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

[Steve Holden]

joanne matthews (RRes-Roth) wrote:
> I'm getting different results when I add up a list of floats depending
> on the order that I list the floats. 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:
>         	print "Your proportions must add up to 1"
>    
>         	return False
>    	return True
> 
> However, if I swap, the 4th and 5th list items like this:
> 
> totalProp=0
> inputs=[0.2,0.2,0.2,0.2,0.1,0,0.1]
> for each in inputs:
>         
>    totalProp+=each
>    print "totalProp=",totalProp
>    if totalProp != 1:
>         print "Your proportions must add up to 1"
>    
>         return False
>    return True
> 
> I get True returned. Can anyone tell me whats going on and how I can
> avoid the problem. Thanks
> 
> Joanne Matthews

I take it by now you understand that you need to test that the sum of 
your numbers is with some small delta of the value you actually require?

I seem to remember this question has been asked on a number of threads, 
just don't know whether this is the same question as the others.

regards
  Steve
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