std::ranges::fold_left_first

Since C++23

Header: <algorithm>

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.

# Declarations

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 );

(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 );

(since C++23)

Helper concepts

template< class F, class T, class I >
concept /*indirectly-binary-left-foldable*/ = /* see description */;

(exposition only*)

# Parameters

# Return value

An object of type std::optional 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(*first), f)).

# Notes

The following table compares all constrained folding algorithms:

(1) ranges::in_value_result<I, U>

(2) ranges::in_value_result<BR, U>,

where BR is ranges::borrowed_iterator_t

(1) ranges::in_value_result<I, std::optional>

(2) ranges::in_value_result<BR, std::optional>

where BR is ranges::borrowed_iterator_t

# Example

#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';
}

# See also