[Numpy-discussion] Importance of order when summing values in an array

Hanni Ali hanni.ali@gmail....
Tue Dec 9 08:07:34 CST 2008


Hi All,

I have encountered a puzzling issue and I am not certain if this is a
mistake of my own doing or not. Would someone kindly just look over this
issue to make sure I'm not doing something very silly.

So, why would the sum of an array have a different value depending on the
order I select the indices of the array?

>>> vector[[39, 46, 49, 50, 6, 9, 12, 14, 15, 17, 21]].sum()
8933281.8757099733
>>> vector[[6, 9, 12, 14, 15, 17, 21, 39, 46, 49, 50]].sum()
8933281.8757099714
>>> sum(vector[[39, 46, 49, 50, 6, 9, 12, 14, 15, 17, 21]])
8933281.8757099733
>>> sum(vector[[6, 9, 12, 14, 15, 17, 21, 39, 46, 49, 50]])
8933281.8757099714

Any thoughts?

Cheers,

Hanni
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