Counting number of same values in map

Question

is there any command that counts the number of same values in a map?

like:

map<int, string> m;
m[1] = "A";
m[22] = "A";
m[53] = "C";
m[12] = "A";
m[6] = "A";

int count = m.count("A");// 4

or should i just write it myself, since its not too hard?

Answer 1

You can use the count_if algorithm with a custom predicate function object:

template <typename Pair>
struct second_equal_to
    : std::unary_function<const Pair&, bool>
{
    second_equal_to(const typename Pair::second_type& value)
        : value_(value) { }

    bool operator()(const Pair& p) const
    {
        return p.second == *value_;
    }

private:
    typename Pair::second_type value_;
};

Usage:

typedef std::map<int, std::string> Map;
typedef Map::value_type MapEntry;
std::count_if(m.begin(), m.end(), second_equal_to<MapEntry>("A"));

Or, for a more generic solution, you can write an apply_to_second predicate transformer:

template <typename Pair, typename Predicate>
struct apply_to_second_f
    : std::unary_function<const Pair&, bool>
{
    apply_to_second_f(const Predicate& p)
        : predicate_(p) { }

    bool operator()(const Pair& p) const
    {
        return predicate_(p.second);
    }

    Predicate predicate_;
};

template <typename Pair, typename Predicate>
apply_to_second_f<Pair, Predicate> apply_to_second(const Predicate& p)
{
    return apply_to_second_f<Pair, Predicate>(p);
}

Usage:

std::count_if(m.begin(), m.end(), 
    apply_to_second<MapEntry>(std::bind2nd(std::equal_to<std::string>(), "A")));

If you have a compiler that supports lambda expressions, you don't need any custom predicate functor at all; you can use a much simpler lambda:

std::count_if(m.begin(), m.end(), [](const MapEntry& e) { 
    return e.second == "A";
});