[ https://issues.apache.org/jira/browse/MATH373?page=com.atlassian.jira.plugin.system.issuetabpanels:alltabpanel ]
Bill Barker resolved MATH373.

Resolution: Fixed
This will be fixed in the 3.0 build.
> StatUtils.sum returns NaN for zerolength arrays
> 
>
> Key: MATH373
> URL: https://issues.apache.org/jira/browse/MATH373
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 2.1
> Reporter: Roman Werpachowski
> Fix For: 3.0
>
>
> StatUtils.sum returns NaN for zerolength arrays, which is:
> 1. inconsistent with the mathematical notion of sum: in maths, sum_{i=0}^{N1} a_i will be 0 for N=0. In particular, the identity
> sum_{i=0}^{k1} a_i + sum_{i=k}^{N1} = sum_{i=0}^{N1}
> is broken for k = 0, since NaN + x = NaN, not x.
> 2. introduces hard to debug erros (returning a NaN is one of the worst forms of reporting an exceptional condition, as NaNs propagate silently and require manual tracing during the debugging)
> 3. enforces "special case" handling when the user expects that the summed array can have a zero length.
> The correct behaviour is, in my opinion, to return 0.0, not NaN in the above case.

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