std::ranges::fold_right_last
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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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 fold_right_last( I first, S last, F f ); |
(1) | (since C++23) |
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 fold_right_last( R&& r, F f ); |
(2) | (since C++23) |
| Helper concepts |
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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.
1) The range is
[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)));
2) Same as (1), except that uses
r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.3) Equivalent to:
| Helper concepts |
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template< class F, class T, class I, class U > concept /*indirectly-binary-left-foldable-impl*/ = 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>>>; |
(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 |
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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 |
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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:
- 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.
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 |
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.
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;
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Complexity
Exactly ranges::distance(first, last) applications of the function object f.
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) (2) where |
ranges::fold_left_first_with_iter |
left | first element |
(1) (2) where |
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ranges_fold |
202207L |
(C++23) | std::ranges fold algorithms
|
Example
Run this code
#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
References
- C++23 standard (ISO/IEC 14882:2024):
- 27.6.18 Fold [alg.fold]
See also
(C++23) |
right-folds a range of elements (algorithm function object) |
(C++23) |
left-folds a range of elements (algorithm function object) |
(C++23) |
left-folds a range of elements using the first element as an initial value (algorithm function object) |
(C++23) |
left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object) |
| left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (algorithm function object) | |
| sums up or folds a range of elements (function template) | |
(C++17) |
similar to std::accumulate, except out of order (function template) |