NAME¶

std::ranges::fold_left_first - std::ranges::fold_left_first

Synopsis¶

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>, (1) (since C++23)
std::iter_reference_t<I>>
constexpr auto

fold_left_first( I first, S last, F f );
template< ranges::input_range R,

/*indirectly-binary-left-foldable*/<
ranges::range_value_t<R>, ranges::iterator_t<R>>
F >
requires std::constructible_from< (2) (since C++23)
ranges::range_value_t<R>,
ranges::range_reference_t<R>>
constexpr auto

fold_left_first( R&& r, F f );
Helper concepts
template< class F, class T, class I > (exposition
concept /*indirectly-binary-left-foldable*/ = /* see description (3) only*)
*/;

Left-folds the elements of given range, that is, returns the result of evaluation of
the chain expression:
f(f(f(f(x[1], x[2]), x[3]), ...), x[n]), where x[1], x[2], ..., x[n] 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> && (3A) (exposition only*)
std::invocable<F&, U, std::iter_reference_t<I>> &&
std::assignable_from<U&,

std::invoke_result_t<F&, U,
std::iter_reference_t<I>>>;
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, (3B) (exposition only*)
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>>>>;

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¶

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 Initial Return type
from value
ranges::fold_left left init U
ranges::fold_left_first left first std::optional<U>
element
ranges::fold_right right init U
ranges::fold_right_last right last std::optional<U>
element
(1) ranges::in_value_result<I, U>

ranges::fold_left_with_iter left init (2) ranges::in_value_result<BR, U>,

where BR is
ranges::borrowed_iterator_t<R>
(1) ranges::in_value_result<I,
std::optional<U>>

ranges::fold_left_first_with_iter left first (2) ranges::in_value_result<BR,
element 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 14882:2023):

* 27.6.18 Fold [alg.fold]

See also¶

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