std::iota(3)

NAME
std::iota - std::iota

Synopsis
   Defined in header <numeric>
   template< class ForwardIt, class T > (since C++11)
   void iota( ForwardIt first, ForwardIt last, T value ); (constexpr since C++20)
   Fills the range [first, last) with sequentially increasing values, starting with
   value and repetitively evaluating ++value.
   Equivalent operation (assuming ++value returns the incremented value):
 *first = value;
 *++first = ++value;
 *++first = ++value;
 *++first = ++value;
 // repeats until “last” is reached
   If any of the following conditions is satisfied, the program is ill-formed:
     * T is not convertible to the value type of ForwardIt.
     * The expression ++val is ill-formed, where val is a variable of type T.

Parameters
   first, last - the range of elements to fill with sequentially increasing values
                 starting with value
   value - initial value to store

Complexity
   Exactly std::distance(first, last) increments and assignments.

Possible implementation
   template<class ForwardIt, class T>
   constexpr // since C++20
   void iota(ForwardIt first, ForwardIt last, T value)
   {
       for (; first != last; ++first, ++value)
           *first = value;
   }

Notes
   The function is named after the integer function ⍳ from the programming language
   APL. It was one of the STL components that were not included in C++98, but made it
   into the standard library in C++11.

Example
   The following example applies std::shuffle to a vector of std::lists' iterators.
   std::iota is used to populate containers. // Run this code
 #include <algorithm>
 #include <iomanip>
 #include <iostream>
 #include <list>
 #include <numeric>
 #include <random>
 #include <vector>
 class BigData // inefficient to copy
 {
     int data[1024]; /* some raw data */
 public:
     explicit BigData(int i = 0) { data[0] = i; /* ... */ }
     operator int() const { return data[0]; }
     BigData& operator=(int i) { data[0] = i; return *this; }
     /* ... */
 };
 int main()
 {
     std::list<BigData> l(10);
     std::iota(l.begin(), l.end(), -4);
     std::vector<std::list<BigData>::iterator> v(l.size());
     std::iota(v.begin(), v.end(), l.begin());
     // Vector of iterators (to original data) is used to avoid expensive copying,
     // and because std::shuffle (below) cannot be applied to a std::list directly.
     std::shuffle(v.begin(), v.end(), std::mt19937{std::random_device{}()});
     std::cout << "Original contents of the list l:\t";
     for (const auto& n : l)
         std::cout << std::setw(2) << n << ' ';
     std::cout << '\n';
     std::cout << "Contents of l, viewed via shuffled v:\t";
     for (const auto i : v)
         std::cout << std::setw(2) << *i << ' ';
     std::cout << '\n';
 }

Possible output:
 Original contents of the list l: -4 -3 -2 -1 0 1 2 3 4 5
 Contents of l, viewed via shuffled v: -1 5 -4 0 2 1 4 -2 3 -3

See also
   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)
   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 fills a range with successive increments of the starting value
   (C++23) (niebloid)