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!

Answer 1

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]])