std::ranges::fold_left_first
Defined in header <algorithm>
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Call signature |
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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 |
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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
return ranges::fold_left_first_with_iter(std::move(first), last, f).value. Helper concepts |
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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 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>> 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; |
[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::ranges fold algorithms
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[edit] Example
#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 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) { return x + y + 13; }); std::cout << "*val: " << *val << '\n'; }
Output:
*sum: 36 *mul: 40320 *sec: 42 *val: 127
[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) |