In Competitive programming, learning to use the standard library than coding from scratch saves a lot of time. In this post we will use C++ STL(Standard Template Library) binary search algorithm with an example.

Given a sorted array of numbers, How to find the frequency of a given number?

Here is an efficient algorithm to do it. Use binary search to find the left-most(L), and right-most(R) occurrence of the given number. Then R-L+1 gives us the result.

C++ STL has two functions namely lower_bound, upper_bound which finds out the left-most and right-most occurrences of a given number in a sorted array.

The lower_bound() function returns an iterator to the left-most element, and the upper_bound() function returns an iterator to one element beyond right-most occurrence. So the difference of these two gives the count.

Here is the C++ code.

#include <iostream>

#include <string>

#include <vector>

#include <algorithm>

using namespace std;

int getCount(vector<int> & numVector, int d)

{

int result = 0;

vector<int>::iterator left, right;

//find the left-most occurence

left = lower_bound( numVector.begin(), numVector.end(), d );

//find right-most occurence only if the number is present

if( left != numVector.end() )

{

right = upper_bound( numVector.begin(), numVector.end(), d );

result = right-left; //right points to one element beyond.

}

return result;

}

int main()

{

int n;

//read array size

cin>>n;

int i,num;

vector<int> nums;

//read the array

for( i = 0 ; i < n ; i++ )

{

cin>>num;

nums.push_back(num);

}

//read the number to be counted

cin>>num;

cout<<getCount(nums,num);

return 0;

}