View source for cpp/algorithm/ranges/fold left with iter

{{cpp/ranges/title|fold_left_with_iter|fold_left_with_iter_result}}
{{cpp/algorithm/ranges/navbar}}
{{dcl begin}}
{{dcl header|algorithm}}
{{dcl h|Call signature}}
{{dcl rev begin|num=1}}
{{dcla|anchor=1|since=c++23|until=c++26|1=
template< std::input_iterator I, std::sentinel_for<I> S, class T,
          /* indirectly-binary-left-foldable */<T, I> F >
constexpr /* see description */
    fold_left_with_iter( I first, S last, T init, F f );
}}
{{dcl|since=c++26|1=
template< std::input_iterator I, std::sentinel_for<I> S,
          class T = std::iter_value_t<I>,
          /* indirectly-binary-left-foldable */<T, I> F >
constexpr /* see description */
    fold_left_with_iter( I first, S last, T init, F f );
}}
{{dcl rev end}}
{{dcl rev begin|num=2}}
{{dcl|since=c++23|until=c++26|1=
template< ranges::input_range R, class T,
          /* indirectly-binary-left-foldable */
              <T, ranges::iterator_t<R>> F >
constexpr /* see description */
    fold_left_with_iter( R&& r, T init, F f );
}}
{{dcl|since=c++26|1=
template< ranges::input_range R, class T = ranges::range_value_t<R>,
          /* indirectly-binary-left-foldable */
              <T, ranges::iterator_t<R>> F >
constexpr /* see description */
    fold_left_with_iter( R&& r, T init, F f );
}}
{{dcl rev end}}
{{dcl h|Helper concepts}}
{{dcl|num=3|notes={{mark expos}}|1=
template< class F, class T, class I >
concept /* indirectly-binary-left-foldable */ = /* see description */;
}}
{{dcl h|Helper class template}}
{{dcl|num=4|since=c++23|1=
template< class I, class T >
using fold_left_with_iter_result = ranges::in_value_result<I, T>;
}}
{{dcl end}}

Left-{{enwiki|Fold (higher-order function)|folds}} the elements of given range, that is, returns the result of evaluation of the chain expression:<br>{{tt|f(f(f(f(init, x{{sub|1}}), x{{sub|2}}), ...), x{{sub|n}})}}, where {{tt|x{{sub|1}}}}, {{tt|x{{sub|2}}}}, ..., {{tt|x{{sub|n}}}} are elements of the range.

Informally, {{tt|ranges::fold_left_with_iter}} behaves like {{lc|std::accumulate}}'s overload that accepts a binary predicate.

The behavior is undefined if {{range|first|last}} is not a valid range.

@1@ The range is {{range|first|last}}.

@2@ Same as {{v|1}}, except that uses {{c|r}} as the range, as if by using {{c|ranges::begin(r)}} as {{c|first}} and {{c|ranges::end(r)}} as {{c|last}}.

{{ranges_fold_algos_helper_concepts}}
@4@ The return type alias. See "{{ls|#Return value}}" section for details.

{{cpp/ranges/niebloid}}

===Parameters===
{{par begin}}
{{par|first, last|the range of elements to fold}}
{{par|r|the range of elements to fold}}
{{par|init|the initial value of the fold}}
{{par|f|the binary function object}}
{{par end}}

===Return value===
Let {{c|U}} be {{c|std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>}}.

@1@ An object of type {{c|ranges::fold_left_with_iter_result<I, U>}}.
* The member {{c|ranges::in_value_result::in}} holds an iterator to the end of the range.
* The member {{c|ranges::in_value_result::value}} holds the result of the left-{{enwiki|Fold (higher-order function)|fold}} of given range over {{c|f}}.
If the range is empty, the return value is obtained via the expression equivalent to {{c|return {std::move(first), U(std::move(init))};}}.

@2@ Same as {{v|1}} except that the return type is {{c|ranges::fold_left_with_iter_result<ranges::borrowed_iterator_t<R>, U>}}.

===Possible implementations===
{{eq fun|1=
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<I>>>;
        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<I> S, class T = std::iter_value_t<I>,
             /* indirectly-binary-left-foldable */<T, I> F>
    constexpr auto operator()(I first, S last, T init, F f) const
    {
        return impl<I>(std::move(first), std::move(last), std::move(init), std::ref(f));
    }
    
    template<ranges::input_range R, class T = ranges::range_value_t<R>,
             /* indirectly-binary-left-foldable */<T, ranges::iterator_t<R>> F>
    constexpr auto operator()(R&& r, T init, F f) const
    {
        return impl<ranges::borrowed_iterator_t<R>>
        (
            ranges::begin(r), ranges::end(r), std::move(init), std::ref(f)
        );
    }
};

inline constexpr fold_left_with_iter_fn fold_left_with_iter;
}}

===Complexity===
Exactly {{c|ranges::distance(first, last)}} applications of the function object {{c|f}}.

===Notes===
{{ranges_fold_algos_table}}

{{ftm begin}}
{{ftm|__cpp_lib_ranges_fold|std=C++23|value=202207L|{{tt|std::ranges}} [[cpp/algorithm/ranges#Constrained fold operations|fold algorithms]]}}
{{ftm|__cpp_lib_algorithm_default_value_type|value=202403L|std=C++26|[[cpp/language/list initialization|List-initialization]] for algorithms {{vl|1,2}}}}
{{ftm end}}

===Example===
{{example
|code=
#include <algorithm>
#include <cassert>
#include <complex>
#include <functional>
#include <ranges>
#include <utility>
#include <vector>

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<int>());
    assert(sum.value == 42);
    assert(sum.in == v.end());
    
    auto mul = ranges::fold_left_with_iter(v, 0X69, std::multiplies<int>());
    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<double>;
    std::vector<CD> 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}));
}
}}

===References===
{{ref std c++23}}
{{ref std|title=Fold|id=alg.fold|section=27.6.18}}
{{ref std end}}

===See also===
{{dsc begin}}
{{dsc inc|cpp/algorithm/ranges/dsc fold_left}}
{{dsc inc|cpp/algorithm/ranges/dsc fold_left_first}}
{{dsc inc|cpp/algorithm/ranges/dsc fold_right}}
{{dsc inc|cpp/algorithm/ranges/dsc fold_right_last}}
{{dsc inc|cpp/algorithm/ranges/dsc fold_left_first_with_iter}}
{{dsc inc|cpp/algorithm/dsc accumulate}}
{{dsc inc|cpp/algorithm/dsc reduce}}
{{dsc end}}

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