Haskell: 9.3 - (2 * 4.5) => 0.3000000000000007

@dragolov:

I expected 0.3

?

Welcome to IEEE 754 double-precision floating point arithmetic. http://en.wikipedia.org/wiki/IEEEfloatingpoint Basic gist is don't expect anything to ever be exact :) .

It's been a long time since I touched Haskell in university, but in most languages, I'd say "Don't use double if you need a precise answer, use the language's decimal implementation."

Curiously,

Python 2.6.5 (r265:79063, Oct  1 2012, 22:04:36) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 9.3 - (2 * 4.5)
0.30000000000000071

$ irb -v
irb 0.9.5(05/04/13)
$ irb
irb(main):001:0> 9.3 - (2 * 4.5)
=> 0.300000000000001

$ perl -v

This is perl, v5.10.1 (*) built for x86_64-linux-gnu-thread-multi
...
$ perl -e "print 9.3 - (2 * 4.5);"
0.300000000000001

$ bc -v bc 1.06 Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc. $ echo 'scale=100; 9.3 - (2 * 4.5)' | bc .3

:)

IIRC the latest version of Haskell now makes all internal calculations using Roman numerals instead of binary, negating this problem.

James

That sounds about right for Haskell.