Weird result returned from adding floats depending on order I add them

[Grant Edwards]

On 2007-02-20, joanne matthews (RRes-Roth) <joanne.matthews at bbsrc.ac.uk> wrote:

> I'm getting different results when I add up a list of floats depending
> on the order that I list the floats.

That's how floats work.

> For example, the following returns
> False:
> def check():
> 	totalProp=0
> 	inputs=[0.2,0.2,0.2,0.1,0.2,0,0.1]
> 	for each in inputs:
>         
>    		totalProp+=each
>    	print "totalProp=",totalProp
>    	if totalProp != 1:

Floating point operations are not exact.  This test requires
them to be.

[...]

> Can anyone tell me whats going on

IEEE floating point can not exactly represent 0.2 nor 0.1, so
you get approximations.

> and how I can avoid the problem.

Don't use floating point if you expect exact results.

-- 
Grant Edwards                   grante             Yow!  I feel better about
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