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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 overload that accepts a binary predicate, except that the *first is used inter- nally 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) (exposition 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) (exposition 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 call- ing 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 in- hibited. In practice, they may be implemented as function objects, or with special 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-fold- able*/<std::iter_value_t<I>, I> F> requires std::constructible_from<std::iter_value_t<I>, std::iter_ref- erence_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::bor- rowed_iterator_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::bor- rowed_iterator_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::multi- plies<int>()); // (2) std::cout << "*mul: " << mul.value() << '\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 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) { re- turn 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 us- ing the first ranges::fold_left_first_with_iter element as an initial value, and returns a pair (C++23) (iterator, optional) (niebloid) accumulate sums up or folds a range of ele- ments (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)
NAME | Synopsis | Parameters | Return value | Possible implementations | Complexity | Notes | Example | Output: | References | See also
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