Ytukyg

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!

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).