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>, std::iter_reference_t<I>>
constexpr /* see description */
fold_left_first_with_iter( I first, S last, F f );
| (1) | (since C++23) |
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 /* see description */
fold_left_first_with_iter( R&& r, F f );
| (2) | (since C++23) |
| Helper concepts | ||
template< class F, class T, class I >
concept __indirectly_binary_left_foldable = // exposition only
/* see description */;
| (3) | (since C++23) |
| Helper class template | ||
template< class I, class T >
using fold_left_first_with_iter_result = ranges::in_value_result<I, T>;
| (4) | (since C++23) |
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), ...), xn), where x1, x2, ..., xn are elements of the range.
Informally, ranges::fold_left_first_with_iter 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.
[first, last).r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.template< class F, class T, class I, class U >
concept __indirectly_binary_left_foldable_impl = // exposition only
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>>>;
template< class F, class T, class I >
concept __indirectly_binary_left_foldable = // exposition only
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>>>>;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.
| first, last | - | the range of elements to fold |
| r | - | the range of elements to fold |
| f | - | the binary function object |
Let U be decltype(ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f)).
ranges::fold_left_first_with_iter_result<I, std::optional<U>>. ranges::in_value_result::in holds an iterator to the end of the range. ranges::in_value_result::value holds the result of the left-fold of given range over f. {std::move(first), std::optional<U>()}. ranges::fold_left_first_with_iter_result<ranges::borrowed_iterator_t<R>, std::optional<U>>. class fold_left_first_with_iter_fn
{
template<class O, class I, class S, class F>
constexpr auto impl(I&& first, S&& last, F f) const
{
using U = decltype(
ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f)
);
using Ret = ranges::fold_left_first_with_iter_result<O, std::optional<U>>;
if (first == last)
return Ret{std::move(first), 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 Ret{std::move(first), std::move(init)};
}
public:
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
{
return impl<I>(std::move(first), std::move(last), std::ref(f));
}
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 impl<ranges::borrowed_iterator_t<R>>(
ranges::begin(r), ranges::end(r), std::ref(f)
);
}
};
inline constexpr fold_left_first_with_iter_fn fold_left_first_with_iter; |
Exactly ranges::distance(first, last) - 1 (assuming the range is not empty) applications of the function object f.
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) (2) where |
ranges::fold_left_first_with_iter | left | first element | (1) (2) where |
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__cpp_lib_ranges_fold | 202207L | (C++23) |
std::ranges fold algorithms |
#include <algorithm>
#include <cassert>
#include <functional>
#include <iostream>
#include <ranges>
#include <utility>
#include <vector>
int main()
{
std::vector<int> v {1, 2, 3, 4, 5, 6, 7, 8};
auto sum = std::ranges::fold_left_first_with_iter
(
v.begin(), v.end(), std::plus<int>()
);
std::cout << "sum: " << sum.value.value() << '\n';
assert(sum.in == v.end());
auto mul = std::ranges::fold_left_first_with_iter(v, std::multiplies<int>());
std::cout << "mul: " << mul.value.value() << '\n';
assert(mul.in == v.end());
// 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', 7.f}};
auto sec = std::ranges::fold_left_first_with_iter
(
data | std::ranges::views::values, std::multiplies<>()
);
std::cout << "sec: " << sec.value.value() << '\n';
// use a program defined function object (lambda-expression):
auto lambda = [](int x, int y) { return x + y + 2; };
auto val = std::ranges::fold_left_first_with_iter(v, lambda);
std::cout << "val: " << val.value.value() << '\n';
assert(val.in == v.end());
}Output:
sum: 36 mul: 40320 sec: 42 val: 50
|
(C++23) | left-folds a range of elements (niebloid) |
|
(C++23) | left-folds a range of elements using the first element as an initial value (niebloid) |
|
(C++23) | right-folds a range of elements (niebloid) |
|
(C++23) | right-folds a range of elements using the last element as an initial value (niebloid) |
|
(C++23) | left-folds a range of elements, and returns a pair (iterator, value) (niebloid) |
| sums up or folds a range of elements (function template) |
|
|
(C++17) | similar to std::accumulate, except out of order (function template) |
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