CODEWITHSUNDEEP
c++lambdastd
Luke nullptr Krupa
Luke nullptr Krupa

Reputation: 343

C++ - Add map to map with lambda

I just wonder if there is any sexy way in C++ (using standard library functions) to do something like that:

I've got two maps (std::map), both same type. I'd like to add both maps together, but also decide which elements add and which not with some lambda predicate.

Any solution? Thanks.

Upvotes: 3

Views: 250

Answers (1)

TartanLlama
TartanLlama

Reputation: 65600

You can use std::copy_if in combination with std::inserter. This example only adds elements from b into a if the value is even:

std::copy_if(b.begin(), b.end(), std::inserter(a, a.end()),
             [](auto&& e){return e.second%2 == 0;});

You could factor this out into a helper function if you find yourself needing this a few times:

template <typename T, typename F>
void merge_maps (T& a, const T& b, const F& filter) { 
    std::copy_if(b.begin(), b.end(), std::inserter(a, a.end()), filter);
}

merge_maps(a, b, [](auto&& e){return e.second%2 == 0;});

Upvotes: 6

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