How to keep using values from a list until the diagonal of a matrix is full using itertools

Question

So I am trying to use a smaller list to populate the diagonal of a larger matrix. I thought using the cycle function in itertools would make this an easy task but I can't seem to get it to work. Here is what I tried

a = np.zeros((10,10))
b = [1, 2, 3, 4, 5]
for i in range(len(a.shape[0])):
     a[i, i] = list(itertools.cycle(b))

but this makes it endlessly iterate. I am hoping that it will stop once the diagonal has been filled. Other options that are more pythonic are greatly appreciated!

what are you trying? to set the diags to 1 then 2, then 3 etc ? — Patrick Artner, Nov 19 '18 at 20:15

Jean-François Fabre · Accepted Answer · 2018-11-19T20:28:28.237

you mean to use itertools.cycle, not repeat. The latter repeats the element (the list), good luck setting that into a value, specially if you force iteration (since it runs forever)

I'd create a reference on a cycle object outside the loop and assign a value to the diagonal iterating over it manually (the only proper way with cycle). Also note that your loop range was wrong. a.shape[0] is a dimension, no need for len

import numpy as np,itertools
a = np.zeros((10,10))
b = [1, 2, 3, 4, 5]
iterator = itertools.cycle(b)
for i in range(a.shape[0]):
     a[i, i] = next(iterator)

result:

>>> a
array([[ 1.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  2.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  3.,  0.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  4.,  0.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  5.,  0.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  1.,  0.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  2.,  0.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  3.,  0.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  4.,  0.],
       [ 0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  0.,  5.]])

As they loop forever, cycle and repeat should not be used in a context of forced iteration (repeat has an optional parameter to limit the repeats, though).

I mean't to put cycle in my original post, oops. But my issue was that I wasn't using it as an iterator outside the loop and not using next! Thank you. — Amanda.py, Nov 19 '18 at 20:25

How to keep using values from a list until the diagonal of a matrix is full using itertools

1 Answers1