[Numpy-discussion] the fast way to loop over ndarray elements?
Chao YUE
chaoyuejoy@gmail....
Sat Nov 17 07:43:04 CST 2012
Yes, both the "base" and "target" are ascending. Thanks!
Chao
On Sat, Nov 17, 2012 at 2:40 PM, Benjamin Root <ben.root@ou.edu> wrote:
>
>
> On Saturday, November 17, 2012, Chao YUE wrote:
>
>> Dear all,
>>
>> I need to make a linear contrast of the 2D numpy array "data" from an
>> interval to another, the approach is:
>> I have another two list: "base" & "target", then I check for each ndarray
>> element "data[i,j]",
>> if base[m] =< data[i,j] <= base[m+1], then it will be linearly
>> converted to be in the interval of (target[m], target[m+1]),
>> using another function called "lintrans".
>>
>>
>> #The way I do is to loop each row and column of the 2D array, and finally
>> loop the intervals constituted by base list:
>>
>> for row in range(data.shape[0]):
>> for col in range(data.shape[1]):
>> for i in range(len(base)-1):
>> if data[row,col]>=base[i] and data[row,col]<=base[i+1]:
>>
>> data[row,col]=lintrans(data[row,col],(base[i],base[i+1]),(target[i],target[i+1]))
>> break #use break to jump out of loop as the data have to
>> be ONLY transferred ONCE.
>>
>>
>> Now the profiling result shows that most of the time has been used in
>> this loop over the array ("plot_array_transg"),
>> and less time in calling the linear transformation fuction "lintrans":
>>
>> ncalls tottime percall cumtime percall
>> filename:lineno(function)
>> 18047 0.110 0.000 0.110 0.000
>> mathex.py:132(lintrans)
>> 1 12.495 12.495 19.061 19.061
>> mathex.py:196(plot_array_transg)
>>
>>
>> so is there anyway I can speed up this loop? Thanks for any suggestions!!
>>
>> best,
>>
>> Chao
>>
>>
> If the values in base are ascending, you can use searchsorted() to find
> out where values from data can be placed into base while maintaining order.
> Don't know if it is faster, but it would certainly be easier to read.
>
> Cheers!
> Ben Root
>
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>
--
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Chao YUE
Laboratoire des Sciences du Climat et de l'Environnement (LSCE-IPSL)
UMR 1572 CEA-CNRS-UVSQ
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Tel: (33) 01 69 08 29 02; Fax:01.69.08.77.16
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