C++ set - lower_bound() Function

The C++ set::lower_bound function returns an iterator pointing to the first element in the set container which is not considered to go before the specified value (it could be either same or goes after the specified value). If all elements of the set are considered to go before the specified value, then the iterator points to set::end.

Syntax

• C++98
• C++11

iterator lower_bound (const value_type& val) const;

iterator lower_bound (const value_type& val);
const_iterator lower_bound (const value_type& val) const;

Parameters

Return Value

An iterator pointing to the first element in the set container which is not considered to go before the specified value, or set::end if all elements of the set is considered to go before the specified value.

Time Complexity

Logarithmic i.e, Θ(log(n))

Example:

In the example below, the set::lower_bound function is used with set::upper_bound to specify the lower and upper bound in the set called MySet.

#include <iostream>
#include <set>
using namespace std;
 
int main (){
  set<int> MySet{10, 20, 30, 40, 50, 60, 70, 80};
  set<int>::iterator it, itlower, itupper;

  itlower = MySet.lower_bound(25);
  itupper = MySet.upper_bound(50);
  MySet.erase(itlower, itupper);

  cout<<"MySet contains: ";
  for(it = MySet.begin(); it != MySet.end(); ++it)
    cout<<" "<<*it;

  return 0;
}

The output of the above code will be:

MySet contains:  10 20 60 70 80