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std::ranges..._left_first(3)   C++ Standard Libary  std::ranges..._left_first(3)

NAME
     std::ranges::fold_left_first - std::ranges::fold_left_first

Synopsis
	Defined in header <algorithm>
	Call signature
	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>,	     (1)
     (since C++23)
	std::iter_reference_t<I>>
	constexpr auto

	    fold_left_first( I first, S last, F f );
	template< ranges::input_range R,

		  /*indirectly-binary-left-foldable*/<
			ranges::range_value_t<R>, ranges::iterator_t<R>>
	F >
	requires  std::constructible_from<				     (2)
     (since C++23)
		     ranges::range_value_t<R>,
	ranges::range_reference_t<R>>
	constexpr auto

	    fold_left_first( R&& r, F f );
	Helper concepts
	template<      class	  F,	  class      T,      class	I      >
     (exposition
	concept /*indirectly-binary-left-foldable*/ = /*  see  description   (3)
     only*)
	*/;

	Left-folds  the  elements of given range, that is, returns the result of
     evaluation of
	the chain expression:
	f(f(f(f(x[1], x[2]), x[3]), ...), x[n]), where x[1], x[2], ..., x[n] are
     elements of
	the range.

	Informally, ranges::fold_left_first behaves like std::accumulate's over-
     load 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<U> &&
	    std::convertible_to<T, U> &&			     (3A) (expo-
     sition only*)
	    std::invocable<F&, U, std::iter_reference_t<I>> &&
	    std::assignable_from<U&,

		std::invoke_result_t<F&, U,
	std::iter_reference_t<I>>>;
	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,	     (3B) (expo-
     sition only*)
	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>>>>;

	The  function-like  entities  described on this page are 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  inhib-
     ited.

	In  practice,  they may be implemented as function objects, or with spe-
     cial compiler
	extensions.

Parameters
	first, last - the range of elements to fold
	r	    - the range of elements to fold
	f	    - the binary function object

Return value
	An object of type std::optional<U> that contains the result of left-fold
     of the
	given range over f, where U is equivalent to
	decltype(ranges::fold_left(std::move(first),			   last,
     std::iter_value_t<I>(*first),
	f)).

	If the range is empty, std::optional<U>() is returned.

Possible implementations
	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_refer-
     ence_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;

Complexity
	Exactly  ranges::distance(first,  last)  -  1 (assuming the range is not
     empty)
	applications of the function object f.

Notes
	The following table compares all constrained folding algorithms:

	     Fold function template	  Starts Initial	     Return type
					   from   value
	ranges::fold_left		  left	 init	 U
	ranges::fold_left_first 	  left	 first	 std::optional<U>
						 element
	ranges::fold_right		  right  init	 U
	ranges::fold_right_last 	  right  last	 std::optional<U>
						 element
							 (1)
     ranges::in_value_result<I, U>

	ranges::fold_left_with_iter		left	      init	     (2)
     ranges::in_value_result<BR, U>,

							 where BR is
							 ranges::borrowed_itera-
     tor_t<R>
							 (1)
     ranges::in_value_result<I,
							 std::optional<U>>

	ranges::fold_left_first_with_iter	 left	       first	     (2)
     ranges::in_value_result<BR,
						 element std::optional<U>>

							 where BR is
							 ranges::borrowed_itera-
     tor_t<R>

	 Feature-test macro    Value	Std		Feature
	__cpp_lib_ranges_fold 202207L (C++23) std::ranges fold algorithms

Example
     // Run this code

      #include <algorithm>
      #include <functional>
      #include <iostream>
      #include <ranges>
      #include <utility>
      #include <vector>

      int main()
      {
	  std::vector v{1, 2, 3, 4, 5, 6, 7, 8};

	  auto	 sum	=    std::ranges::fold_left_first(v.begin(),	v.end(),
     std::plus<int>()); // (1)
	  std::cout << "*sum: " << sum.value() << '\n';

	  auto mul = std::ranges::fold_left_first(v, std::multiplies<int>()); //
     (2)
	  std::cout << "*mul: " << mul.value() << '\n';

	  //  get  the product of the std::pair::second of all pairs in the vec-
     tor:
	  std::vector<std::pair<char, float>> data  {{'A',  3.f},  {'B',  3.5f},
     {'C', 4.f}};
	  auto sec = std::ranges::fold_left_first
	  (
	      data | std::ranges::views::values, std::multiplies<>()
	  );
	  std::cout << "*sec: " << *sec << '\n';

	  // use a program defined function object (lambda-expression):
	  auto val = std::ranges::fold_left_first(v, [](int x, int y) { return x
     + y + 13; });
	  std::cout << "*val: " << *val << '\n';
      }

Output:
      *sum: 36
      *mul: 40320
      *sec: 42
      *val: 127

References
	  * C++23 standard (ISO/IEC 14882:2023):

	  * 27.6.18 Fold [alg.fold]

See also
	ranges::fold_left		  left-folds a range of elements
	(C++23) 			  (niebloid)
	ranges::fold_right		  right-folds a range of elements
	(C++23) 			  (niebloid)
	ranges::fold_right_last 	   right-folds a range of elements using
     the last
	(C++23) 			  element as an initial value
					  (niebloid)
	ranges::fold_left_with_iter	  left-folds a range  of  elements,  and
     returns a pair
	(C++23) 			  (iterator, value)
					  (niebloid)
					  left-folds  a  range of elements using
     the first
	ranges::fold_left_first_with_iter element as an initial value,	and  re-
     turns a pair
	(C++23) 			  (iterator, optional)
					  (niebloid)
	accumulate			  sums up or folds a range of elements
					  (function template)
	reduce				  similar to std::accumulate, except out
     of order
	(C++17) 			  (function template)

http://cppreference.com 	   2024.06.10	    std::ranges..._left_first(3)
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