map lower_bound() function in C++ STL

The map::lower_bound(k) is a built-in function in C++ STL which returns an iterator pointing to the key in the container which is equivalent to k passed in the parameter.

Syntax:

map_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 key in the map container which is equivalent to k passed in the parameter. In case k is not present in the map 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 key in the container, then the iterator returned points to number of elements in the map as key and element=0.

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// C++ function for illustration
// map::lower_bound() function
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
  
    // initialize container
    map<int, int> mp;
  
    // insert elements in random order
    mp.insert({ 2, 30 });
    mp.insert({ 1, 10 });
    mp.insert({ 5, 50 });
    mp.insert({ 4, 40 });
    for (auto it = mp.begin(); it != mp.end(); it++) {
        cout << (*it).first << " " << (*it).second << endl;
    }
  
    // when 2 is present
    auto it = mp.lower_bound(2);
    cout << "The lower bound of key 2 is ";
    cout << (*it).first << " " << (*it).second << endl;
  
    // when 3 is not present
    // points to next greater after 3
    it = mp.lower_bound(3);
    cout << "The lower bound of key 3 is ";
    cout << (*it).first << " " << (*it).second;
  
    // when 6 exceeds
    it = mp.lower_bound(6);
    cout << "\nThe lower bound of key 6 is ";
    cout << (*it).first << " " << (*it).second;
    return 0;
}

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