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Re^7: Help me make a test case for Math::BigFloat

by fizbin (Chaplain)
on Mar 07, 2006 at 13:31 UTC ( #534914=note: print w/replies, xml ) Need Help??


in reply to Re^6: Help me make a test case for Math::BigFloat
in thread Help me make a test case for Math::BigFloat

I've never been able to work out an algorithm for calculating exp for rationals.
That's probably because exp(r) is irrational for every rational r except for 0. However, saying use bigrat sprinkles some magic crazy dust over the whole program in a way similar to use bignum; witness:
use bigrat; use strict; my $lnev = - 7 / (10 ** 17); print "lnev is $lnev\n"; my $ev = exp($lnev); print "ev is $ev\n"; print "1-ev is ", 1-$ev, "\n";
Again, both values are 1.
--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/; map{y/X_/\n /;print}map{pop@$_}@/for@/

Replies are listed 'Best First'.
Re^8: Help me make a test case for Math::BigFloat
by spiritway (Vicar) on Mar 07, 2006 at 23:20 UTC
    That's probably because exp(r) is irrational for every rational r except for 0.

    Not quite; what I'd be looking for, of course, would be an algorithm that would *approximate* exp. Basically, any numeric representation in a computer is going to be a sort of "rational", in that it uses a finite number of bytes, bits, digits, or whatever to represent any number.

    The problem isn't that approximation algorithms don't exist; it's that I haven't been able to find one that converges rapidly. I know they're out there - I just haven't had the time to dig around and find them.

    There is certainly something wrong with bigrat or its friends. I'm not sure who's maintaining the modules any more - it used to be Tels.

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