Can't compile on std::min_element's return

Question

#include <map> #include <iostream> #include <algorithm> using namespace std; int main() { std::map<double, double> A; const auto it = std::min_element(A.begin(), A.end(), [](decltype(A)::value_type& l, decltype(A)::value_type& r) -> bool { return l.second < r.second; }); std::cout << *it << std::endl; }

I wish to compute the minimum in the map.

This code failed to compile. I thought the way to use std::min_element's returned iterator is by referencing it. No?

The error message on the std::cout line is "invalid operands to binary expression".

In your case if you constructed the map with the keys as your values and values as your keys, the minimum value would just be *A.begin() since std::map stores its elements in sorted order. — SirGuy
– SirGuy, Commented Sep 19, 2018 at 13:26
Using double as key for std::map is usually not a good idea — Slava
– Slava, Commented Sep 19, 2018 at 13:53

SirGuy · Accepted Answer · 2018-09-19 13:32:12Z

std::map::iterator::value_type (ie. the type of *it) is std::pair<const double,double>, and there is no standard overload of operator<<(std::ostream &, std::pair<const double,double>).

You could either define one, or do something like std::cout << it->first << ' ' << it->second << std::endl;.

SirGuy · Accepted Answer · 2018-09-19 13:45:33Z

The element type of std::map is a std::pair<const key_type, mapped_type>. *it will give you a reference to that. There is no output output operator defined for std::pair so the code fails to compile.

You will either have to add an overload for it like

std::ostream& operator <<(std::ostream& os, const std::pair<const double, double>& e) { return os << "{" << e.first << ", " << e.second << "}\n"; }

or just print what you want like

std::cout << it->first << " " << it->second << "\n";

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