std::ranges::fold_left_first - cppreference.com (original) (raw)

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. 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.

2. Equivalent to:

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

• 1 Parameters
• 2 Return value
• 3 Possible implementations
• 4 Complexity
• 5 Notes
• 6 Example
• 7 References
• 8 See also

[edit] Parameters

[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 todecltype(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 S, /indirectly-binary-left-foldable/<std::iter_value_t, I> F> requires std::constructible_from<std::iter_value_t, std::iter_reference_t> constexpr auto operator()(I first, S last, F f) const { using U = decltype( ranges::fold_left(std::move(first), last, std::iter_value_t(*first), f) ); if (first == last) return std::optional(); std::optional 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, ranges::iterator_t> F> requires std::constructible_from<ranges::range_value_t, ranges::range_reference_t> 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:

[edit] Example

#include #include #include #include #include   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 ); }

[edit] References

• C++23 standard (ISO/IEC 14882:2024):

• 27.6.18 Fold [alg.fold]

[edit] See also