Concatenating 2 dimensional numpy arrays in Python

Question

I want to concatenate these two arrays

a = np.array([[1,2,3],[3,4,5],[6,7,8]])  
b = np.array([9,10,11])

such that

a = [[1,2,3,9],[3,4,5,10],[6,7,8,11]]

Tried using concatenate

for i in range(len(a)):
  a[i] = np.concatenate(a[i],[b[i]])

Got an error:

TypeError: 'list' object cannot be interpreted as an integer

Tried using append

for i in range(len(a)):
  a[i] = np.append(a[i],b[i])

Got another error:

ValueError: could not broadcast input array from shape (4,) into shape (3,)

(New to stackoverflow, sorry if I didn't format this well)

You can use extend(): possible duplicate of this question — D_00
– D_00, Commented Apr 15, 2021 at 13:58
Does this answer your question? Concatenate a NumPy array to another NumPy array — alex
– alex, Commented Apr 15, 2021 at 14:05
seems like extend() only works for list. I can convert them to lists, extend them, then convert it back to a numpy array. But there gotta be a better way to do it right? — N_ E
– N_ E, Commented Apr 15, 2021 at 14:07

Jérôme Richard · Accepted Answer · 2021-04-15 14:09:11Z

1

You can use hstack and vector broadcasting for that:

a = np.array([[1,2,3],[3,4,5],[6,7,8]])  
b = np.array([9,10,11])
res = np.hstack((a, b[:,None]))
print(res)

Output:

[[ 1  2  3  9]
 [ 3  4  5 10]
 [ 6  7  8 11]]

Note that you cannot use concatenate because the array have different shapes. hstack stack horizontally the multi-dimentional arrays so it just add a new line at the end here. A broadcast operation (b[:,None]) is needed so that the appended vector is a vertical one.

edited Apr 15, 2021 at 14:09

answered Apr 15, 2021 at 14:02

Jérôme Richard

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XtianP · Accepted Answer · 2021-04-15 14:05:52Z

1

You can do it like this:

np.append(a,b.reshape(-1,1),axis=1)

answered Apr 15, 2021 at 14:05

XtianP

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