std::iota() algorithm

od C++20

// (1)template< class ForwardIt, class T >constexpr void iota( ForwardIt first, ForwardIt last, T value );

od C++11

// (1)template< class ForwardIt, class T >void iota( ForwardIt first, ForwardIt last, T value );

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 second	The range of elements to fill with sequentially increasing values starting from value.
value	Initial value to store.

Type requirements

++value

Must be well-formed.

Return value

(none)

Complexity

Exactly last - first increments and assignments.

Exceptions

(none)

Possible implementation

iota(1)

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

Examples

Main.cpp

#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 (auto const& n : l) std::cout << std::setw(2) << n << ' ';    std::cout << '\n';    std::cout << "Contents of l, viewed via shuffled v:\t";    for (auto const i : v) std::cout << std::setw(2) << *i << ' ';    std::cout << '\n';}

Output

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