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

< cpp‎ | algorithm‎ | ranges
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutation operations
Fold operations
Numeric operations
Operations on uninitialized storage
Return types
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 [firstlast) with sequentially increasing values, starting with value and repetitively evaluating ++value.

Equivalent operation:

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


[edit] 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

[edit] Return value

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

[edit] Complexity

Exactly last - first increments and assignments.

[edit] 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);
        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;

[edit] 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

[edit] 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.

#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

[edit] See also

copy-assigns the given value to every element in a range
(function template) [edit]
assigns a range of elements a certain value
assigns the results of successive function calls to every element in a range
(function template) [edit]
saves the result of a function in a range
a view consisting of a sequence generated by repeatedly incrementing an initial value
(class template) (customization point object)[edit]
fills a range with successive increments of the starting value
(function template) [edit]