Append a 1d array to a 2d array in Numpy Python

Question

I have a numpy 2D array [[1,2,3]]. I need to append a numpy 1D array,( say [4,5,6]) to it, so that it becomes [[1,2,3], [4,5,6]]

This is easily possible using lists, where you just call append on the 2D list.

But how do you do it in Numpy arrays?

np.concatenate and np.append dont work. they convert the array to 1D for some reason.

Thanks!

vstack does np.concatenate([np.atleast_2d(m) for m in tup], 0) - in other words - make sure all inputs are 2d and then concatenate. — hpaulj
– hpaulj, Commented Feb 17, 2016 at 21:20

Padraic Cunningham · Accepted Answer · 2016-02-17 20:04:08Z

20

You want vstack:

In [45]: a = np.array([[1,2,3]])

In [46]: l = [4,5,6]

In [47]: np.vstack([a,l])
Out[47]: 
array([[1, 2, 3],
       [4, 5, 6]])

You can stack multiple rows on the condition that The arrays must have the same shape along all but the first axis.

In [53]: np.vstack([a,[[4,5,6], [7,8,9]]])
Out[53]: 
array([[1, 2, 3],
       [4, 5, 6],
       [4, 5, 6],
       [7, 8, 9]])

answered Feb 17, 2016 at 20:04

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2 Comments

it doesn't work similarly with hstack though... col_vector_to_append = np.column_stack([4, 5, 6]).T is needed

@Mehdi surely vstack is the transposed version of hstack right?

Wattanapong Suttapak · Accepted Answer · 2020-04-20 17:55:31Z

1

Try this:

np.concatenate(([a],[b]),axis=0)

when

a = np.array([1,2,3])
b = np.array([4,5,6])

then result should be:

array([[1, 2, 3], [4, 5, 6]])

answered Apr 20, 2020 at 17:55

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