What does the c underscore expression `c_` do exactly?

Question

It seems to be some kind of horizontal concatenation, but I could not find any documentation online. Here a minimal working example:

In [1]: from numpy impo         


        

Answer 1

It took me a lot of time to understand but it seems I finally got it.

All you have to do is add along second axis.

let's take :

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

But there isn't second axis. So we mentally add one.

so shape of both array becomes (3,1).

So resultant shape would be (3,1+1) which is (3,2). which is the shape of result -

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

Another ex.:

np.c_[np.array([[1,2,3]]), 0, 0, np.array([[4,5,6]])]

shapes:

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

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

0 so we can think of it as [[0]] = 1,1

So result 1,3+1+1+3 = 1,8

which is the shape of result : array([[1, 2, 3, 0, 0, 4, 5, 6]])