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Bill Barker edited comment on MATH373 at 6/10/10 10:58 PM:

The consensus of the commonsmath developers is that, since the current behavior is documented
in 2.x, that this will have to wait for 3.0. Fixing this in 2.x would introduce a too large
incompatibility change to include in 2.x.
I can attach a patch against 2.x that fixes this, as long as anybody using the patch understands
that it isn't supported.
was (Author: billbarker):
The consensus of the commonsmath developers is that, since the current behavior is documented
in 2.x, that this will have to wait for 3.0. Fixing this would introduce a too large incompatibility
change to include in 2.x.
I can attach a patch against 2.x that fixes this, as long as anybody using the patch understands
that it isn't supported.
> 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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