std::iota

Defined in header `<numeric>`
template< class ForwardIt, class T > void iota( ForwardIt first, ForwardIt last, T value );		(since C++11) (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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::iota_viewviews::iota (C++20)	a `view` consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)
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 (C++23)	fills a range with successive increments of the starting value (niebloid)