Return the lowest index in the string where substring is found in a range using Python index()

The numpy.char.index() method returns the lowest index where a substring is found within string arrays. It searches within a specified range and raises ValueError if the substring is not found.

Syntax

numpy.char.index(a, sub, start=0, end=None)

Parameters

The method accepts the following parameters:

• a − Input array of strings
• sub − Substring to search for
• start − Starting position for search (optional)
• end − Ending position for search (optional)

Basic Example

Let's find the index of substring 'AT' in string arrays −

import numpy as np

# Create array of strings
arr = np.array(['KATIE', 'KATE', 'CRATE'])
print("Array:", arr)

# Find index of 'AT' substring
result = np.char.index(arr, 'AT')
print("Index of 'AT':", result)

Array: ['KATIE' 'KATE' 'CRATE']
Index of 'AT': [1 1 2]

Using Start and End Parameters

You can specify a range to search within using start and end parameters −

import numpy as np

arr = np.array(['KATIE', 'KATE', 'CRATE'])
print("Array:", arr)

# Search for 'AT' between positions 1 and 4
result = np.char.index(arr, 'AT', start=1, end=4)
print("Index of 'AT' (range 1-4):", result)

Array: ['KATIE' 'KATE' 'CRATE']
Index of 'AT' (range 1-4): [1 1 2]

How It Works

The method searches each string in the array:

• KATIE − 'AT' found at index 1
• KATE − 'AT' found at index 1
• CRATE − 'AT' found at index 2

Error Handling

The method raises ValueError if the substring is not found −

import numpy as np

arr = np.array(['HELLO', 'WORLD'])

try:
    result = np.char.index(arr, 'XYZ')
    print(result)
except ValueError as e:
    print("Error: Substring not found")

Error: Substring not found

Conclusion

Use numpy.char.index() to find substring positions in string arrays. It returns the lowest index for each string and raises ValueError when substring is not found.