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

[Bruce Southey]

Nadav Horesh wrote:
> 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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>   
Also, increase the numerical precision as that may depend on your 
platform especially given the input values above are ints. Numpy has 
float128 and int64 that will minimize rounding error.

Bruce