indexing with arrays of indexes and +=

Vincent Schut schut at sarvision.nl
Wed Oct 25 02:18:54 CDT 2006


David Huard wrote:
> 2006/10/24, Vincent Schut <schut at sarvision.nl 
> <mailto:schut at sarvision.nl>>:
>
>     It is clear to me that the numpy += operator in combination with
>     the use
>     of arrays of indexes, as is explained in the Tentative Numpy Tutorial
>     (
>     http://www.scipy.org/Tentative_NumPy_Tutorial#head-3f4d28139e045a442f78c5218c379af64c2c8c9e),
>     the limitation being that indexes that appear more than 1 time in the
>     indexes-array will get incremented only once.
>
>     Does anybody know a way to work around this? 
>
>
>     I am using this to fill up a custom nd-histogram, and obviously
>     each bin
>     should be able to get incremented more than once. Looping over the
>     entire array and incrementing each bin succesively takes waaay to long
>     (these are pretty large arrays, like 4000x2000 items, or even larger)
>
>
> I don't know the answer to the first question, but I'd like to ask if 
> you tried histogramdd ? If its lacking some features, i'd be willing 
> to implement them.
>
> David
No, haven't tried that, but actually my nd-histogram is a bit special. 
Firstly, it is not a histogram in the usual sample counting sense, but I 
need it to give me the average of values that are inserted in a certain 
bin (so actually I'm summing and counting, and finally divide the sums 
by the counts). Then, the bin number a sample goes into is not 
determined by the position of the sample in the array, but by the value 
of cells with the same position in other arrays. Lastly, not only these 
arrays are large, but there are many, and I need the final average, so I 
need to be able to update my histogram many times in a row.
I think these needs are a bit too peculiar to have them added to 
histogramdd. I have already got my own nd-averagingHistogram class which 
does exactly what I want, only point it that it is slow because of the 
loop. So that's why I asked here if anyone knows a workaround.

Vincent.
>   
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