std::ranges::fold_right_last - 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-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. 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.

2. 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 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.

[edit] Possible implementations

struct fold_right_last_fn { template<std::bidirectional_iterator I, std::sentinel_for S, /indirectly-binary-right-foldable/<std::iter_value_t, I> F> requires std::constructible_from<std::iter_value_t, std::iter_reference_t> constexpr auto operator()(I first, S last, F f) const { using U = decltype( ranges::fold_right(first, last, std::iter_value_t(*first), f));   if (first == last) return std::optional(); I tail = ranges::prev(ranges::next(first, std::move(last))); return std::optional(std::in_place, ranges::fold_right(std::move(first), tail, std::iter_value_t(*tail), std::move(f))); }   template<ranges::bidirectional_range R, /*indirectly_binary_right_foldable*/< ranges::range_value_t, ranges::iterator_t> F> requires std::constructible_from<ranges::range_value_t, ranges::range_reference_t> 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;

[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   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

[edit] References

• C++23 standard (ISO/IEC 14882:2024):

• 27.6.18 Fold [alg.fold]

[edit] See also