set::lower_bound() function in C++ STL
Last Updated :
05 Jun, 2022
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 element beyond last element in the set container.
Time Complexity of set::lower_bound() is O(logn), where n is the size of the set.
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 is equivalent to s.end() (A special iterator points beyond the last element).
Below program illustrate the above function:
CPP
#include <bits/stdc++.h>
using namespace std;
int main()
{
set< int > s;
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 << " " ;
auto it = s.lower_bound(2);
if (it != s.end()) {
cout << "\nThe lower bound of key 2 is " ;
cout << (*it) << endl;
}
else
cout << "The element entered is larger than the "
"greatest element in the set"
<< endl;
it = s.lower_bound(3);
if (it != s.end()) {
cout << "The lower bound of key 3 is " ;
cout << (*it) << endl;
}
else
cout << "The element entered is larger than the "
"greatest element in the set"
<< endl;
it = s.lower_bound(8);
if (it != s.end()) {
cout << "The lower bound of key 8 is " ;
cout << (*it) << endl;
}
else
cout << "The element is larger than the greatest "
"element in the set"
<< endl;
return 0;
}
|
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 element is larger than the greatest element in the set
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