On 13/04/2016 18:05, Random832 wrote:
.........
No, it doesn't. Sum works on any type that can be added (except
strings), it can't make any assumptions about the characteristics of
floating point types. For non-numeric types, the addition operator may
not be semantically commutative or associative.
I thought as much. My problem was that the sum of an array of small floats was
being used to compute a grid of points by subtraction like this
height = sum(H)
pos = [height]
for h in H:
height -= h
pos.append(height)
the value of height[0] came out negative which was a problem. I could reduce the
error by using Kahan summation instead of sum, but that required Kahan style
subtraction as well. In the end it just seemed better to reverse the loop and
compute pos by addition.
Look at
http://code.activestate.com/recipes/393090-binary-floating-point-summation-accurate-to-full-p/
for an example of a more accurate algorithm, but note that, for example,
this algorithm wouldn't work on complex numbers (you'd have to sum the
real and imaginary components separately)
yes indeed summation is hard :(
--
Robin Becker
--
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