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Re: sumskipnan(nan) = 0 ?


From: Thomas Shores
Subject: Re: sumskipnan(nan) = 0 ?
Date: Fri, 7 Sep 2007 14:43:56 -0500
User-agent: KMail/1.9.7

> >> sumskipnan(nan) = 0  ?
> >
> >I'm guessing that the sum of the input with all the NaN's removed
> > is zero. That is the sum of no numbers is considered 0.
>
> That seems logical.  Take the set of arguments, remove NaNs, get
> an empty set.  I think that the sum of an empty set of numbers
> should be 0.  Why should it be different?

Actually, it's not only logical, it's the only choice.  Look at it 
this way: the sum of elements over the union of disjoint index sets 
A and B should be the sum of the sum over indices in A plus the sum 
over indices in B. Now if B is the empty set, the union of the sets 
is just A, and A and B are disjoint.   Hence the sum over A union B 
should be the sum over A.  Hence the sum over B must be zero.

Tom Shores



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