How can i create 2 for loops to replace values in Numpy array?

Question

How can i use two for loops to replace the values in x with the row number, starting at 1, so it should be [[1,1,1,1,1],[2,2,2,2,2] … [5,5,5,5,5]]

x=np.ones((5,5)) print(x)

Thanks

Why would you use for loops with numpy? It's an anti-pattern. — mozway
– mozway, Commented Dec 12, 2022 at 21:06

mozway · Accepted Answer · 2022-12-12 21:16:43Z

2

Don't use for loops in numpy, use broadcasting:

x=np.ones((5,5))

x[:] = np.arange(x.shape[0])[:, None]+1

Updated x:

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

Alternatives:

x[:] = np.arange(x.shape[0])[:, np.newaxis]+1

Or:

x[:] = np.arange(x.shape[0]).reshape(-1, 1)+1

answered Dec 12, 2022 at 21:11

mozway

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2 Comments

one nit -- use np.newaxis instead of None so it makes it a bit clearer what's happening

@jkr you're probably right, although np.newaxis just points to None ;)