std::ranges::fold_left_with_iter, std::ranges::fold_left_with_iter_result - 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(init, x1), x2), ...), xn), where x1, x2, ..., xn are elements of the range.

Informally, ranges::fold_left_with_iter behaves like std::accumulate's overload that accepts a binary predicate.

The behavior is undefined if [first, last) is not a valid range.

1. The range is [first, last).

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:

1. The return type alias. See "Return value" section for details.

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

Let U be std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>.

1. An object of type ranges::fold_left_with_iter_result<I, U>.

• The member ranges::in_value_result::in holds an iterator to the end of the range.
• The member ranges::in_value_result::value holds the result of the left-fold of given range over f.

If the range is empty, the return value is obtained via the expression equivalent to return {std::move(first), U(std::move(init))};.

1. Same as (1) except that the return type is ranges::fold_left_with_iter_result<ranges::borrowed_iterator_t<R>, U>.

[edit] Possible implementations

class fold_left_with_iter_fn { template<class O, class I, class S, class T, class F> constexpr auto impl(I&& first, S&& last, T&& init, F f) const { using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t>>; using Ret = ranges::fold_left_with_iter_result<O, U>; if (first == last) return Ret{std::move(first), U(std::move(init))}; U accum = std::invoke(f, std::move(init), *first); for (++first; first != last; ++first) accum = std::invoke(f, std::move(accum), first); return Ret{std::move(first), std::move(accum)}; } public: template<std::input_iterator I, std::sentinel_for S, class T = std::iter_value_t, / indirectly-binary-left-foldable /<T, I> F> constexpr auto operator()(I first, S last, T init, F f) const { return impl(std::move(first), std::move(last), std::move(init), std::ref(f)); }   template<ranges::input_range R, class T = ranges::range_value_t, / indirectly-binary-left-foldable */<T, ranges::iterator_t> F> constexpr auto operator()(R&& r, T init, F f) const { return impl<ranges::borrowed_iterator_t> ( ranges::begin(r), ranges::end(r), std::move(init), std::ref(f) ); } };   inline constexpr fold_left_with_iter_fn fold_left_with_iter;

[edit] Complexity

Exactly ranges::distance(first, last) applications of the function object f.

[edit] Notes

The following table compares all constrained folding algorithms:

[edit] Example

#include #include #include #include #include #include #include   int main() { namespace ranges = std::ranges;   std::vector v{1, 2, 3, 4, 5, 6, 7, 8};   auto sum = ranges::fold_left_with_iter(v.begin(), v.end(), 6, std::plus()); assert(sum.value == 42); assert(sum.in == v.end());   auto mul = ranges::fold_left_with_iter(v, 0X69, std::multiplies()); assert(mul.value == 4233600); assert(mul.in == v.end());   // Get the product of the std::pair::second of all pairs in the vector: std::vector<std::pair<char, float>> data {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}}; auto sec = ranges::fold_left_with_iter ( data | ranges::views::values, 2.0f, std::multiplies<>() ); assert(sec.value == 42);   // Use a program defined function object (lambda-expression): auto lambda = [](int x, int y){ return x + 0B110 + y; }; auto val = ranges::fold_left_with_iter(v, -42, lambda); assert(val.value == 42); assert(val.in == v.end());   using CD = std::complex; std::vector nums{{1, 1}, {2, 0}, {3, 0}}; #ifdef __cpp_lib_algorithm_default_value_type auto res = ranges::fold_left_with_iter(nums, {7, 0}, std::multiplies{}); #else auto res = ranges::fold_left_with_iter(nums, CD{7, 0}, std::multiplies{}); #endif assert((res.value == CD{42, 42})); }

[edit] References

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

• 27.6.18 Fold [alg.fold]

[edit] See also