Binary Search functions in C++ STL (binary_search, lower_bound and upper_bound) - GeeksforGeeks

31 May, 2021
Binary search is an important component in competitive programming or any algorithmic competition, having knowledge of shorthand functions reduces the time to code them. This searching only works when container is sorted. Related functions are discussed below.1.binary_search(start_ptr, end_ptr, num) : This function returns boolean true if the element is present in the container, else returns false. 
CPP
// C++ code to demonstrate the working of binary_search() #include<bits/stdc++.h>using namespace std; int main(){    // initializing vector of integers    vector<int> arr = {10, 15, 20, 25, 30, 35};         // using binary_search to check if 15 exists    if (binary_search(arr.begin(), arr.end(), 15))       cout << "15 exists in vector";    else       cout << "15 does not exist";          cout << endl;         // using binary_search to check if 23 exists    if (binary_search(arr.begin(), arr.end(), 23))         cout << "23 exists in vector";    else         cout << "23 does not exist";          cout << endl;   }
Output:  
15 exists in vector
23 does not exist
2. lower_bound(start_ptr, end_ptr, num) : Returns pointer to “position of num” if container contains 1 occurrence of num. Returns pointer to “first position of num” if container contains multiple occurrence of num. Returns pointer to “position of next higher number than num” if container does not contain occurrence of num. Subtracting the pointer to 1st position i.e “vect.begin()” returns the actual index. 
CPP
// C++ code to demonstrate the working of lower_bound()#include<bits/stdc++.h>using namespace std; int main(){    // initializing vector of integers    // for single occurrence    vector<int> arr1 = {10, 15, 20, 25, 30, 35};         // initializing vector of integers    // for multiple occurrences    vector<int> arr2 = {10, 15, 20, 20, 25, 30, 35};         // initializing vector of integers    // for no occurrence    vector<int> arr3 = {10, 15, 25, 30, 35};            // using lower_bound() to check if 20 exists    // single occurrence    // prints 2    cout << "The position of 20 using lower_bound "            " (in single occurrence case) : ";    cout << lower_bound(arr1.begin(), arr1.end(), 20)            - arr1.begin();          cout << endl;         // using lower_bound() to check if 20 exists    // multiple occurrence    // prints 2    cout << "The position of 20 using lower_bound "             "(in multiple occurrence case) : ";    cout << lower_bound(arr2.begin(), arr2.end(), 20)            - arr2.begin();          cout << endl;          // using lower_bound() to check if 20 exists    // no occurrence    // prints 2 ( index of next higher)    cout << "The position of 20 using lower_bound "             "(in no occurrence case) : ";    cout << lower_bound(arr3.begin(), arr3.end(), 20)            - arr3.begin();          cout << endl;   }
Output:  
The position of 20 using lower_bound (in single occurrence case) : 2
The position of 20 using lower_bound (in multiple occurrence case) : 2
The position of 20 using lower_bound (in no occurrence case) : 2
3. upper_bound(start_ptr, end_ptr, num) : Returns pointer to “position of next higher number than num” if container contains 1 occurrence of num. Returns pointer to “first position of next higher number than last occurrence of num” if container contains multiple occurrence of num. Returns pointer to “position of next higher number than num” if container does not contain occurrence of num. Subtracting the pointer to 1st position i.e “vect.begin()” returns the actual index.
Binary search is the most efficient search algorithm. 
CPP
// C++ code to demonstrate the working of upper_bound()#include<bits/stdc++.h>using namespace std; int main(){    // initializing vector of integers    // for single occurrence    vector<int> arr1 = {10, 15, 20, 25, 30, 35};         // initializing vector of integers    // for multiple occurrences    vector<int> arr2 = {10, 15, 20, 20, 25, 30, 35};         // initializing vector of integers    // for no occurrence    vector<int> arr3 = {10, 15, 25, 30, 35};              // using upper_bound() to check if 20 exists    // single occurrence    // prints 3    cout << "The position of 20 using upper_bound"           " (in single occurrence case) : ";    cout << upper_bound(arr1.begin(), arr1.end(), 20)            - arr1.begin();          cout << endl;         // using upper_bound() to check if 20 exists    // multiple occurrence    // prints 4    cout << "The position of 20 using upper_bound "             "(in multiple occurrence case) : ";    cout << upper_bound(arr2.begin(), arr2.end(), 20)            - arr2.begin();          cout << endl;          // using upper_bound() to check if 20 exists    // no occurrence    // prints 2 ( index of next higher)    cout << "The position of 20 using upper_bound"            " (in no occurrence case) : ";    cout << upper_bound(arr3.begin(), arr3.end(), 20)           - arr3.begin();          cout << endl;   }
Output:  
The position of 20 using upper_bound (in single occurrence case) : 3
The position of 20 using upper_bound (in multiple occurrence case) : 4
The position of 20 using upper_bound (in no occurrence case) : 2
Time Complexity :O(logN) -where N is number of elements in array.
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