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c++algorithmc++11iteratorprojection

Is there a variant of min_element which takes a projection function?

std::min_element will return the smallest element as defined either by operator<(T,T) or by a custom predicate bool Pred(T,T). Is there a similar function which returns the element for which the projection function f(T)->R takes on the minimal value?

Obviously I can define bool Pred(t1,t2) { return f(t1) < f(t2); } but that's a bit inconvenient when f is a lambda.

Solution

  • Why not just define a predicate generator less_by which, taking a lambda, returns a functor that does the job for you?

    template <typename Proj>
struct less_by_t {
    Proj p;

    template <typename T>
    bool operator ()(T const& a, T const& b) const {
        return p(a) < p(b);
    }
};

template <typename Proj>
less_by_t<Proj> less_by(Proj p) {
    return {p};
}

    auto result = std::min_element(begin, end, less_by([](T const& x){return …;}));