unordered_map pair of values c++

Question

I am trying to use the unordered_map in C++, such that, for the key I have an int, while for the value there is a pair of floats. But, I am not sure how to access the pair of values. I am just trying to make sense of this data structure. I know to access the elements we need an iterator of the same type as this unordered map declaration. I tried using iterator->second.first and iterator->second.second. Is this the correct way to do access elements?

typedef std::pair<float, float> Wkij; tr1::unordered_map<int, Wkij> sWeight; tr1::unordered_map<int, Wkij>:: iterator it; it->second.first // access the first element of the pair it->second.second // access the second element of the pair

Thanks for your help and time.

unordered_map is part of the C++11 standard , you can use std:: instead of tr1:: — Alejandro
– Alejandro, Commented Apr 23, 2015 at 11:14
you could also use std::get<0>(it->second), or std::get<0>(std::get<1>(*it)) (both gives it->second.first, which is perfectly valid) — Piotr Skotnicki
– Piotr Skotnicki, Commented Apr 23, 2015 at 11:31

Nikolay K · Accepted Answer · 2015-04-23 11:52:26Z

Yes, this is correct, but don't use tr1, write std, since unordered_map is already part of STL.

Use iterators like you said

for(auto it = sWeight.begin(); it != sWeight.end(); ++it) { std::cout << it->first << ": " << it->second.first << ", " << it->second.second << std::endl; }

Also in C++11 you can use range-based for loop

for(auto& e : sWeight) { std::cout << e.first << ": " << e.second.first << ", " << e.second.second << std::endl; }

And if you need it you can work with std::pair like this

for(auto it = sWeight.begin(); it != sWeight.end(); ++it) { auto& p = it->second; std::cout << it->first << ": " << p.first << ", " << p.second << std::endl; }

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