2008/8/22 Catherine Moroney <[EMAIL PROTECTED]>: > I'm looking for a way to acccomplish the following task without lots > of loops involved, which are really slowing down my code. > > I have a 128x512 array which I want to break down into 2x2 squares. > Then, for each 2x2 square I want to do some simple calculations > such as finding the maximum value, which I then store in a 64x256 > array.
You should be able to do some of this with reshape and transpose: In [1]: import numpy as np In [3]: A = np.zeros((128,512)) In [4]: B = np.reshape(A,(64,2,256,2)) In [5]: C = np.transpose(B,(0,2,1,3)) In [6]: C.shape Out[6]: (64, 256, 2, 2) Now C[i,j] is the 2 by 2 array [ [A[2*i,2*j], A[2*i,2*j+1]], [A[2*i+1,2*j], A[2*i+1, 2*j+1]] ] (or maybe I got those indices the wrong way around.) Now most operations you want to do on the two-by-two matrices can be done, using ufuncs, on the whole array at once. For example if you wanted the max of each two-by-two matrix, you'd write: In [7]: D = np.amax(np.amax(C,axis=-1),axis=-1) In [8]: D.shape Out[8]: (64, 256) Anne _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
