NAME¶

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

Synopsis¶

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&> (1) (since C++23)
constexpr iota_result<O, T>

iota( O first, S last, T value );
template< std::weakly_incrementable T, ranges::output_range<const
T&> R >
(2) (since C++23)
constexpr iota_result<ranges::borrowed_iterator_t<R>, T>

iota( R&& r, T value );

Helper types¶

template< class O, class T > (3) (since C++23)
using iota_result = ranges::out_value_result<O, T>;

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 Feature
__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.

// Run this code

#include <algorithm>
#include <functional>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>

template <typename Proj = std::identity>
void println(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);
println("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 {}()});
println("List viewed via vector: ", vec, [](auto it) { return *it; });
}

Possible output:¶

List: 0 1 2 3 4 5 6 7
List viewed via vector: 5 7 6 0 1 3 4 2

See also¶

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