How to add a column to a NumPy array in Python

By Nikhil Vaddipati

Hello folks, in this tutorial we are going to learn how to add a column to a NumPy array in Python language. Two methods are discussed below.

It is obvious that we need to import NumPy library before we begin.

To know more about NumPy arrays,refer this

Using append or insert to add a column to a NumPy array in Python

Method-1 (Using append method):

Append method requires three parameters:

1. Original array – The array to which we wanted to add a column
2. New Values – The values of the column we wanted to add
3. Axis – The axis by which we wanted to append, this is always 1 in this case

Let’s look into an example

arr = [[1,2,3] , [4,5,6] , [7,8,9]]

newcolumn = [[99],[100],[101]]

import numpy as np
original = [[1,2,3],[4,5,6],[7,8,9]]
arr = np.array(original)
print("The original array is :")
print(arr)
print("--------------------------")
newcolumn = [[99],[100],[101]]
newarray=np.append(arr,newcolumn,axis=1)
print("Array after adding a column:")
print(newarray)

Output:

The original array is : 

[[1 2 3] 

[4 5 6] 

[7 8 9]] 

-------------------------- 

Array after adding a column:

[[ 1 2 3 99] 

[ 4 5 6 100]

[ 7 8 9 101]]

Method – 2 (Using insert method):

Insert method helps in adding the column at any position as per the requirement. It requires 4 parameters. They are:

1. Original array – The array to which we wanted to add a column
2. Index – The position where we want to add a column
3. Values – Values of the new column
4. Axis-The axis by which we wanted to append, this is always 1 in this case

Example:

arr = [[1,2,3] , [4,5,6] , [7,8,9]]

newcolumn = [99,100,101]

To add the newcolumn in the second position, we need to assign 1 for index parameter

index = 1

import numpy as np
original = [[1,2,3],[4,5,6],[7,8,9]]
arr = np.array(original)
print("The original array is :")
print(arr)
print("--------------------------")
newcolumn = [99,100,101]
index=1
newarray=np.insert(arr,index,newcolumn,axis=1)
print("Array after adding a column:")
print(newarray)

Output:

The original array is :
[[1 2 3] 
[4 5 6] 
[7 8 9]]
--------------------------
Array after adding a column:
[[ 1 99 2 3] 
[ 4 100 5 6] 
[ 7 101 8 9]]

Also read: How to use numpy.percentile() in Python