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upper_bound and lower_bound for non increasing vector in c++

  • Difficulty Level : Easy
  • Last Updated : 20 May, 2020

The lower_bound() and upper_bound() functions, by default works on non-decreasing array. The lower_bound() function finds iterator of first element that does not compare less to given element. The upper_bound() function returns iterator to the first element that is greater.

Given an array converting this to a non-increasing vector, applying the std:: upper_bound and std:: lower_bound function to the vector

For array sorted in a non-increasing array, lower_bound() finds iterator to the first element that does not compare greater than to given element. upper_bound() finds iterator to the first element that is smaller than the given element. We use greater() for this purpose.

// C++ program to demonstrate the working of lower_bound()
// and upper_bound() for array sorted in non-decreasing
// array,
#include <algorithm>
#include <iostream>
#include <vector>
int main()
    int unsorted[10] = { 3, 3, 2, 1, 5, 5, 4, 3, 7, 8 };
    std::vector<int> v(unsorted, unsorted + 10);
    // sorting vector in non increasing order. Vector
    // becomes {8, 7, 5, 5, 4, 3, 3, 3, 2, 1}
    std::sort(v.begin(), v.end(), std::greater<int>());
    std::vector<int>::iterator low, up;
    low = std::lower_bound(v.begin(), v.end(), 3, std::greater<int>());         
    up = std::upper_bound(v.begin(), v.end(), 5, std::greater<int>());          
    std::cout << "lower_bound at position " << (low - v.begin()) << '\n';
    std::cout << "upper_bound at position " << (up - v.begin()) << '\n';
    return 0;
lower_bound at position 5
upper_bound at position 4
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