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std::ranges::fold_right

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
       
       
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
       
       
Permutation operations
Fold operations
(C++23)
(C++23)  
fold_right
(C++23)
(C++23)  
Numeric operations
(C++23)            
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
(1)
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)
(2)
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 */;
(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 [firstlast) is not a valid range.

1) The range is [firstlast).
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:
Helper concepts
template< class F, class T, class I, class U >

concept /*indirectly-binary-left-foldable-impl*/ =
    std::movable<T> &&
    std::movable<U> &&
    std::convertible_to<T, U> &&
    std::invocable<F&, U, std::iter_reference_t<I>> &&
    std::assignable_from<U&,

        std::invoke_result_t<F&, U, std::iter_reference_t<I>>>;
(3A) (exposition only*)
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:
Helper concepts
template< class F, class T, class I >

concept /*indirectly-binary-right-foldable*/ =

    /*indirectly-binary-left-foldable*/</*flipped*/<F>, T, I>;
(4A) (exposition only*)
Helper class templates
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 niebloids, that is:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] 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

[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 = 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.

[edit] 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) ranges::in_value_result<I, U>

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

where BR is ranges::borrowed_iterator_t<R>

ranges::fold_left_first_with_iter left first element

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

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

where BR is ranges::borrowed_iterator_t<R>

Feature-test macro Value Std Feature
__cpp_lib_ranges_fold 202207L (C++23) std::ranges fold algorithms
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

[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):
  • 27.6.18 Fold [alg.fold]

[edit] See also

right-folds a range of elements using the last element as an initial value
(niebloid)[edit]
left-folds a range of elements
(niebloid)[edit]
left-folds a range of elements using the first element as an initial value
(niebloid)[edit]
left-folds a range of elements, and returns a pair (iterator, value)
(niebloid)[edit]
left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional)
(niebloid)[edit]
sums up or folds a range of elements
(function template) [edit]
(C++17)
similar to std::accumulate, except out of order
(function template) [edit]