StatUtils.sum returns NaN for zero-length arrays
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Key: MATH-373
URL: https://issues.apache.org/jira/browse/MATH-373
Project: Commons Math
Issue Type: Bug
Affects Versions: 2.1
Reporter: Roman Werpachowski
StatUtils.sum returns NaN for zero-length arrays, which is:
1. inconsistent with the mathematical notion of sum: in maths, sum_{i=0}^{N-1}
a_i will be 0 for N=0. In particular, the identity
sum_{i=0}^{k-1} a_i + sum_{i=k}^{N-1} = sum_{i=0}^{N-1}
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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