std::ranges::fold_right
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| (1) | ||
template< std::bidirectional_iterator I, std::sentinel_for<I> S, class T, /* indirectly-binary-right-foldable */<T, I> F > constexpr auto fold_right( I first, S last, T init, F f ); |
(since C++23) (until C++26) |
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template< std::bidirectional_iterator I, std::sentinel_for<I> S, class T = std::iter_value_t<I>, /* indirectly-binary-right-foldable */<T, I> F > constexpr auto fold_right( I first, S last, T init, F f ); |
(since C++26) | |
| (2) | ||
template< ranges::bidirectional_range R, class T, /* indirectly-binary-right-foldable */ <T, ranges::iterator_t<R>> F > constexpr auto fold_right( R&& r, T init, F f ); |
(since C++23) (until C++26) |
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template< ranges::bidirectional_range R, class T = ranges::range_value_t<R>, /* indirectly-binary-right-foldable */ <T, ranges::iterator_t<R>> F > constexpr auto fold_right( R&& r, T init, F f ); |
(since C++26) | |
| 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, init))), where x1, x2, ..., xn are elements of the range.
Informally, ranges::fold_right behaves like ranges::fold_left(views::reverse(r), init, /*flipped*/(f)).
The behavior is undefined if [first, last) is not a valid range.
1) The range is
[first, last).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 |
| init | - | the initial value of the fold |
| f | - | the binary function object |
Return value
An object of type U that contains the result of right-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, std::iter_reference_t<I>, T>>;.
If the range is empty, U(std::move(init)) is returned.
Possible implementations
struct fold_right_fn
{
template<std::bidirectional_iterator I, std::sentinel_for<I> S,
class T = std::iter_value_t<I>,
/* indirectly-binary-right-foldable */<T, I> F>
constexpr auto operator()(I first, S last, T init, F f) const
{
using U = std::decay_t<std::invoke_result_t<F&, std::iter_reference_t<I>, T>>;
if (first == last)
return U(std::move(init));
I tail = ranges::next(first, last);
U accum = std::invoke(f, *--tail, std::move(init));
while (first != tail)
accum = std::invoke(f, *--tail, std::move(accum));
return accum;
}
template<ranges::bidirectional_range R, class T = ranges::range_value_t<R>,
/* indirectly-binary-right-foldable */<T, ranges::iterator_t<R>> F>
constexpr auto operator()(R&& r, T init, F f) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f));
}
};
inline constexpr fold_right_fn fold_right;
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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
|
__cpp_lib_algorithm_default_value_type |
202403L |
(C++26) | List-initialization for algorithms (1,2) |
Example
Run this code
#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <ranges>
#include <string>
#include <utility>
#include <vector>
using namespace std::literals;
namespace ranges = std::ranges;
int main()
{
auto v = {1, 2, 3, 4, 5, 6, 7, 8};
std::vector<std::string> vs{"A", "B", "C", "D"};
auto r1 = ranges::fold_right(v.begin(), v.end(), 6, std::plus<>()); // (1)
std::cout << "r1: " << r1 << '\n';
auto r2 = ranges::fold_right(vs, "!"s, std::plus<>()); // (2)
std::cout << "r2: " << r2 << '\n';
// Use a program defined function object (lambda-expression):
std::string r3 = ranges::fold_right
(
v, "A", [](int x, std::string s) { return s + ':' + std::to_string(x); }
);
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', 2.f}, {'B', 3.f}, {'C', 3.5f}};
float r4 = ranges::fold_right
(
data | ranges::views::values, 2.0f, std::multiplies<>()
);
std::cout << "r4: " << r4 << '\n';
using CD = std::complex<double>;
std::vector<CD> nums{{1, 1}, {2, 0}, {3, 0}};
#ifdef __cpp_lib_algorithm_default_value_type
auto r5 = ranges::fold_right(nums, {7, 0}, std::multiplies{});
#else
auto r5 = ranges::fold_right(nums, CD{7, 0}, std::multiplies{});
#endif
std::cout << "r5: " << r5 << '\n';
}
Output:
r1: 42
r2: ABCD!
r3: A:8:7:6:5:4:3:2:1
r4: 42
r5: (42,42)
References
- C++23 standard (ISO/IEC 14882:2024):
- 27.6.18 Fold [alg.fold]
See also
(C++23) |
right-folds a range of elements using the last element as an initial value (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) |