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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))
Matrix.code
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2 Answers2

4

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);
D Drmmr
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    Thanks D Drmmr ,it worked .. But complexity is linear... ? This may be give me TLE ... Again Do you know how to merge to multi set ? – Matrix.code Jan 18 '15 at 12:56
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
Alper
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    Can we do it in constant time ? `distance` take linear time ! – Matrix.code Jan 18 '15 at 14:28
  • @Matrix.code, constant time only for random-access iterators, it is not possible with `std::multiset`. I don't know the details but depending to your requirements, it could be possible to implement your own data structure. – Alper Jan 18 '15 at 15:02