How to vertically concatenate two arrays in Python? [duplicate]

Question

I want to concatenate two arrays vertically in Python using the NumPy package:

a = array([1,2,3,4])
b = array([5,6,7,8])

I want something like this:

c = array([[1,2,3,4],[5,6,7,8]])

How we can do that using the concatenate function? I checked these two functions but the results are the same:

c = concatenate((a,b),axis=0)
# or
c = concatenate((a,b),axis=1)

We have this in both of these functions:

c = array([1,2,3,4,5,6,7,8])

Answer 1

The problem is that both a and b are 1D arrays and so there's only one axis to join them on.

Instead, you can use vstack (v for vertical):

>>> np.vstack((a,b))
array([[1, 2, 3, 4],
       [5, 6, 7, 8]])

Also, row_stack is an alias of the vstack function:

>>> np.row_stack((a,b))
array([[1, 2, 3, 4],
       [5, 6, 7, 8]])

It's also worth noting that multiple arrays of the same length can be stacked at once. For instance, np.vstack((a,b,x,y)) would have four rows.

Under the hood, vstack works by making sure that each array has at least two dimensions (using atleast_2D) and then calling concatenate to join these arrays on the first axis (axis=0).