About numpy's concatenate, hstack, vstack functions?

Question

See some examples

import numpy as np

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

print(np.vstack((a,b))) # [[1,2,3],[4,5,6]]
print(np.concatenate((a,b), axis=1)) # IndexError: axis 1 out of bounds [0, 1)

The result of hstack is the same as concatenate along axis=0, but the api document says hstack=concatenate along axis=1, please look at the https://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.hstack.html#numpy.hstack

And concatenating along the axis=1 raise an IndexError, the api document says hstack=concatenate along axis=0, please look at the https://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.vstack.html#numpy.vstack

Can anybody explain it?By the way, can anybody explain how to broadcast when the ndarray's dimension is less than 2 and concatenating along axis=1?

Have a look at this: stackoverflow.com/questions/35401041/… — Stijn Van Daele
Possible duplicate of Concatenation of 2 1D numpy arrays along 2nd axis — Stijn Van Daele

hpaulj hpaulj · Accepted Answer · 2017-03-25T17:44:47

Look at the actual code for hstack:

arrs = [atleast_1d(_m) for _m in tup]
# As a special case, dimension 0 of 1-dimensional arrays is "horizontal"
if arrs[0].ndim == 1:
    return _nx.concatenate(arrs, 0)
else:
    return _nx.concatenate(arrs, 1)

I don't see anything in the docs about axis=1. The term it uses is 'stack them horizontally'.

As I noted a year ago, Concatenation of 2 1D numpy arrays along 2nd axis, earlier versions don't raise an error if the axis is too high. But in 1.12 we get an error.

There is a newish np.stack that can add a dimension where needed:

In [46]: np.stack((np.arange(3), np.arange(4,7)),axis=1)
Out[46]: 
array([[0, 4],
       [1, 5],
       [2, 6]])

The base function is concatenate. The various stack functions adjust array dimensions in one way or other, and then do concatenate. Look at their code to see the details. (I've summarized the differences in earlier posts as well).

About numpy's concatenate, hstack, vstack functions?

2 Answers