Python Bisect: Find the first occurrence of a given number in a sorted list using Binary Search

Last update on April 01 2025 13:14:28 (UTC/GMT +8 hours)

4. First Occurrence with Binary Search

Write a Python program to find the first occurrence of a given number in a sorted list using Binary Search (bisect).

Sample Solution:

Python Code:

from bisect import bisect_left   
def Binary_Search(a, x): 
    i = bisect_left(a, x) 
    if i != len(a) and a[i] == x: 
        return i 
    else: 
        return -1

nums = [1, 2, 3, 4, 8, 8, 10, 12] 
x = 8 
num_position = Binary_Search(nums, x) 
if num_position == -1: 
    print(x, "is not present.") 
else: 
    print("First occurrence of", x, "is present at index", num_position)

Sample Output:

First occurrence of 8 is present at index 4

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to locate the first occurrence of a target number in a sorted list using bisect_left and then print the index.
• Write a Python script to modify a binary search function so that it returns the index of the first occurrence of a duplicate target in a sorted array.
• Write a Python program that uses bisect_left to find the leftmost index of a target value in a sorted list and then verifies the result by checking adjacent elements.
• Write a Python function to implement a binary search that returns the first occurrence index of a target and handles cases when the target is not present.

Python Code Editor:

Contribute your code and comments through Disqus.

Previous: Write a Python program to insert items into a list in sorted order.