std::ranges::fold_right_last
| Defined in header <algorithm>
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| Call signature |
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| template< std::bidirectional_iterator I, std::sentinel_for<I> S, |
(1) | (since C++23) |
| template< ranges::bidirectional_range R, |
(2) | (since C++23) |
| Helper concepts |
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| template< class F, class T, class I > concept __indirectly_binary_left_foldable = // exposition only |
(3) | (since C++23) |
| template< class F, class T, class I > concept __indirectly_binary_right_foldable = // exposition only |
(4) | (since C++23) |
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 std::fold_left(ranges::reverse(r), *--last, __flipped(f)) (assuming the range is not empty).
The behavior is undefined if [first, last) is not a valid range.
[first, last). Given U as decltype(ranges::fold_right(first, last, std::iter_value_t<I>(*first), f)), equivalent to:
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)));
r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.template< class F, class T, class I, class U > concept __indirectly_binary_left_foldable_impl = // exposition only std::movable<T> && std::movable<U> && std::convertible_to<T, U> && std::invocable<F&, U, std::iter_reference_t<I>> && std::assignable_from<U&, std::invoke_result_t<F&, U, std::iter_reference_t<I>>>; template< class F, class T, class I > concept __indirectly_binary_left_foldable = // exposition only 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>>>>;
template< class F, class T, class I > concept __indirectly_binary_right_foldable = // exposition only __indirectly_binary_left_foldable<__flipped<F>, T, I>;
The helper class template flipped is equivalent to:
template< class F > class __flipped // exposition only { 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&& ); };
The function-like entities described on this page are niebloids, that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
In practice, they may be implemented as function objects, or with special compiler extensions.
Contents |
[edit] Parameters
| first, last | - | 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:
| Fold function template | Starts from | Initial value | Return type |
|---|---|---|---|
| ranges::fold_left | left | init | U |
| ranges::fold_left_first | left | first element | std::optional<U> |
| ranges::fold_right | right | init | U |
| ranges::fold_right_last | right | last element | std::optional<U> |
| ranges::fold_left_with_iter | left | init |
(1) std::in_value_result<I, U> (2) std::in_value_result<BR, U>, where BR is ranges::borrowed_iterator_t<R> |
| ranges::fold_left_first_with_iter | left | first element |
(1) std::in_value_result<I, std::optional<U>> (2) std::in_value_result<BR, std::optional<U>> where BR is ranges::borrowed_iterator_t<R> |
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__cpp_lib_ranges_fold |
202207L | (C++23) | std::ranges fold algorithms
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[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
- C++23 standard (ISO/IEC 14882:2023):
- 27.6.18 Fold [alg.fold]
[edit] See also
| (C++23) |
right-folds a range of elements (niebloid) |
| (C++23) |
left-folds a range of elements (niebloid) |
| (C++23) |
left-folds a range of elements using the first element as an initial value (niebloid) |
| (C++23) |
left-folds a range of elements, and returns a pair (iterator, value) (niebloid) |
| left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (niebloid) | |
| sums up or folds a range of elements (function template) | |
| (C++17) |
similar to std::accumulate, except out of order (function template) |
