numpy, replace column with array

Question

I am trying to replace one or several columns with a new array with the same length.

a = np.array([[1,2,3],[1,2,3],[1,2,3]])
b = np.array([[0,0,0])
a[:, 0] = b

I got an error of ValueError: could not broadcast input array from shape (3,1) into shape (3). However this works when b has multiple columns.

a = np.array([[1,2,3],[1,2,3],[1,2,3]])
b = np.array([[0,7],[0,7],[0,7]])
a[:, 0:2] = b

array([[0, 7, 3],
       [0, 7, 3],
       [0, 7, 3]])

How can I efficiently replace a column with another array?

Thanks

J

Supply flattened version - a[:, 0] = b.ravel() or with b[:,0]. — Divakar
– Divakar, Commented Apr 12, 2019 at 15:30

Sheldore · Accepted Answer · 2019-04-12 15:31:53Z

3

Your example will work fine if you use the following just like you are using a[:, 0:2] = b. [:, 0:1] is effectively just the first column

a = np.array([[1,2,3],[1,2,3],[1,2,3]])
b = np.array([[0],[0],[0]])
a[:, 0:1] = b

# array([[0, 2, 3],
#        [0, 2, 3],
#        [0, 2, 3]])

answered Apr 12, 2019 at 15:31

Sheldore

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Comments

vurmux · Accepted Answer · 2019-04-12 15:33:16Z

2

You have an incorrect shape of b. You should pass an ordinary 1D array to it if you want to replace only one column:

a = np.array([[1,2,3],[1,2,3],[1,2,3]])
b = np.array([0,0,0])
a[:, 0] = b
a

Returns:

array([[0, 2, 3],
       [0, 2, 3],
       [0, 2, 3]])

answered Apr 12, 2019 at 15:33

vurmux

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