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Index a 2D NumPy array with 2 lists of indices

Learn, how to index a 2D NumPy array with 2 lists of indices in Python?
By Pranit Sharma Last updated : December 25, 2023

NumPy is an abbreviated form of Numerical Python. It is used for different types of scientific operations in python. Numpy is a vast library in python which is used for almost every kind of scientific or mathematical operation. It is itself an array which is a collection of various methods and functions for processing the arrays.

Problem statement

Suppose that we are given a 2D numpy array and we have 2 indexers one with indices for the rows, and one with indices for the column, we need to index this 2-dimensional numpy array with these 2 indexers.

Indexing a 2D NumPy array with 2 lists of indices

For this purpose, we will use the numpy.ix_() with indexing arrays. This function is used to construct an open mesh from multiple sequences. This function takes N 1-D sequences and returns N outputs with N dimensions each, such that the shape is 1 in all but one dimension and the dimension with the non-unit shape value cycles through all N dimensions.

Below is the syntax of numpy.ix_() method:

numpy.ix_(*args)

Let us understand with the help of an example,

Python code to index a 2D NumPy array with 2 lists of indices

# Import numpy
import numpy as np

# Creating a numpy array
arr =  [[17, 39, 88, 14, 73, 58, 17, 78],[88, 92, 46, 67, 44, 81, 17, 67],[31, 70, 47, 90, 52, 15, 24, 22]]

# Display original array
print("Original Array:\n",arr,"\n")

# Defining indexers
row = [4, 2, 5, 4, 1]

col = [1, 2]

# Indexing 2D array
res = np.ix_(row,col)

# Display result
print("Result:\n",res,"\n")

Output

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