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 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.
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_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) std::in_value_result<I, U> (2) std::in_value_result<BR, U>, where BR is ranges::borrowed_iterator_t<R> |
| ranges::fold_left_first_with_iter | left | first element |
(1) std::in_value_result<I, std::optional<U>> (2) std::in_value_result<BR, std::optional<U>> where BR is ranges::borrowed_iterator_t<R> |
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__cpp_lib_ranges_fold |
202207L | (C++23) | std::ranges fold algorithms
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Example
#include <algorithm> #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(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
References
- C++23 standard (ISO/IEC 14882:2023):
- 27.6.18 Fold [alg.fold]
See also
| (C++23) |
left-folds a range of elements (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) |
| left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (niebloid) | |
| sums up or folds a range of elements (function template) | |
| (C++17) |
similar to std::accumulate, except out of order (function template) |