Perl: the Markov chain saw PerlMonks

### (tye)Re3: In theory, theory and practice are the same...

by tye (Sage)
 on Apr 01, 2001 at 08:09 UTC ( #68782=note: print w/replies, xml ) Need Help??

in reply to Re (tilly) 1: In theory, theory and practice are the same...

I stand by my (quite conservative) statement. It does introduce errors for fairly small values of n. It is also at least twice as slow (it can be orders of magnitude slower).

But the greatest weakness to my mind is the following output:

```200 choose 1: 200 vs. -1.#IND (1.#QNAN).
Rate  easy  nice
easy  1464/s    --  -96%
nice 34687/s 2270%    --
Now that is quite a large error term, don't you think?

Yes, there is a useful space over which its accuracy is quite good (though not always as good), and thanks for exploring that.

Here is the code I used:

```#!/usr/bin/perl -w
use strict;
use Benchmark qw( cmpthese );
sub fact {
my \$fact = 1;
\$fact *= \$_ for 1..\$_[0];
return \$fact;
}
sub nice {
my (\$n, \$r) = @_;
my \$res = 1;
for my \$i (1..\$r) {
\$res *= \$n--;
\$res /= \$i;
}
return \$res;
}
while(  <>  ) {
my( \$n, \$m )= split ' ';
my \$nice= nice(\$n,\$m);
my \$easy= fact(\$n)/fact(\$m)/fact(\$n-\$m);
print "\$n choose \$m: \$nice vs. \$easy (",abs(\$nice-\$easy),").\n";
cmpthese( -3, {
nice=>sub{nice(\$n,\$m)},
easy=>sub{fact(\$n)/fact(\$m)/fact(\$n-\$m)},
} );
}
- tye (but my friends call me "Tye")

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Re (tilly) 3: In theory, theory and practice are the same...
by tilly (Archbishop) on Apr 01, 2001 at 18:56 UTC
It is a valid performance note. True But it isn't producing (visibly) wrong answers to take the naive approach.

And the improvement is subtle. For instance consider the following slightly different version of your code:

```sub not_so_nice {
my (\$n, \$r) = @_;
my \$res = 1;
for my \$i (1..\$r) {
\$res *= (\$n-- / \$i);
}
return \$res;
}
Someone was trying to simplify. But oops. In Perl you have just slowed down instead. In other languages you are going to get wrong answers. It takes a fair amount of knowledge to understand what changed. In fact it takes a fair amount of knowledge to understand why the original formula can be sped up by using your alternate approach.

Now I appreciate that you have both pieces of knowledge. In fact the necessary analysis is probably almost a reflex for you, just like it is for me. But not everyone has the math background that we do...

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