log(0.5) with bignum blows up

jimav has asked for the wisdom of the Perl Monks concerning the following question:

Is there something about log(0.5) which is a mathematical singularity?

use bugnum;
say log(0.5);
[download]

Gives: Can't locate object method "_log" via package "Math::BigInt" at /home/jima/perl5/lib/perl5/Math/BigFloat.pm line 5737.

Any other value works, such as

say log(0.5000000000000000000000000000000000000000000001);
say log(0.4999999999999999999999999999999999999999999999);
[download]

(which both print '-0.6931471805599453094172321214581765680755')

Comment on log(0.5) with bignum blows up Select or Download Code

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Re: log(0.5) with bignum blows up by talexb (Chancellor) on Jan 04, 2024 at 02:59 UTC
`use bugnum; say log(0.5);` [download] Hmm .. I suppose you want Math::BigNum and not the unknown bugnum module. And .. after that, this module does have log functions .. `... $x->blog(); # logarithm of $x to base e (Euler's number) $x->blog($base); # logarithm of $x to base $base (e.g., base 2) $x->bexp(); # calculate e ** $x where e is Euler's number $x->bilog2(); # log2($x) rounded down to nearest int $x->bilog10(); # log10($x) rounded down to nearest int $x->bclog2(); # log2($x) rounded up to nearest int $x->bclog10(); # log19($x) rounded up to nearest int ...` [download] The `log` functions are called `blog`. If I run this .. `tab@music4:~/Pianoforte/Perlmonks 21:54:45 $ cat !$ cat 11156670.pl #!/usr/bin/perl use strict; use warnings; # use Math::BigNum; { print "Log of .5 is " . log(0.5) . "\n"; } tab@music4:~/Pianoforte/Perlmonks 21:54:51 $ ./11156670.pl Log of .5 is -0.693147180559945 tab@music4:~/Pianoforte/Perlmonks 21:54:57 $` [download] .. I get a good value. That presumably uses Perl's built in `log` function. I can strongly recommend a complete, self-contained example that shows the output .. this will help you solve things faster. Alex / talexb / Toronto Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.	[reply] [d/l] [select]
Re: log(0.5) with bignum blows up by Anonymous Monk on Jan 04, 2024 at 08:21 UTC
I've had the same error message, for a short while, but after upgrading (note previous versions): --> Working on bignum Fetching http://www.cpan.org/authors/id/P/PJ/PJACKLAM/bignum-0.67.tar. +gz ... OK Configuring bignum-0.67 ... OK ==> Found dependencies: Math::BigRat --> Working on Math::BigRat Fetching http://www.cpan.org/authors/id/P/PJ/PJACKLAM/Math-BigInt-2.00 +3001.tar.gz ... OK Configuring Math-BigInt-2.003001 ... OK Building and testing Math-BigInt-2.003001 ... OK Successfully installed Math-BigInt-2.003001 (upgraded from 0.2614) Building and testing bignum-0.67 ... OK Successfully installed bignum-0.67 (upgraded from 0.51) 2 distributions installed [download] there is no error afterwards. The previous versions mentioned are not part of Perl standard distribution (no error with unpacked original Perl), but perhaps installed some time ago. Now I have erroneous versions overwritten and, honestly, no desire to dig/debug further.	[reply] [d/l]