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 ); (until C++20)
template< class ForwardIt, class T > (since C++20)
constexpr 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, 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

Return value

(none)

Complexity

Exactly last - first increments and assignments.

Possible implementation

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.

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(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';
}

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