The lower_bound function in C++ for a multiset is employed to retrieve an iterator that points to the key in the multiset container that is equal to the specified val provided as an argument.
If a value is not found in the multiset container, it will return an iterator that points to the next element in the container which is greater than the specified value.
Syntax
iterator lower_bound (const value_type& val) const; //until C++ 11
iterator lower_bound (const value_type& val); //since C++ 11
const_iterator lower_bound (const value_type& val) const; //since C++ 11
Parameter
val : The value that will be sought in the multiset container.
Return value
It yields an iterator that references the value within the multiset structure that matches the provided val parameter. If no such element exists, it returns end.
Complexity
Logarithmic in size.
Iterator validity
No changes.
Data Races
Accessing the container does not result in any modifications to the multiset, whether it is the const version or the non-const version.
Simultaneously retrieving the elements of a container is considered to be a secure operation.
Exception Safety
If an error is raised, the multiset remains unchanged.
Example 1
Let's examine a straightforward example to retrieve the minimum bound of a specified key:
#include <iostream>
#include <set>
using namespace std;
int main(void) {
multiset<char> m = {'a','b','c','a','c'};
auto it = m.lower_bound('c');
cout << "Lower bound(=) of c is: " << *it;
return 0;
}
Output:
Lower bound(=) of c is: c
In the above example, lower bound of c is c.
Example 2
Let's explore a basic illustration of removing the element in a multiset from the lower bound to the upper bound:
#include <iostream>
#include <set>
using namespace std;
int main ()
{
multiset<int> mymultiset;
multiset<int>::iterator itlow,itup;
for (int i=1; i<10; i++) mymultiset.insert(i*10); // 10 20 30 40 50 60 70 80 90
itlow=mymultiset.lower_bound (30); // ^
itup=mymultiset.upper_bound (60); // ^
mymultiset.erase(itlow,itup); // 10 20 70 80 90
std::cout << "mymultiset contains:";
for (multiset<int>::iterator it=mymultiset.begin(); it!=mymultiset.end(); ++it)
cout << ' ' << *it;
cout << '\n';
return 0;
}
Output:
mymultiset contains: 10 20 70 80 90
The erase method removes the elements in a multiset from the lower bound (inclusive) to the upper bound (exclusive) and then displays the remaining elements.
Example 3
Let's see a simple example:
#include <set>
#include <iostream>
using namespace std;
int main( )
{
using namespace std;
multiset <int> s1;
multiset <int> :: const_iterator s1_AcIter, s1_RcIter;
s1.insert( 10 );
s1.insert( 20 );
s1.insert( 30 );
s1_RcIter = s1.lower_bound( 20 );
cout << "The element of multiset s1 with a key of 20 is: "
<< *s1_RcIter << "." << endl;
s1_RcIter = s1.lower_bound( 40 );
// If no match is found for the key, end( ) is returned
if ( s1_RcIter == s1.end( ) )
cout << "The multiset s1 doesn't have an element "
<< "with a key of 40." << endl;
else
cout << "The element of multiset s1 with a key of 40 is: "
<< *s1_RcIter << "." << endl;
// The element at a specific location in the multiset can be found
// by using a dereferenced iterator that addresses the location
s1_AcIter = s1.end( );
s1_AcIter--;
s1_RcIter = s1.lower_bound( *s1_AcIter );
cout << "The element of s1 with a key matching "
<< "that of the last element is: "
<< *s1_RcIter << "." << endl;
return 0;
}
Output:
The element of multiset s1 with a key of 20 is: 20.
The multiset s1 doesn't have an element with a key of 40.
The element of s1 with a key matching that of the last element is: 30.
Example 4
Let's see a simple example:
#include<set>
#include<iostream>
using namespace std;
int main()
{
multiset<int> mp;
// insert elements in random order
mp.insert( 2 );
mp.insert( 1 );
mp.insert( 5 );
mp.insert( 4 );
cout<<"Elements are: \n";
for (auto it = mp.begin(); it != mp.end(); it++) {
cout << (*it)<< endl;
}
// when 2 is present
auto it = mp.lower_bound(2);
cout << "The lower bound of key 2 is ";
cout << (*it)<< 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)<< endl;
// when 6 exceeds
it = mp.lower_bound(6);
cout << "The lower bound of key 6 is ";
cout << (*it);
return 0;
}
Output:
Elements are:
1
2
4
5
The lower bound of key 2 is 2
The lower bound of key 3 is 4
The lower bound of key 6 is 4
When attempting to determine the lower boundary of a value that surpasses the container, or in other words, a value not existing within the multiset container, the function will return an iterator pointing to the end position.
