std::ranges::fold_right - cppreference.com (original) (raw)

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:

1. Equivalent to:

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.

Contents

• 1 Parameters
• 2 Return value
• 3 Possible implementations
• 4 Complexity
• 5 Notes
• 6 Example
• 7 References
• 8 See also

[edit] Parameters

[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 S, class T = std::iter_value_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, 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, / indirectly-binary-right-foldable */<T, ranges::iterator_t> 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.

[edit] Notes

The following table compares all constrained folding algorithms:

[edit] Example

#include #include #include #include #include #include #include #include   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; std::vector 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):

• 27.6.18 Fold [alg.fold]

[edit] See also