# std::ranges::fold_left_first_with_iter, std::ranges::fold_left_first_with_iter_result

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< cpp‎ | algorithm‎ | ranges

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 > requires std::constructible_from< std::iter_value_t<I>, std::iter_reference_t<I>> constexpr /* see description */ fold_left_first_with_iter( I first, S last, F f ); | (1) | (since C++23)
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 /* see description */ fold_left_first_with_iter( R&& r, F f ); | (2) | (since C++23)
Helper concepts |  |
template< class F, class T, class I > concept __indirectly_binary_left_foldable = /* see description */; | (3) | (exposition only*)
Helper class template |  |
template< class I, class T > using fold_left_first_with_iter_result = ranges::in_value_result<I, T>; | (4) | (since C++23)

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_with_iter` 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) 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:

Helper concepts |  |
template< class F, class T, class I, class U > concept /*indirectly-binary-left-foldable-impl*/ = std::movable<T> && std::movable<U> && std::convertible_to<T, U> && std::invocable<F&, U, std::iter_reference_t<I>> && std::assignable_from<U&, std::invoke_result_t<F&, U, std::iter_reference_t<I>>>; | (3A) | (exposition only*)
template< class F, class T, class I > concept /*indirectly-binary-left-foldable*/ = std::copy_constructible<F> && std::indirectly_readable<I> && std::invocable<F&, T, std::iter_reference_t<I>> && std::convertible_to<std::invoke_result_t<F&, T, std::iter_reference_t<I>>, std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>> && /*indirectly-binary-left-foldable-impl*/<F, T, I, std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>>; | (3B) | (exposition only*)

4) The return type alias. See Return value section for details.

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

Let U be decltype(ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f)).

1) An object of type ranges::fold_left_first_with_iter_result<I, std::optional<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 {std::move(first), std::optional<U>()}.
2) Same as (1) except that the return type is ranges::fold_left_first_with_iter_result<ranges::borrowed_iterator_t<R>, std::optional<U>>.

### Possible implementations

    class fold_left_first_with_iter_fn
    {
        template<class O, class I, class S, class F>
        constexpr auto impl(I&& first, S&& last, F f) const
        {
            using U = decltype(
                ranges::fold_left(std::move(first), last, std::iter_value_t<I>(*first), f)
            );
            using Ret = ranges::fold_left_first_with_iter_result<O, std::optional<U>>;
            if (first == last)
                return Ret{std::move(first), 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 Ret{std::move(first), std::move(init)};
        }

    public:
        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
        {
            return impl<I>(std::move(first), std::move(last), std::ref(f));
        }

        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 impl<ranges::borrowed_iterator_t<R>>(
                ranges::begin(r), ranges::end(r), std::ref(f)
            );
        }
    };

    inline constexpr fold_left_first_with_iter_fn fold_left_first_with_iter;

### 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

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #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_with_iter
        (
            v.begin(), v.end(), std::plus<int>()
        );
        std::cout << "sum: " << sum.value.value() << '\n';
        assert(sum.in == v.end());

        auto mul = std::ranges::fold_left_first_with_iter(v, std::multiplies<int>());
        std::cout << "mul: " << mul.value.value() << '\n';
        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', 7.f}};
        auto sec = std::ranges::fold_left_first_with_iter
        (
            data | std::ranges::views::values, std::multiplies<>()
        );
        std::cout << "sec: " << sec.value.value() << '\n';

        // use a program defined function object (lambda-expression):
        auto lambda = [](int x, int y) { return x + y + 2; };
        auto val = std::ranges::fold_left_first_with_iter(v, lambda);
        std::cout << "val: " << val.value.value() << '\n';
        assert(val.in == v.end());
    }

Output:

    sum: 36
    mul: 40320
    sec: 42
    val: 50

### References

- C++23 standard (ISO/IEC 14882:2023):

  - 27.6.18 Fold [alg.fold]

### See also

ranges::fold_left (C++23) | left-folds a range of elements (niebloid)
ranges::fold_left_first (C++23) | left-folds a range of elements using the first element as an initial value (niebloid)
ranges::fold_right (C++23) | right-folds a range of elements (niebloid)
ranges::fold_right_last (C++23) | right-folds a range of elements using the last element as an initial value (niebloid)
ranges::fold_left_with_iter (C++23) | left-folds a range of elements, and returns a pair (iterator, value) (niebloid)
accumulate | sums up or folds a range of elements (function template)
reduce (C++17) | similar to std::accumulate, except out of order (function template)

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