Access Elements of Array - Numpy

NumPy - Access elements of array

To access elements of a numpy array, you can use indexing operator [].

The indexing operator takes an integer or a list of integers representing the indices of elements you want to access.

index starts with a value of 0 for first element, and increments by one for subsequent elements.

If arr is numpy array, then the following expression accesses the element at index=5.

arr[5]

If arr is numpy array, then the following expression accesses the elements from start index of 5 to a stop index of 10 (stop not included in the returned object).

arr[5:10]

We can also specify a step value, with the following syntax.

arr[start:stop:step]

If the array is multi-dimensional then we can specify the indices for as many dimensions as required.

arr[index_1][index_2]

Accessing a range of elements is called slicing, and please refer the tutorial - NumPy Array Slicing.

Examples

1. Access element at index=5

In the following program, we take a numpy array (one dimensional), and access the element at index=5.

Python Program

import numpy as np

arr = np.array([1, 2, 4, 9, 16, 25, 36, 49, 64])
element = arr[5]
print(element)

Output

Explanation

[1, 2, 4, 9, 16, 25, 36, 49, 64] <- arr
 0  1  2  3   4   5   6   7   8  <- indices
              arr[5] is 25

2. Access elements from index=2 until index=6

In the following program, we take a numpy array (one dimensional), and access the elements from index=2 until index=6.

Python Program

import numpy as np

arr = np.array([1, 2, 4, 9, 16, 25, 36, 49, 64])
elements = arr[2:6]
print(elements)

Output

[ 4  9 16 25]

Explanation

[1, 2, 4, 9, 16, 25, 36, 49, 64] <- arr
 0  1  2  3   4   5   6   7   8  <- indices
      |--------------|
       4, 9, 16, 25 <-  arr[2:6]

3. Access elements from index=2 until index=8 in steps of 2

In the following program, we take a numpy array (one dimensional), and access the elements from index=2 until index=8 in steps of 2.

Python Program

import numpy as np

arr = np.array([1, 2, 4, 9, 16, 25, 36, 49, 64])
elements = arr[2:8:2]
print(elements)

Output

[ 4 16 36]

Explanation

[1, 2, 4, 9, 16, 25, 36, 49, 64] <- arr
 0  1  2  3   4   5   6   7   8  <- indices
      |-      -       -     |
       4,    16,     36       <-  arr[2:8:2]

Summary

In this NumPy Tutorial, we learned how to access elements of a numpy array using indexing operator.