Get the Last N Columns of a 2D Numpy Array

The Numpy library in Python comes with a number of useful methods and techniques to work with and manipulate data in arrays. In this tutorial, we will look at how to get the last N columns of a two-dimensional Numpy Array with the help of some examples.

How to get the last n columns of a 2D Numpy array?

You can use slicing to get the last N columns of a 2D array in Numpy. Here, we use column indices to specify the range of columns we’d like to slice. To get the last n columns, use the following slicing syntax –

# last n columns of numpy array
ar[:, -n:]

It returns the array’s last n columns (including all the rows).

Steps to get the last n columns of 2D array

Let’s now look at a step-by-step example of using the above syntax on a 2D Numpy array.

Step 1 – Create a 2D Numpy array

First, we will create a 2D Numpy array that we’ll operate on.

import numpy as np

# create a 2D array
ar = np.array([
    ['Tim', 181, 86],
    ['Peter', 170, 68],
    ['Isha', 158, 59],
    ['Rohan', 168, 81],
    ['Yuri', 171, 65],
    ['Emma', 166, 64],
    ['Michael', 175, 78],
    ['Jim', 190, 87],
    ['Pam', 168, 57],
    ['Dwight', 187, 84]
])
# display the array
print(ar)

Output:

[['Tim' '181' '86']
 ['Peter' '170' '68']
 ['Isha' '158' '59']
 ['Rohan' '168' '81']
 ['Yuri' '171' '65']
 ['Emma' '166' '64']
 ['Michael' '175' '78']
 ['Jim' '190' '87']
 ['Pam' '168' '57']
 ['Dwight' '187' '84']]

Here, we used the numpy.array() function to create a 2D Numpy array with 10 rows and 3 columns. The array contains information on the height (in cm) and weight (in kg) of some employees in an office.

Step 2 – Slice the array to get the last n columns

To get the last n columns of the above array, slice the array starting from the nth last column up to the last column of the array. You can use negative indexing to get the index of the nth column from the end (which will be -n).

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For example, you can use -5 as the index of the 5th column from the end.

For example, let’s get the last 2 columns from the array that we created in step 1.

# last 2 columns
print(ar[:, -2:])

Output:

[['181' '86']
 ['170' '68']
 ['158' '59']
 ['168' '81']
 ['171' '65']
 ['166' '64']
 ['175' '78']
 ['190' '87']
 ['168' '57']
 ['187' '84']]

We get the last 2 columns of the 2D array.

For more on slicing Numpy arrays, refer to its documentation.

Summary

In this tutorial, we looked at how to get the last n columns of a two-dimensional Numpy array using slicing. The following is a short summary of the steps mentioned in the tutorial.

Create a 2D Numpy array (skip this step if you already have an array to operate on).
Slice the array from the nth last column up to the last column of the array. Using a negative index can be helpful here.

Author

Piyush Raj

Piyush is a data professional passionate about using data to understand things better and make informed decisions. He has experience working as a Data Scientist in the consulting domain and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.

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