How to simplify a for loop in python

Question

I have this piece of Python code that fills up a 2d matrix in a for loop

img=zeros((len(bins_x),len(bins_y)))

for i in arange(0,len(ix)):
   img[ix[i]][iy[i]]=dummy[i]

Is it possible to use a vectorial operation for the last two lines of code? Is there also something that might speed up the calculation?

Can you post some sample contents of ix, iy, dummy, bins_x, bins_y? — jterrace
– jterrace, Commented Aug 20, 2012 at 15:33
This is not really an answer (that's why it's a comment) but while someone comes up with something more "decent", you may want to take a look to the itertools module (docs.python.org/library/itertools.html) See if there's something interesting for you there :) — Savir
– Savir, Commented Aug 20, 2012 at 15:40
as far as speeding it up, img[ix[i], iy[i] ] = dummy[i] is likely to be faster as it only need to do indexing once instead of twice. Also note that you can pretty this up a little by for x,y,z in zip(ix,iy,dummy): img[x,y] = z, but I don't know if that will be faster. — mgilson
– mgilson, Commented Aug 20, 2012 at 15:46

jfs · Accepted Answer · 2012-08-20 15:50:39Z

2

If ix, iy are index sequences:

img[ix, iy] = dummy

answered Aug 20, 2012 at 15:50

jfs

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6 Comments

I just discovered this about 3 seconds ago. (+1)

Although, technically, you might want to slice them in case they're not the same length: img[ix, iy[:len(ix)] ] = dummy[:len(ix)]

As @mgilson pointed out, you may want to make sure that ix, iy and dummy have all the same size beforehand...

Is this possible with python 2.7? I keep getting TypeError: list indices must be integers, not tuple.

@Kevin: the OP likely uses numpy.zeros() function that returns a numpy array that supports ix,iy indexing.

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Glorfindel · Accepted Answer · 2023-03-30 18:00:23Z

0

It might be useful to use numpy. In particular, the reshape method might be useful. Here is an example (adapted from the second link):

>>> import numpy as np
>>> a = np.array([1,2,3,4,5,6])
>>> np.reshape(a, (3,2))
array([[1, 2],
       [3, 4],
       [5, 6]])

Glorfindel

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answered Aug 20, 2012 at 15:48

HerrKaputt

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