Check out example codes for "lower_bound c++". It will help you in understanding the concepts better.

Code Example 1

// lower_bound/upper_bound example
#include <iostream>     // std::cout
#include <algorithm>    // std::lower_bound, std::upper_bound, std::sort
#include <vector>       // std::vector

int main () {
  int myints[] = {10,20,30,30,20,10,10,20};
  std::vector<int> v(myints,myints+8);           // 10 20 30 30 20 10 10 20

  std::sort (v.begin(), v.end());                // 10 10 10 20 20 20 30 30

  std::vector<int>::iterator low,up;
  low=std::lower_bound (v.begin(), v.end(), 20); //          ^
  up= std::upper_bound (v.begin(), v.end(), 20); //                   ^

  std::cout << "lower_bound at position " << (low- v.begin()) << '\n'; 
  std::cout << "upper_bound at position " << (up - v.begin()) << '\n';

  return 0;
}

// Output
// lower_bound at position 3
// upper_bound at position 6

Code Example 2

std::vector<int> v{ 10, 20, 30, 40, 50 }; 
low1 = std::lower_bound(v.begin(), v.end(), 35); 
cout << (low1 - v.begin());

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