timing 10 iterations of Root, Root2...
Root: 104 wallclock secs (104.38 usr + 0.01 sys = 104.39 CPU) @ 0.10/s (n=10)
Root2: 15 wallclock secs (14.75 usr + 0.00 sys = 14.75 CPU) @ 0.68/s (n=10)
Root2: 97 wallclock secs (92.67 usr + 0.16 sys = 92.83 CPU) @ 0.11/s (n=10)

UPDATE :
Corrected my code! I misplaced the \$t calculation outside the loop,
which is wrong beccause \$guess change inside the loop.
As you can see it's no more so fast...

Replies are listed 'Best First'.
Re: Re: Help w/ Code Optimization
by sifukurt (Hermit) on Dec 26, 2001 at 22:58 UTC
When I try to run your code, I get an incorrect answer for the root. I think the problem is in the exponent. The exponents are \$root and (\$root - 1), respectively.

When I run this code:
```timethese( 100, {
'Root' => '\$x = Root( 100, 3 )',
'Root2' => '\$y = Root2( 100, 3 )',
});

print <<END;

Root: \$x
-----
Root2: \$y
END
I get this for output:
```Benchmark: timing 100 iterations of Root, Root2...
Root: 29 wallclock secs (28.39 usr +  0.00 sys = 28.39 CPU) @  3
+.52/s (n=1 00)
Root2: 20 wallclock secs (20.03 usr +  0.00 sys = 20.03 CPU) @  4
+.99/s (n=1 00)

Root: 4.6415888336127788924100763542328778501807372125002714
-----
Root2: 0.6664902595019951166742874561807256134735

___________________
Kurt

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