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Get the First 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 first N columns of a two-dimensional Numpy Array with the help of some examples.

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

get the first n columns of a 2d numpy array

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

# first n columns of numpy array
ar[:, :n]

It returns the first n columns (including all the rows) of the given array.

Steps to get the first 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 first n columns

To get the first n columns of the above array, slice the array starting from the first column (0th index) up to (but not including) the column with the nth index.

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

# first 2 columns
print(ar[:, 0:2])

Output:

[['Tim' '181']
 ['Peter' '170']
 ['Isha' '158']
 ['Rohan' '168']
 ['Yuri' '171']
 ['Emma' '166']
 ['Michael' '175']
 ['Jim' '190']
 ['Pam' '168']
 ['Dwight' '187']]

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

In the above code, you don’t need to explicitly specify 0 (since you’re slicing starting from the very first column of the array).

# first 2 columns
print(ar[:, :2])

Output:

[['Tim' '181']
 ['Peter' '170']
 ['Isha' '158']
 ['Rohan' '168']
 ['Yuri' '171']
 ['Emma' '166']
 ['Michael' '175']
 ['Jim' '190']
 ['Pam' '168']
 ['Dwight' '187']]

We get the same result as above.

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

Summary

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

  1. Create a 2D Numpy array (skip this step if you already have an array to operate on).
  2. Slice the array from the column with index 0 to the column with index n (but not including it) to get the first n columns of the array.

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Author

  • Piyush

    Piyush is a data scientist passionate about using data to understand things better and make informed decisions. In the past, he's worked as a Data Scientist for ZS and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.