find element with max value from std::map

Question

I'm trying to get the element with max value from std::map,

int main() {
    map<int, int> m;
    m[1] = 100;
    m[2] = -1;

    auto x = std::max_element(m.begin(), m.end(), m.value_comp());

    cout << x->first << " : " << x->second << endl;
}

why it prints the second element 2 : -1 ?

Hmmm... `std::max_element()` seems to be comparing the keys of the map — Levi, Jun 03 '15 at 05:48

score 22 · Accepted Answer · edited May 23 '17 at 12:34

22

Taken from here:

auto x = std::max_element(m.begin(), m.end(),
    [](const pair<int, int>& p1, const pair<int, int>& p2) {
        return p1.second < p2.second; });

This, rather than using std::map::value_comp() (which compares the key values) looks at the second member in the pair, which contains the value. This uses a lambda expression, so you will have to compile with C++11 support

edited May 23 '17 at 12:34

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answered Jun 03 '15 at 05:55

Levi

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I agree with this answer. It could be written a bit more generic, though. If you know `m` is of type `M`, you might write `[](const M::value_type& p1, const M::value_type& p2)`. It is robust to type changes in `M` but has an unnecessary indirection here – IceFire Jun 01 '17 at 06:58

score 4 · Answer 2 · answered Jun 03 '15 at 05:49

http://www.cplusplus.com/reference/map/map/value_comp/

Returns a comparison object that can be used to compare two elements to get whether
the key of the first one goes before the second.

and 2 > 1. value_comp compares the key values, not the value values. Because that's how C++ rolls.

find element with max value from std::map

2 Answers2