How to make a map sort by value C++

Question

I was trying to make a map sort by value using a custom comparator but I couldn\'t figure out why I kept getting the error of \"no matching call to compareByVal\"

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Answer 1

Well, first, the reason you're getting the error: "no matching call to compareByVal" is because map's comparator works only with the keys. So the comparator should like:

struct compareByVal {
  template 
  bool operator()(const T& a, const T& b) const
    return a < b;
}

Coming on to what you want to achieve, I see two ways of doing so:

1. Copy all the elements of the map to a std::vector and sort that:

std::vector > v(hash.begin(), hash.end());
std::sort(v.begin(), v.end(), [](const auto& a, const auto& b) { return a.second < b.second; });

2. Copy all the elements of the map to another map with keys as values and values as keys. If values of your map are not unique, you can use a std::multimap instead.