Basic occurrence
Input
find_substring_index('hello', 'll')
Output
2
The substring 'll' starts at index 2 in 'hello'.
Full lesson preview
Find the index of a substring
Return the first index where a substring appears in a string, or -1 if not present.
Python practice6 minString PatternsBeginnerLast updated March 18, 2026
Problem statement
Write a function find_substring_index(s, sub) that returns the index of the first occurrence of substring sub in string s. If sub is not found, return -1. If sub is an empty string, return 0 (the position where an empty substring is considered to occur). Do not use print; return the integer index.
Task
Implement a function to locate the first occurrence of a substring inside a string with expected behavior for empty substrings and non-matches.
Examples
Input format
A call to find_substring_index(s, sub) where s and sub are strings.
Output format
An integer index (or -1) returned by the function.
Constraints
Do not use any external libraries. You may use built-in string methods. Time complexity should be O(n*m) in the worst case (where n and m are lengths of s and sub).
Samples
Sample 1
Input
find_substring_index('abc', 'a')
Output
0
'a' occurs at index 0.
Sample 2
Input
find_substring_index('abc', 'z')
Output
-1
'z' is not present, so return -1.