bisect in Python

Binary Search 是一种用于搜索排序列表中元素的technique。在本文中，我们将研究执行Binary Search 的库函数。

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) 

Output:

Last occurrence of 4 is present at 3