Target in middle
Input
arr = [5, 3, 7, 3, 9], target = 3
Output
1
3 first appears at index 1.
Full lesson preview
Find the first index of a target
Return the index of the first occurrence of a target value in an array.
Python practice8 minArrays – Fundamentals & PatternsBeginnerLast updated March 25, 2026
Problem statement
Given an array of integers and a target integer, return the index (0-based) of the first occurrence of the target in the array. If the target does not exist in the array, return -1. Aim for O(n) time and O(1) extra space.
Task
Traverse the array and find the first position where the element equals the target; return -1 if not found.
Examples
Input format
Function call: first_index_of_target(arr, target), where arr is a list of integers and target is an integer.
Output format
An integer index (0-based) or -1 if target not present.
Constraints
0 <= len(arr) <= 10^5. Elements and target are integers. Use O(n) time and O(1) extra space.
Samples
Sample 1
Input
first_index_of_target([1,2,3], 2)
Output
1
2 is at index 1.