std::ranges::fold_left_first

Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(x₁, x₂), x₃), ...), x_n), where x₁, x₂, ..., x_n are elements of the range.

Informally, ranges::fold_left_first behaves like std::accumulate 's overload that accepts a binary predicate, except that the *first is used internally as an initial element.

The behavior is undefined if [first, last) is not a valid range.

1) The range is [first, last). Equivalent to return ranges::fold_left_first_with_iter (std::move(first), last, f).value.

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 &&
std::convertible_to <T, U> &&
std::invocable <F&, U, std::iter_reference_t > &&
std::assignable_from <U&,

std::invoke_result_t <F&, U, std::iter_reference_t >>;

(3A) (exposition only*)

template< class F, class T, class I >

concept /*indirectly-binary-left-foldable*/ =
std::copy_constructible <F> &&
std::indirectly_readable &&
std::invocable <F&, T, std::iter_reference_t > &&
std::convertible_to <std::invoke_result_t <F&, T, std::iter_reference_t >,
std::decay_t <std::invoke_result_t <F&, T, std::iter_reference_t >>> &&
/*indirectly-binary-left-foldable-impl*/<F, T, I,

std::decay_t <std::invoke_result_t <F&, T, std::iter_reference_t >>>;

(3B) (exposition only*)

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.

struct fold_left_first_fn
{
 template<std::input_iterator I, std::sentinel_for <I> S,
 /*indirectly-binary-left-foldable*/<std::iter_value_t <I>, I> F>
 requires
 std::constructible_from <std::iter_value_t <I>, std::iter_reference_t <I>>
 constexpr auto operator()(I first, S last, F f) const
 {
 using U = decltype(
 ranges::fold_left (std::move(first), last, std::iter_value_t <I>(*first), f)
 );
 if (first == last)
 return std::optional <U>();
 std::optional <U> init(std::in_place, *first);
 for (++first; first != last; ++first)
 *init = std::invoke (f, std::move(*init), *first);
 return std::move(init);
 }
 
 template<ranges::input_range R,
 /*indirectly-binary-left-foldable*/<
 ranges::range_value_t <R>, ranges::iterator_t <R>> F>
 requires
 std::constructible_from <ranges::range_value_t <R>, ranges::range_reference_t <R>>
 constexpr auto operator()(R&& r, F f) const
 {
 return (*this)(ranges::begin (r), ranges::end (r), std::ref (f));
 }
};
 
inline constexpr fold_left_first_fn fold_left_first;

[edit] Complexity

Exactly ranges::distance (first, last) - 1 (assuming the range is not empty) 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

[edit] Example

Run this code

#include <algorithm>
#include <array>
#include <functional>
#include <ranges>
#include <utility>
 
int main()
{
 constexpr std::array v{1, 2, 3, 4, 5, 6, 7, 8};
 static_assert
 (
 *std::ranges::fold_left_first(v.begin(), v.end(), std::plus {}) == 36
 && *std::ranges::fold_left_first(v, std::multiplies {}) == 40320
 );
 
 constexpr std::array w
 {
 1, 2, 3, 4, 13,
 1, 2, 3, 4, 13,
 1, 2, 3, 4, 13,
 1, 2, 3, 4,
 };
 static_assert
 (
 "Find the only value that (by precondition) occurs odd number of times:"
 && *std::ranges::fold_left_first(w, [](int p, int q){ return p ^ q; }) == 13
 );
 
 constexpr auto pairs = std::to_array <std::pair <char, float>>
 ({
 {'A', 3.0f},
 {'B', 3.5f},
 {'C', 4.0f}
 });
 static_assert
 (
 "Get the product of all pair::second in pairs:"
 && *std::ranges::fold_left_first
 (
 pairs | std::ranges::views::values, std::multiplies {}
 ) == 42
 );
}