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++20
void 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  5
Contents of l, viewed via shuffled v:   -1  5 -4  0  2  1  4 -2  3 -3