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

The numpy.char.index() method returns the lowest index where a substring is found within each string element of a NumPy array. It raises a ValueError if the substring is not found in any string.

Syntax

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

Parameters

• a − Input array of strings
• sub − Substring to search for
• start − Starting position (optional)
• end − Ending position (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'])
print("Array:", arr)

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

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

Using Start and End Parameters

You can specify start and end positions to search within a range ?

import numpy as np

# Create array of strings
arr = np.array(['PYTHON', 'PROGRAM'])
print("Array:", arr)

# Find 'O' starting from index 2
result = np.char.index(arr, 'O', start=2)
print("Index of 'O' from position 2:", result)

Array: ['PYTHON' 'PROGRAM']
Index of 'O' from position 2: [4 2]

Error Handling

The method raises ValueError when substring is not found ?

import numpy as np

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

try:
    result = np.char.index(arr, 'XYZ')
    print("Index found:", result)
except ValueError as e:
    print("Error:", e)

Error: substring not found

Comparison with find() Method

Conclusion

Use numpy.char.index() to find the first occurrence index of a substring in string arrays. Handle ValueError exceptions when the substring might not exist, or use find() for safer operations.