[erlang-questions] erlang float comparison

Angel J. Alvarez Miguel <>
Mon May 14 10:47:19 CEST 2012


Well we just picked the wrong example


that's is the problem

:~> erl
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4] [async-threads:0] 
[hipe] [kernel-poll:false]

[QuickCheck] [PropEr] [Erl0MQ] 
Eshell V5.9.1  (abort with ^G)
1> 1.000000000000000002 > 1.0.  
false


On Lunes, 14 de Mayo de 2012 09:50:57 usted escribió:
> 1> 0.99999998 < 1.000000.
> true
> 2> 1.99999976 < 1.99999988.
> true
> 
> What's the problem?
> 
> On Mon, May 14, 2012 at 9:41 AM, Angel J. Alvarez Miguel 
<>wrote:
> > **
> > 
> > 
> > Hi guys
> > 
> > 
> > 
> > I need to compare two floats something like 0.99999998... vs 1.000000
> > 
> > and we came across accuracy problems when testing dihedral angles on a
> > molecule...
> > 
> > 
> > 
> > so we read
> > http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm
> > 
> > and wanted to implement something like:
> > 
> > 
> > 
> > if (*(int*)&f1 < *(int*)&f2)...
> > 
> > 
> > 
> > when ef1 and f2 are floats... (f1 about 0.9999899.. and f2=1.0
> > 
> > 
> > 
> > is think we should start with...
> > 
> >  <<Padding:16,Myint:64>> = term_to_binary(1.9999976,[{minor_version,1}]).
> > 
> > and let Padding swallow the external format and tag while Myint gets the
> > ieee754 float
> > 
> > 
> > 
> > But MyInt = 4611686017346523993 instead of 1073741822
> > 
> > these conversion should follow ieee754 and being lexicografic ordered
> > 
> > but ...
> > 
> > term_to_binary(1.99999976,[{minor_version,1}]).
> > 
> > <<131,70,63,255,255,255,191,147,83,89>>
> > 
> > term_to_binary(1.99999988,[{minor_version,1}]).
> > 
> > <<131,70,63,255,255,255,223,201,169,173>>
> > 
> > doesnt seem to be the same thatn you spect to see after reading that
> > page...
> > 
> >  ..What im doing wrong....?
> > 
> > ieee754 layout....
> > 
> >   +1.99999976
> > 
> > 0x3FFFFFFE
> > 
> > 1073741822
> > 
> > +1.99999988
> > 
> > 0x3FFFFFFF
> > 
> > 1073741823
> > 
> > +2.00000000
> > 
> > 0x40000000
> > 
> > 1073741824
> > 
> > +2.00000024
> > 
> > 0x40000001
> > 
> > 1073741825
> > 
> > +2.00000048
> > 
> > 0x40000002
> > 
> > 1073741826
> > 
> > Thanks!..
> > 
> > /Angel
> > 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://erlang.org/mailman/listinfo/erlang-questions



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