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

The first, simple problem is

struct compareByVal {
  bool operator[](const std::pair & a, const std::pair & b)
    return a.second < b.second;
}

should be

struct compareByVal {
  bool operator()(const std::pair & a, const std::pair & b) const {
    return a.second < b.second;
  }
};

The second, serious problem is the signature of the compare is wrong. It should be

struct compareByVal {
  bool operator()(const int leftKey, const int rightKey) const;
}

You can't access the value in the compare function. There is no (simple) way to sort a map by value.