std::ranges::fold_right
| Defined in header <algorithm>
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| Call signature |
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| template< std::bidirectional_iterator I, std::sentinel_for<I> S, class T, __indirectly_binary_right_foldable<T, I> F > |
(1) | (since C++23) |
| template< ranges::bidirectional_range R, class T, __indirectly_binary_right_foldable<T, ranges::iterator_t<R>> 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, init))), where x1, x2, ..., xn are elements of the range.
Informally, ranges::fold_right behaves like std::fold_left(ranges::reverse(r), init, __flipped(f)).
The behavior is undefined if [first, last) is not a valid range.
[first, last).| Helper concepts |
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| template< class F, class T, class I, class U > concept /*indirectly-binary-left-foldable-impl*/ = |
(3A) | (exposition only*) |
| template< class F, class T, class I > concept /*indirectly-binary-left-foldable*/ = |
(3B) | (exposition only*) |
| Helper concepts |
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| template< class F, class T, class I > concept /*indirectly-binary-right-foldable*/ = |
(4A) | (exposition only*) |
| Helper class templates |
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| template< class F > class /*flipped*/ |
(4B) | (exposition only*) |
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.
Parameters
| first, last | - | 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, __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 = invoke(f, *--tail, std::move(accum)); return accum; } template<ranges::bidirectional_range R, class T, __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; |
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) 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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Example
#include <algorithm> #include <functional> #include <iostream> #include <ranges> #include <string> #include <utility> #include <vector> using namespace std::literals; 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(v.begin(), v.end(), 6, std::plus<>()); // (1) std::cout << "r1: " << r1 << '\n'; auto r2 = std::ranges::fold_right(vs, "!"s, std::plus<>()); // (2) std::cout << "r2: " << r2 << '\n'; // Use a program defined function object (lambda-expression): std::string r3 = std::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 = std::ranges::fold_right ( data | std::ranges::views::values, 2.0f, std::multiplies<>() ); std::cout << "r4: " << r4 << '\n'; }
Output:
r1: 42 r2: ABCD! r3: A:8:7:6:5:4:3:2:1 r4: 42
References
- C++23 standard (ISO/IEC 14882:2023):
- 27.6.18 Fold [alg.fold]
See also
| (C++23) |
right-folds a range of elements using the last element as an initial value (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) |