python: multiple arithmetic operations for numpy array

Question

I have this code (need to subtract sum of previous elements from current):

arr = np.zeros((N,M,T))
for it in xrange(T):
     sum_arr = np.zeros((M,N))
     for tt in xrange(it): sum_arr += arr[:,:,tt]
     arr[:,:,it] -= sum_arr

Question: Is it way to write this code in pythonic way (prefer one line)? Thx in advance.

Your indentation in the code seems to be off. Can you please fix it? — mgilson
– mgilson, Commented Oct 10, 2013 at 7:12

mgilson · Accepted Answer · 2013-10-10 07:32:15Z

2

I think you can get the sum to be done more efficiently at least:

arr = np.zeros((N, M, T))
for it in xrange(T):
    arr[:,:,it] -= np.sum(arr[:,:,:it], axis=2)

which is almost a 1-liner:

for it in xrange(T): arr[:,:,it] -= np.sum(arr[:,:,:it], axis=2)

I assume that your real data arr is not all zeros -- Otherwise, the sum will be an array of zeros which you then subtract from an array of zeros leaving you with ... and array of zeros (which isn't very interesting).

answered Oct 10, 2013 at 7:32

mgilson

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1 Comment

Serenity Over a year ago

It works perfectly. I just forgot about np.sum, thanks for clue.

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