A RetroSearch Logo

Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Search Query:

Showing content from http://en.cppreference.com/w/cpp/algorithm/../symbol_index/../algorithm/ranges/fold_right_last.html below:

std::ranges::fold_right_last - cppreference.com

Call signature

(1) (since C++23) (2) (since C++23)

Helper concepts

template< class F, class T, class I >
concept /*indirectly-binary-left-foldable*/ = /* see description */;

(3) (exposition only*)

template< class F, class T, class I >
concept /*indirectly-binary-right-foldable*/ = /* see description */;

(4) (exposition only*)

Right-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(x1, f(x2, ...f(xn-1, xn))), where x1, x2, ..., xn are elements of the range.

Informally, ranges::fold_right_last behaves like ranges::fold_left(views::reverse(r), *--last, /*flipped*/(f)) (assuming the range is not empty).

The behavior is undefined if [first, last) is not a valid range.

3)

Equivalent to:

Helper concepts

(3A) (exposition only*) template< class F, class T, class I >

concept /*indirectly-binary-left-foldable*/ =
    std::copy_constructible<F> &&
    std::indirectly_readable<I> &&
    std::invocable<F&, T, std::iter_reference_t<I>> &&
    std::convertible_to<std::invoke_result_t<F&, T, std::iter_reference_t<I>>,
        std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>> &&
    /*indirectly-binary-left-foldable-impl*/<F, T, I,

        std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>>; (3B) (exposition only*) 4)

Equivalent to:

Helper concepts

template< class F, class T, class I >

concept /*indirectly-binary-right-foldable*/ =

    /*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>; (4A) (exposition only*)

Helper class templates

template< class F >

class /*flipped*/
{
    F f;    // exposition only
public:
    template< class T, class U >
        requires std::invocable<F&, U, T>
    std::invoke_result_t<F&, U, T> operator()( T&&, U&& );

}; (4B) (exposition only*)

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

[edit] Parameters first, last - the iterator-sentinel pair defining the range of elements to fold r - the range of elements to fold f - the binary function object [edit] Return value

An object of type std::optional<U> that contains the result of right-fold of the given range over f.

If the range is empty, std::optional<U>() is returned.

[edit] Possible implementations 
struct fold_right_last_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             /*indirectly-binary-right-foldable*/<std::iter_value_t<I>, I> F>
    requires
        std::constructible_from<std::iter_value_t<I>, std::iter_reference_t<I>>
    constexpr auto operator()(I first, S last, F f) const
    {
        using U = decltype(
            ranges::fold_right(first, last, std::iter_value_t<I>(*first), f));
 
        if (first == last)
            return std::optional<U>();
        I tail = ranges::prev(ranges::next(first, std::move(last)));
        return std::optional<U>(std::in_place,
            ranges::fold_right(std::move(first), tail, std::iter_value_t<I>(*tail),
                               std::move(f)));
    }
 
    template<ranges::bidirectional_range R,
             /*indirectly_binary_right_foldable*/<
                 ranges::range_value_t<R>, ranges::iterator_t<R>> F>
    requires
        std::constructible_from<ranges::range_value_t<R>, ranges::range_reference_t<R>>
    constexpr auto operator()(R&& r, F f) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::ref(f));
    }
};
 
inline constexpr fold_right_last_fn fold_right_last;
[edit] Complexity

Exactly ranges::distance(first, last) applications of the function object f.

[edit] Notes

The following table compares all constrained folding algorithms:

[edit] Example 
#include <algorithm>
#include <functional>
#include <iostream>
#include <ranges>
#include <utility>
#include <vector>
 
int main()
{
    auto v = {1, 2, 3, 4, 5, 6, 7, 8};
    std::vector<std::string> vs {"A", "B", "C", "D"};
 
    auto r1 = std::ranges::fold_right_last(v.begin(), v.end(), std::plus<>()); // (1)
    std::cout << "*r1: " << *r1 << '\n';
 
    auto r2 = std::ranges::fold_right_last(vs, std::plus<>()); // (2)
    std::cout << "*r2: " << *r2 << '\n';
 
    // Use a program defined function object (lambda-expression):
    auto r3 = std::ranges::fold_right_last(v, [](int x, int y) { return x + y + 99; });
    std::cout << "*r3: " << *r3 << '\n';
 
    // Get the product of the std::pair::second of all pairs in the vector:
    std::vector<std::pair<char, float>> data {{'A', 3.f}, {'B', 3.5f}, {'C', 4.f}};
    auto r4 = std::ranges::fold_right_last
    (
        data | std::ranges::views::values, std::multiplies<>()
    );
    std::cout << "*r4: " << *r4 << '\n';
}

Output:

*r1: 36
*r2: ABCD
*r3: 729
*r4: 42
[edit] References
[edit] See also

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4