# std::ranges::fold_left_first

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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 auto fold_left_first( 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 auto fold_left_first( 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*)

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) The range is `[`first`,` last`)`. Equivalent to return ranges::fold_left_first_with_iter (std::move(first), last, f).value.
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*)

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.

### 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) 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

### Example

Run this code

    #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

### References

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

  - 27.6.18 Fold [alg.fold]

### See also

ranges::fold_left (C++23) | left-folds a range of elements (algorithm function object)
ranges::fold_right (C++23) | right-folds a range of elements (algorithm function object)
ranges::fold_right_last (C++23) | right-folds a range of elements using the last element as an initial value (algorithm function object)
ranges::fold_left_with_iter (C++23) | left-folds a range of elements, and returns a pair (iterator, value) (algorithm function object)
ranges::fold_left_first_with_iter (C++23) | left-folds a range of elements using the first element as an initial value, and returns a pair (iterator, optional) (algorithm function object)
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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