in reply to Very small numbers

One thing you might try (though this may also fail due to floating point errors) is multiplying your number by a few hundred before taking the log. Then subtracting from the result to account for the initial magnification. 
log(a*b) = log a + log b

log10 1000 = 3
log10 6 = 0.77815125
log10 6000 = 3.77815125
...

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UPDATE: Although this makes me think we ought to have a Math::SmallRat and SmallFloat. PDL or Math::FixedPrecision might also work.

UPDATE: Apparently Math::BigFloat is just a bit of a slight misnomer. It's not just for BIG numbers. The Big imples a large number of bits assigned to store the number.

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Re: Very small numbers
by Abigail-II (Bishop) on Feb 09, 2004 at 16:21 UTC
    Well, if your number is small enough that Perl thinks the number is 0, multiplying it with 100, or whatever number you choose, will not do you any good: 
        $ perl -wle 'print log (10**-4950)'
    -11397.7067720196325
    $ perl -wle 'print log (10**-4951)'
    Can't take log of 0 at -e line 1.
    $ perl -wle 'print log (10**-4951 * 100)'
    Can't take log of 0 at -e line 1.

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    Abigail

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Re^2: Very small numbers
by etj (Priest) on Jun 23, 2022 at 16:16 UTC
    PDL unfortunately cannot help with this, since it (currently) only works with C types, in this case probably a double-precision floating point number. However, the techniques discussed elsewhere in this thread to avoid underflow (especially scaling) will work great to overcome that.
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Re: Re: Very small numbers
by Win (Novice) on Feb 09, 2004 at 16:43 UTC
    Maths::BigFloat is not available via ppm.
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      The name is Math::BigFloat, and it is included with perl (though if you have an old version of perl, you may want to update it since there have been bugfixes).
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