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

[Nadav Horesh]

The highest accuracy is obtained when you sum an acceding ordered series, and the lowest accuracy with descending ordered. In between you might get a variety of rounding errors.

  Nadav. 

-----הודעה מקורית-----
מאת: numpy-discussion-bounces at scipy.org בשם Hanni Ali
נשלח: ג 09-דצמבר-08 16:07
אל: Discussion of Numerical Python
נושא: [Numpy-discussion] Importance of order when summing values in anarray
 
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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