Question

I need to create a new (n*m) x 4 matrix (e.g. named b) from a n x m matrix (e.g. named a), but I don’t want to use nested loops for speed reasons. Here how I would do it with a nested loop:

for j in xrange(1,m+1):
    for i in xrange(1,n+1):
        index = (j-1)*n+i
        b[index,1] = a[i,j]
        b[index,2] = index
        b[index,3] = s1*i+s2*j+s3
        b[index,4] = s4*i+s5*j+s6

The question is, therefore, how to create a new matrix with values derived from original matrix indexes? Thanks

Answer 1

If you can use numpy, you could try

import numpy as np
# Create an empty array
b = np.empty((np.multiply(*a.shape), 4), dtype=a.dtype)
# Get two (nxm) of indices
(irows, icols) = np.indices(a)
# Fill the b array
b[...,0] = a.flat
b[...,1] = np.arange(a.size)
b[...,2] = (s1*irows + s2*icols + s3).flat
b[...,3] = (s4*irows + s5*icols + s6).flat