std::ranges::fold_left_first
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 > | (1) | (since C++23) |
template<ranges::input_range R, /*indirectly-binary-left-foldable*/< | (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.
[first, last). Equivalent to returnranges::fold_left_first_with_iter(std::move(first), last, f).value.Helper concepts | ||
template<class F, class T, class I, class U > concept /*indirectly-binary-left-foldable-impl*/= | (3A) | (exposition only*) |
template<class F, class T, class I > concept /*indirectly-binary-left-foldable*/= | (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.
Contents |
[edit]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 |
[edit]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.
[edit]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>>constexprauto 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)returnstd::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>>constexprauto operator()(R&& r, F f)const{return(*this)(ranges::begin(r), ranges::end(r), std::ref(f));}}; inlineconstexpr 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::rangesfold algorithms |
[edit]Example
#include <algorithm>#include <array>#include <functional>#include <ranges>#include <utility> int main(){constexprstd::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); constexprstd::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); constexprauto 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);}
[edit]References
- C++23 standard (ISO/IEC 14882:2024):
- 27.6.18 Fold [alg.fold]
[edit]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) |
