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std::ranges::iota, std::ranges::iota_result

Defined in header `<numeric>`
Call signature
template< std::input_or_output_iterator O, std::sentinel_for<O> S, std::weakly_incrementable T > requires std::indirectly_writable<O, const T&> constexpr iota_result<O, T> iota( O first, S last, T value );	(1)	(since C++23)
template< std::weakly_incrementable T, ranges::output_range<const T&> R > constexpr iota_result<ranges::borrowed_iterator_t<R>, T> iota( R&& r, T value );	(2)	(since C++23)
Helper types
template< class O, class T > using iota_result = ranges::out_value_result<O, T>;	(3)	(since C++23)

Fills the range [first, last) with sequentially increasing values, starting with value and repetitively evaluating ++value.

Equivalent operation:

*(first)     = value;
*(first + 1) = ++value;
*(first + 2) = ++value;
*(first + 3) = ++value;
...

Parameters

first, last	-	the range of elements to fill with sequentially increasing values starting with `value`
value	-	initial value to store; the expression `++value` must be well-formed

Return value

{last, value + ranges::distance(first, last)}

Complexity

Exactly last - first increments and assignments.

Possible implementation

struct iota_fn
{
    template<std::input_or_output_iterator O, std::sentinel_for<O> S,
            std::weakly_incrementable T>
    requires std::indirectly_writable<O, const T&>
    constexpr iota_result<O, T> operator()(O first, S last, T value) const
    {
        while (first != last)
        {
            *first = as_const(value);
            ++first;
            ++value;
        }
        return {std::move(first), std::move(value)};
    }
 
    template<std::weakly_incrementable T, std::ranges::output_range<const T&> R>
    constexpr iota_result<std::ranges::borrowed_iterator_t<R>, T>
    operator()(R&& r, T value) const
    {
        return (*this)(std::ranges::begin(r), std::ranges::end(r), std::move(value));
    }
};
 
inline constexpr iota_fn iota;

Notes

The function is named after the integer function ⍳ from the programming language APL.

Feature-test macro	Value	Std	Comment
`__cpp_lib_ranges_iota`	202202L	(C++23)	`std::ranges::iota`

Example

Uses the vector of iterators (std::vector<std::list<T>::iterator) as a proxy to shuffle the elements of the std::list, because ranges::shuffle cannot be applied to the std::list directly.
A link to run the example online: Compiler Explorer.

#include <algorithm>
#include <functional>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>
 
template <typename Proj = std::identity>
inline void print(auto comment, std::ranges::input_range auto&& range, Proj proj = {})
{
    for (std::cout << comment; auto const &element : range)
        std::cout << proj(element) << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::list<int> list(8);
 
    // Fill the list with ascending values: 0, 1, 2, ..., 7
    std::ranges::iota(list, 0);
    print("Contents of the list: ", list);
 
    // A vector of iterators (see the comment to Example)
    std::vector<std::list<int>::iterator> vec(list.size());
 
    // Fill with iterators to consecutive list's elements
    std::ranges::iota(vec.begin(), vec.end(), list.begin());
 
    std::ranges::shuffle(vec, std::mt19937 {std::random_device {}()});
    print("Contents of the list viewed via vector: ", vec, [](auto it) { return *it; });
}

Possible output:

Contents of the list: 0 1 2 3 4 5 6 7
Contents of the list viewed via vector: 5 7 6 0 1 3 4 2

fill	copy-assigns the given value to every element in a range (function template)
ranges::fill (C++20)	assigns a range of elements a certain value (niebloid)
generate	assigns the results of successive function calls to every element in a range (function template)
ranges::generate (C++20)	saves the result of a function in a range (niebloid)
ranges::iota_viewviews::iota (C++20)	a `view` consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)
iota (C++11)	fills a range with successive increments of the starting value (function template)