How to sum up all the elements in a multimap in c++ having the same key

Question

Please help!. How to sum up all the elements in a multimap in c++ having the same key. I have the follwing multimap:

multimap <string,int> mymap;

and I want to sum all values for a particular string. Any approach would be appreciated.

pair range = mymap.equal_range(s); int x = accumulate(range.first, range.second, 0); — Dref D, Jan 03 '13 at 14:59
Did you try to use simple `for` loop instead of `accumulate`? — Bartek Banachewicz, Jan 03 '13 at 15:02
The problem is that iterators of a map are pairs and you only want to accumulate the `.second` part of them... — leemes, Jan 03 '13 at 15:03
As suggested on : [link]http://stackoverflow.com/questions/247818/stlmultimap-how-do-i-get-groups-of-data?rq=1 — Dref D, Jan 03 '13 at 15:04
If what you tried didn't work, please write *what didn't exactly work*. — Bartek Banachewicz, Jan 03 '13 at 15:07
I'm getting this error: 54 C:\Users\Drafly `Iter' undeclared (first use this function) — Dref D, Jan 03 '13 at 15:07
Well, you have to replace `Iter` with the right type (in this case, `multimap::iterator`). — Gorpik, Jan 03 '13 at 15:17
I think you can read this refence: http://www.cplusplus.com/reference/map/multimap/equal_range/ — hired777, Jan 03 '13 at 15:23

score 3 · Answer 1 · edited May 23 '17 at 12:10

Something like this may work for you:

#include <string>
#include <map>
#include <numeric>
#include <functional>
#include <iostream>

struct plus2nd {
    template <typename T1, typename T2>
    inline T1 operator()(T1 t1, const T2 &t2) const {
        return t1 + t2.second;
    }
};

template <typename Map, typename Key, typename Value>
Value accumulate_range(const Map & map, const Key & key, Value init)
{
    typedef typename Map::const_iterator const_iterator;
    typedef typename Map::value_type value_type;
    std::pair<const_iterator, const_iterator> range = map.equal_range(key);
    return std::accumulate(range.first, range.second, init, plus2nd());
}

int main()
{
    std::multimap<std::string, int> mymap;
    mymap.insert(std::multimap<std::string, int>::value_type("A", 1));
    mymap.insert(std::multimap<std::string, int>::value_type("B", 3));
    mymap.insert(std::multimap<std::string, int>::value_type("A", 4));
    mymap.insert(std::multimap<std::string, int>::value_type("B", 4));
    std::cout << "Sum of A: " << accumulate_range(mymap, "A", 0) << std::endl;
    std::cout << "Sum of B: " << accumulate_range(mymap, "B", 0) << std::endl;
}

See also:

score 0 · Answer 2 · edited May 23 '17 at 10:24

0

your accumulate function wont work that way because your map provides iterators with first and second and you only want to sum up it->second. You will have to write your own sum-function and pass it to accumlute. There already is a topic that describes this, please see multimap accumulate values for the answer of Matthieu M.

edit: The following should solve your problem.

int map_acc(int lhs, const std::pair<std::string, int> & rhs)
{
  return lhs + rhs.second;
}

std::pair<std::multimap<std::string,int>::const_iterator, std::multimap<std::string,int>::const_iterator> range = mymap.equal_range("somekey");
int result = std::accumulate(range.first, range.second, 0, &map_acc);

edit 2: If you want a more generic approach, go for Vlad Lazarenko answer. But considering your question, this might be overkill.

edited May 23 '17 at 10:24

Community

1
1

answered Jan 03 '13 at 15:13

Betagan

121
3

This should have been a comment to my answer I guess. I will remove it as it is obviously wrong. Thank you - I had no time to try that out. – Ivaylo Strandjev Jan 03 '13 at 15:15
but it say buf_map has not been declared!! – Dref D Jan 03 '13 at 15:19
the range from it->first to it->second is correct. all algorithm using stl range iterator pairs follow the convention [first, last) – Jan 03 '13 at 15:20
you have to replace buf_map with the name of your map variable. The answer I linked uses the names relevant to that question :) – Betagan Jan 03 '13 at 15:21
@arrows `accumulate(range.first, range.second, 0);` will try to sum up (*it) while the TO wants to sum up (*it)->second. accumulate just does not work on multimaps out-of-the-box as it can not know what you want to sum up. – Betagan Jan 03 '13 at 15:24

How to sum up all the elements in a multimap in c++ having the same key

2 Answers2