Adding n columns to a numpy array

Question

I'm making a program where I need to make a matrix looking like this:

A = np.array([[ 1.,  2.,  3.],
   [ 1.,  2.,  3.],
   [ 1.,  2.,  3.],
   [ 1.,  2.,  3.]])

So I started thinking about this np.arange(1,4)

But, how to append n columns of np.arange(1,4) to A?

Please include the *complete* code you've tried with `np.arange` — kdbanman, Nov 24 '15 at 22:33
Do you want to append rows to `A` or create `A` that looks like the one in your question? For the latter case use `numpy.repeat` on axis=0. — Ashwini Chaudhary, Nov 24 '15 at 22:35
Looks like he would like (4, 4, 4, 4) (5, 5, 5, 5), ... columns added to the array, but it's a bit vague... — Ricardo Cárdenes, Nov 24 '15 at 22:36

score 3 · Answer 1 · answered Nov 24 '15 at 22:36

As mentioned in docs you can use concatenate

>>> a = np.array([[1, 2], [3, 4]])
>>> b = np.array([[5, 6]])
>>> np.concatenate((a, b), axis=0)
array([[1, 2],
       [3, 4],
       [5, 6]])
>>> np.concatenate((a, b.T), axis=1)
array([[1, 2, 5],
       [3, 4, 6]])

score 3 · Answer 2 · answered Nov 24 '15 at 22:44

3

Here's another way, using broadcasting:

In [69]: np.arange(1,4)*np.ones((4,1))
Out[69]: 
array([[ 1.,  2.,  3.],
       [ 1.,  2.,  3.],
       [ 1.,  2.,  3.],
       [ 1.,  2.,  3.]])

answered Nov 24 '15 at 22:44

hpaulj

201,845
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score 1 · Accepted Answer · answered Nov 24 '15 at 22:39

1

You can get something like what you typed in your question with:

N = 3
A = np.tile(np.arange(1, N+1), (N, 1))

I'm assuming you want a square array?

answered Nov 24 '15 at 22:39

Frank M

1,470
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14

score 0 · Answer 4 · answered Nov 24 '15 at 23:18

0

>>> np.repeat([np.arange(1, 4)], 4, 0)
array([[1, 2, 3],
       [1, 2, 3],
       [1, 2, 3],
       [1, 2, 3]])

answered Nov 24 '15 at 23:18

John La Rooy

281,034
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Adding n columns to a numpy array

4 Answers4