Multiset Index Finding - c++

Question

I have a multi set of int . C++
multiset<int>t;
I need to find the position of the first element which is greater than of equal to val. I used lower_bound for this
multiset<int>::iterator it= lower_bound(t[n].begin(), t[n].end(), val);
but can not find the the relative position from the beginning of the multi set .
As The Cplusplus.com suggests using.. for vector.
// lower_bound/upper_bound example
#include <iostream> // std::cout
#include <algorithm> // std::lower_bound, std::upper_bound, std::sort
#include <vector> // std::vector
int main () {
int myints[] = {10,20,30,30,20,10,10,20};
std::vector<int> v(myints,myints+8); // 10 20 30 30 20 10 10 20
std::sort (v.begin(), v.end()); // 10 10 10 20 20 20 30 30
std::vector<int>::iterator low,up;
low=std::lower_bound (v.begin(), v.end(), 20); // ^
up= std::upper_bound (v.begin(), v.end(), 20); // ^
std::cout << "lower_bound at position " << (low- v.begin()) << '\n';
std::cout << "upper_bound at position " << (up - v.begin()) << '\n';
return 0;
}
Can I do it in multi set .. ?
Another question is : Can I merge to multi set like vectors like shown bellow , v1,v2,v are vectors ?
merge(v1.begin(),v1.end(),v2.begin(),v1.end(),back_inserter(v))

Answer 1

The generic way to get the distance between two iterators is to call std::distance.
auto it = std::lower_bound(t[n].begin(), t[n].end(), val);
const auto pos = std::distance(t[n].begin(), it);

Answer 2

For std::multiset, member types iterator and const_iterator are bidirectional iterator types. Bidirectional iterator does not support arithmetic operators + and - (for details check cppreference).
std::distance can be used to calculate the number of elements between two iterators.
std::distance uses operator- to calculate the number of elements if parameter is a random-access iterator. Otherwise, it uses the increase operator (operator++) repeatedly.
Here is a slightly changed code snippet from cppreference.
#include <iostream>
#include <set>
int main ()
{
std::multiset<int> mymultiset;
std::multiset<int>::iterator itlow, itup;
for (int i = 1; i < 8; i++) mymultiset.insert(i * 10); // 10 20 30 40 50 60 70
itlow = mymultiset.lower_bound(30);
itup = mymultiset.upper_bound(40);
std::cout << std::distance(mymultiset.begin(), itlow) << std::endl;
std::cout << std::distance(mymultiset.begin(), itup) << std::endl;
mymultiset.erase(itlow, itup); // 10 20 50 60 70
std::cout << "mymultiset contains: ";
for (std::multiset<int>::iterator it = mymultiset.begin(); it != mymultiset.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
return 0;
}
Output
2
4
mymultiset contains: 10 20 50 60 70
You can merge the std::multiset with std::multiset::insert member function as following;
#include <iostream>
#include <set>
int main ()
{
std::multiset<int> mset1;
std::multiset<int> mset2;
for (int i = 1; i < 8; i++) mset1.insert(i * 10); // 10 20 30 40 50 60 70
for (int i = 1; i < 8; i++) mset2.insert(i * 10); // 10 20 30 40 50 60 70
mset1.insert(mset2.begin(), mset2.end());
std::cout << "mset1 contains: ";
for (std::multiset<int>::iterator it = mset1.begin(); it != mset1.end(); ++it)
std::cout << ' ' << *it;
std::cout << '\n';
return 0;
}
Output
mset1 contains: 10 10 20 20 30 30 40 40 50 50 60 60 70 70