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set::lower_bound() function in C++ STL

C++ STL set::lower_bound() function: Here, we are going to learn about the lower_bound() function of set in C++ STL (Standard Template Library).
Submitted by Radib Kar, on February 16, 2019

C++ STL set::lower_bound() function

set::lower_bound() function is a predefined function, it is used to get the lower bound of any element in a set.

It finds lower bound of any desired element from the set. Lower bound of any_element means the first number in the set that's not considered to go before any_element. So, if any_element is itself present, then it's any_element else immediate next of any_element.

The function finds lower bound of any desired element from the set. Lower bound of x means the first number in the set that's not considered to go before x. So, if x is itself present, then it's x else immediate next of x.

Syntax

set<T> st; //declaration
st<T> st::iterator it; //iterator declaration

it=st.upper_bound(T key);

Parameter(s)

It accepts a "key" of T type.

Return value

If lower_bound of the key exists in the set, Iterator pointer to the lower bound, Else, st.end()

Sample Input and Output

For a set of integer,
set<int> st;
st.insert(6);
st.insert(4);
st.insert(10);
set content: //sorted always(ordered)
    4
    6
    10

it=st.lower_bound(6)
Print *it; //6
it=st.lower_bound(8)
Print *it; //10

Header file

Header file to be included:

#include <iostream>
#include <set>
OR
#include <bits/stdc++.h>

Example

#include <bits/stdc++.h>
using namespace std;

void printSet(set<int> st) {
  set<int>::iterator it;
  cout << "Set contents are:\n";
  if (st.empty()) {
    cout << "empty set\n";
    return;
  }
  for (it = st.begin(); it != st.end(); it++) cout << *it << " ";
  cout << endl;
}

int main() {
  cout << "Example of lower_bound function\n";
  set<int> st;
  set<int>::iterator it;
  cout << "inserting 4\n";
  st.emplace(4);
  cout << "inserting 6\n";
  st.emplace(6);
  cout << "inserting 10\n";
  st.emplace(10);

  printSet(st);  // printing current set

  cout << "lower bound of 6 is " << *(st.lower_bound(6));

  return 0;
}

Output

Example of lower_bound function
inserting 4
inserting 6
inserting 10
Set contents are:
4 6 10
lower bound of 6 is 6