Method ForEach

catboost/libs/helpers/array_subset.h:850–856

        // f is a visitor function that will be repeatedly called with (index, element) arguments
        template <class F>
        void ForEach(F&& f) {
            SubsetIndexing->ForEach(
                [src = this->Src, f = std::move(f)](TSize index, TSize srcIndex) {
                    f(index, (*src)[srcIndex]);
                }
            );
        };

        template <class F>
        void ForEach(F&& f) const {