std::iota

 

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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 pair of iterators defining 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++20void 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(constauto& n : l)std::cout<<std::setw(2)<< n <<' ';std::cout<<'\n';   std::cout<<"Contents of l, viewed via shuffled v:\t";for(constauto 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 (C++23)	fills a range with successive increments of the starting value (algorithm function 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 (algorithm function object)
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 (algorithm function object)
ranges::iota_viewviews::iota (C++20)	a view consisting of a sequence generated by repeatedly incrementing an initial value (class template)(customization point object)
ranges::enumerate_viewviews::enumerate (C++23)	a view that maps each element of adapted sequence to a tuple of both the element's position and its value (class template)(range adaptor object)