set lower_bound() function in C++ STL

The set::lower_bound() is a built-in function in C++ STL which returns an iterator pointing to the element in the container which is equivalent to k passed in the parameter. In case k is not present in the set container, the function returns an iterator pointing to the immediate next element which is just greater than k. If the key passed in the parameter exceeds the maximum value in the container, then the iterator returned points to the last element in the set container.

Syntax:

set_name.lower_bound(key)

Parameters: This function accepts a single mandatory parameter key which specifies the element whose lower_bound is to be returned.

Return Value: The function returns an iterator pointing to the element in the container which is equivalent to k passed in the parameter. In case k is not present in the set container, the function returns an iterator pointing to the immediate next element which is just greater than k. If the key passed in the parameter exceeds the maximum value in the container, then the iterator returned points to the last element in the set container.

Below program illustrate the above function:

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// CPP program to demonstrate the
// set::lower_bound() function
#include <bits/stdc++.h>
using namespace std;
int main()
{
  
    set<int> s;
  
    // Function to insert elements
    // in the set container
    s.insert(1);
    s.insert(4);
    s.insert(2);
    s.insert(5);
    s.insert(6);
  
    cout << "The set elements are: ";
    for (auto it = s.begin(); it != s.end(); it++)
        cout << *it << " ";
  
    // when 2 is present
    auto it = s.lower_bound(2);
    cout << "\nThe lower bound of key 2 is ";
    cout << (*it) << endl;
  
    // when 3 is not present
    // points to next greater after 3
    it = s.lower_bound(3);
    cout << "The lower bound of key 3 is ";
    cout << (*it) << endl;
  
    // when 8 exceeds the max element in set
    it = s.lower_bound(8);
    cout << "The lower bound of key 8 is ";
    cout << (*it) << endl;
  
    return 0;
}

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Output:

The set elements are: 1 2 4 5 6 
The lower bound of key 2 is 2
The lower bound of key 3 is 4
The lower bound of key 8 is 5


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