| Permutation operations |
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| Fold operations |
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| Operations on uninitialized storage |
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| Return types |
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(1)
(since C++23) (until C++26)
(2)
(since C++23) (until C++26)
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).
3) Equivalent to:
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:
template< class F, class T, class I >
concept /*indirectly-binary-right-foldable*/ =
/*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;
(4A)
(exposition only*)
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
init
-
the initial value of the fold
f
-
the binary function object
[edit] 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.
[edit] 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;
[edit] Complexity
Exactly ranges::distance (first, last) applications of the function object f.
The following table compares all constrained folding algorithms:
[edit] Example
#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)
[edit] References
- C++23 standard (ISO/IEC 14882:2024):
-
[edit] See also
right-folds a range of elements using the last element as an initial value
(algorithm function object)[edit]
left-folds a range of elements
(algorithm function object)[edit]
left-folds a range of elements using the first element as an initial value
(algorithm function object)[edit]
left-folds a range of elements, and returns a
pair (iterator, value)
(algorithm function object)[edit]
left-folds a range of elements using the first element as an initial value, and returns a
pair (iterator,
optional)
(algorithm function object)[edit]
sums up or folds a range of elements
(function template) [edit]