Binary Search (bisect) in Python

Difficulty Level : Basic
Last Updated : 30 Dec, 2020

Binary Search is a technique used to search element in a sorted list. In this article, we will looking at library functions to do Binary Search.

Finding first occurrence of an element.

bisect.bisect_left(a, x, lo=0, hi=len(a)) : Returns leftmost insertion point of x in a sorted list. Last two parameters are optional, they are used to search in sublist.

# Python code to demonstrate working 
# of binary search in library 
from bisect import bisect_left 
  
def BinarySearch(a, x): 
    i = bisect_left(a, x) 
    if i != len(a) and a[i] == x: 
        return i 
    else: 
        return -1
  
a  = [1, 2, 4, 4, 8] 
x = int(4) 
res = BinarySearch(a, x) 
if res == -1: 
    print(x, "is absent") 
else: 
    print("First occurrence of", x, "is present at", res) 

Output:

First occurrence of 4 is present at 2

Finding greatest value smaller than x.

# Python code to demonstrate working 
# of binary search in library 
from bisect import bisect_left 
  
def BinarySearch(a, x): 
    i = bisect_left(a, x) 
    if i: 
        return (i-1) 
    else: 
        return -1
  
# Driver code 
a  = [1, 2, 4, 4, 8] 
x = int(7) 
res = BinarySearch(a, x) 
if res == -1: 
    print("No value smaller than ", x) 
else: 
    print("Largest value smaller than ", x, " is at index ", res) 

Output:

Largest value smaller than  7  is at index  3

Finding rightmost occurrence

bisect.bisect_right(a, x, lo=0, hi=len(a)) Returns rightmost insertion point of x in a sorted list a. Last two parameters are optional, they are used to search in sublist.

# Python code to demonstrate working 
# of binary search in library 
from bisect import bisect_right 
  
def BinarySearch(a, x): 
    i = bisect_right(a, x) 
    if i != len(a)+1 and a[i-1] == x: 
        return (i-1) 
    else: 
        return -1
  
a  = [1, 2, 4, 4] 
x = int(4) 
res = BinarySearch(a, x) 
if res == -1: 
    print(x, "is absent") 
else: 
    print("Last occurrence of", x, "is present at", res) 