C++ sorting unordered_map or map by value

Question

I have python code that I am trying to convert to C++ equivalent.

dict1 = {i:(val1,val2,val3)} 
dict_sorted = sorted(dict1.items(), key=lambda kv: kv[1][2])

This will sort dictionary dict1 by val3 and stored in dict_sorted (which is not a dictionary).

In c++, I have std::unordered_map<int, std::vector<double>> dict1 , or map similar to above. Vector will have val1,val2,val3 . How can I sort this unordered map using val3 ?

Answer 1

As I noted in the comments , the Python code is making a sorted list of two- tuple s, sorting based on the third element of the second element in each tuple (the result is not a dict ). For equivalent C++ code, you're replacing dict with std::unordered_map , list with std::vector , and the two- tuple with std::pair ; to precisely match Python's sorted results, you'd want to use std::stable_sort for the sorting.

So something like (where dict1 is your unordered_map ):

using kv_pair = std::pair<int, std::vector<double>>;

std::vector<kv_pair> dict_sorted;
dict_sorted.reserve(dict1.size());
for (const auto& kv : dict1) {
    dict_sorted.emplace_back(kv.first, kv.second);
}
std::stable_sort(std::begin(dict_sorted), std::end(dict_sorted),
                 [](const kv_pair& a, const kv_pair& b) { return a.second[2] < b.second[2]; });