the_Saint the_Saint - 9 months ago 49
C++ Question

stl::multimap - how do i get groups of data?

Multimap essentially has groups of data sorted by the key. I want a method by which I could access these individual groups and get their aggregate values.
For example, in a

std::multimap< string, int >
I store

{"Group1", 1},
{"Group1", 2},
{"Group1", 3},

{"Group2", 10},
{"Group2", 11},
{"Group2", 12}

Having stored these values, I should be able to iterate this multimap and get the aggregate values of each "group". Problem is there aren't any functions defined in STL to access MultiMaps in such a way. I could use
to manually iterate the multimap and total the group's contents, but I am hoping there could be better ways already defined in STL ? Can anyone propose a solution as to how I could get the aggregate values for a group in the above example.

Answer Source
pair<Iter, Iter> range = my_multimap.equal_range("Group1");
int total = accumulate(range.first, range.second, 0);

Is one way.


If you don't know the group you are looking for, and are just going through each group, getting the next group's range can be done like so:

template <typename Pair>
struct Less : public std::binary_function<Pair, Pair, bool>
    bool operator()(const Pair &x, const Pair &y) const
        return x.first < y.first;

Iter first = mmap.begin();
Iter last = adjacent_find(first, mmap.end(), Less<MultimapType::value_type>());