# std::iota

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< cpp‎ | algorithm

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

Fills the range `[first, last)` with sequentially increasing values, starting with `value` and repetitively evaluating ++value.

Equivalent operation:

    *(d_first)   = value;
    *(d_first+1) = ++value;
    *(d_first+2) = ++value;
    *(d_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 ForwardIterator, class T>
    void iota(ForwardIterator first, ForwardIterator 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 eventually made it into the standard library in C++11.

### Example

The following example applies std::shuffle to a vector of std::list iterators since std::shuffle cannot be applied to a std::list directly. `std::iota` is used to populate both containers.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <list>
    #include <numeric>
    #include <random>
    #include <vector>

    int main()
    {
        std::list<int> l(10);
        std::iota(l.begin(), l.end(), -4);

        std::vector<std::list<int>::iterator> v(l.size());
        std::iota(v.begin(), v.end(), l.begin());

        std::shuffle(v.begin(), v.end(), std::mt19937{std::random_device{}()});

        std::cout << "Contents of the list: ";
        for(auto n: l) std::cout << n << ' ';
        std::cout << '\n';

        std::cout << "Contents of the list, shuffled: ";
        for(auto i: v) std::cout << *i << ' ';
        std::cout << '\n';
    }

Possible output:

    Contents of the list: -4 -3 -2 -1 0 1 2 3 4 5
    Contents of the list, shuffled: 0 -1 3 4 -4 1 -2 -3 2 5

### See also

fill | copy-assigns the given value to every element in a range (function template)
generate | assigns the results of successive function calls to every element in a range (function template)

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