std::ranges::fold_left_first
From cppreference.com
| 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 auto fold_left_first( 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 auto fold_left_first( R&& r, F f ); |
(2) | (since C++23) |
| Helper concepts |
||
template< class F, class T, class I > concept /*indirectly-binary-left-foldable*/ = /* see description */; |
(3) | (exposition only*) |
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 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<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*) |
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.
Parameters
| first, last | - | the iterator-sentinel pair defining 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_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;
|
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 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 | Feature |
|---|---|---|---|
__cpp_lib_ranges_fold |
202207L |
(C++23) | std::ranges fold algorithms
|
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
);
}
References
- C++23 standard (ISO/IEC 14882:2024):
- 27.6.18 Fold [alg.fold]
See also
(C++23) |
left-folds a range of elements (algorithm function object) |
(C++23) |
right-folds a range of elements (algorithm function object) |
(C++23) |
right-folds a range of elements using the last element as an initial value (algorithm function object) |
(C++23) |
left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object) |
| left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (algorithm function object) | |
| sums up or folds a range of elements (function template) | |
(C++17) |
similar to std::accumulate, except out of order (function template) |