std::ranges::fold_right

Since C++26

Header: <algorithm>

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.

# Declarations

Call signature

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)

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)

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)

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

template< class F, class T, class I >
concept /* indirectly-binary-left-foldable */ = /* see description */;

(exposition only*)

template< class F, class T, class I >
concept /* indirectly-binary-right-foldable */ = /* see description */;

(exposition only*)

# Parameters

# 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, T»;.

# Notes

The following table compares all constrained folding algorithms:

(1) ranges::in_value_result<I, U>

(2) ranges::in_value_result<BR, U>,

where BR is ranges::borrowed_iterator_t

(1) ranges::in_value_result<I, std::optional>

(2) ranges::in_value_result<BR, std::optional>

where BR is ranges::borrowed_iterator_t

# 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';
}

# See also