[Numpy-discussion] speeding up getting a subset of a data array

Brennan Williams brennan.williams@visualreservoir....
Mon Aug 10 20:10:29 CDT 2009


josef.pktd@gmail.com wrote:
> On Mon, Aug 10, 2009 at 8:52 PM, Brennan
> Williams<brennan.williams@visualreservoir.com> wrote:
>   
>> Hi
>>
>> No doubt asked many times before so apologies....
>>
>> I'm pulling a subset array out of a data array where I have a list of
>> the indices I want (could be an array rather than a list actually - I
>> have it in both).
>>
>> Potentially the number of points and the number of times I do this can
>> get very large so any saving in time is good.
>>
>> So, paraphrasing what I've currently got.... say I have...
>>
>> subsetpointerlist=[0,1,2,5,8,15,25...]
>> subsetsize=len(subsetpointerlist)
>> subsetarray=zeros(subsetsize,dtype=float)
>> for index,pos in enumerate(subsetpointerlist):
>>  subsetarray[index]=dataarray[pos]
>>
>> How do I speed this up in numpy, i.e. by removing the for loop?
>>
>> Do I set up some sort of a subsetpointerarray as a mask and then somehow
>> apply that to dataarray to get the values into subsetarray?
>>
>> Thanks
>>
>> Brennan
>>
>>
>>     
>
> looks to me like
>
> subsetarray = dataarray[subsetpointerlist]
>
> or with type conversion
>
> subsetarray = dataarray[subsetpointerlist].astype(float)
>
> Josef
>   
Thanks, with a little bit of googling/rtfm I'm getting there. Think I 
overdid my thinking on mask based on something else that Robert Kern 
helped me out with.
>   
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