Concatenate two NumPy arrays vertically

Question

I tried the following    gt  gt  gt  a   np array  1 2 3    gt  gt  gt  b   np array  4 5 6    gt  gt  gt  np concatenate  a b   axis 0  array  1  2  3  4  5  6    gt  gt  gt  np concatenate  a b   axis 1  array  1  2  3  4  5  6     However  I d expect at least that one result looks like this  array   1  2  3           4  5  6      Why is it not concatenated vertically

User · Answer

A not well known feature of numpy is to use r_. This is a simple way to build up arrays quickly:

import numpy as np
a = np.array([1,2,3])
b = np.array([4,5,6])
c = np.r_[a[None,:],b[None,:]]
print(c)
#[[1 2 3]
# [4 5 6]]

The purpose of a[None,:] is to add an axis to array a.

User · Answer

Because both a and b have only one axis  as their shape is  3   and the axis parameter specifically refers to the axis of the elements to concatenate   this example should clarify what concatenate is doing with axis  Take two vectors with two axis  with shape  2 3    a   np array   1 5 9    2 6 10    b   np array   3 7 11    4 8 12      concatenates along the 1st axis  rows of the 1st  then rows of the 2nd    np concatenate  a b   axis 0  array    1   5   9            2   6  10            3   7  11            4   8  12      concatenates along the 2nd axis  columns of the 1st  then columns of the 2nd    np concatenate  a  b   axis 1  array    1   5   9   3   7  11            2   6  10   4   8  12      to obtain the output you presented  you can use vstack  a   np array  1 2 3   b   np array  4 5 6   np vstack  a  b   array   1  2  3           4  5  6      You can still do it with concatenate  but you need to reshape them first   np concatenate  a reshape 1 3   b reshape 1 3    array   1  2  3           4  5  6      Finally  as proposed in the comments  one way to reshape them is to use newaxis   np concatenate  a np newaxis     b np newaxis

User · Answer

a   np array  1 2 3   b   np array  4 5 6   np array  a b     works just as well as  np array   1 2 3    4 5 6      Regardless of whether it is a list of lists or a list of 1d arrays  np array tries to create a 2d array   But it s also a good idea to understand how np concatenate and its family of stack functions work   In this context concatenate needs a list of 2d arrays  or any anything that np array will turn into a 2d array  as inputs     np vstack first loops though the inputs making sure they are at least 2d  then does concatenate   Functionally it s the same as expanding the dimensions of the arrays yourself   np stack is a new function that joins the arrays on a new dimension   Default behaves just like np array   Look at the code for these functions   If written in Python you can learn quite a bit   For vstack   return  nx concatenate  atleast 2d  m  for  m in tup   0

User · Answer

If the actual problem at hand is to concatenate two 1-D arrays vertically  and we are not fixated on using concatenate to perform this operation  I would suggest the use of np column stack   In     a   np array  1 2 3   In     b   np array  4 5 6   In     np column stack  a  b   array   1  4           2  5           3  6

[python] Concatenate two NumPy arrays vertically

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