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Populate a vector with all multimap values with a given key

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伪装坚强ぢ
伪装坚强ぢ 2021-02-08 13:10

Given a multimap M what\'s a neat way to create a vector of all values in M with a specific key.

e.g given a multimap how c

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  • 野趣味
    野趣味 (楼主)
    2021-02-08 13:35

    Just some addenda to the other answers here…

    std::mem_fn (from #include ) can be used as a shorthand for the transform operator:

    // previously we might've used this longhand
[](pair element){return element.second;}

    And we can use vector::resize and std::distance to allocate space for the vector in one go, rather than repeatedly resizing it with back_inserter.

    #include 
#include 
#include 
#include 
#include 
#include 
#include 

using namespace std;

typedef multimap MapType;

int main()
{
    MapType multimap;
    vector valuesForKey123;

    multimap.emplace(0,   "red");
    multimap.emplace(123, "hello");
    multimap.emplace(123, "world");
    multimap.emplace(0,   "herring");

    MapType::iterator lower{multimap.lower_bound(123)};
    MapType::iterator upper{multimap.upper_bound(123)};
    valuesForKey123.resize(distance(lower, upper));

    transform(
        lower,
        upper,
        valuesForKey123.begin(),
        mem_fn(&MapType::value_type::second));

    copy(
        valuesForKey123.begin(),
        valuesForKey123.end(),
        ostream_iterator(cout, " "));
}
// outputs "hello world "

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