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# std::generate_n

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class OutputIt, class Size, class Generator > void generate_n( OutputIt first, Size count, Generator g ); |  | (until C++11)
template< class ForwardIt, class Size, class Generator > ForwardIt generate_n( ForwardIt first, Size count, Generator g ); |  | (since C++11)
template< class ExecutionPolicy, class OutputIt, class Size, class Generator > OutputIt generate_n( ExecutionPolicy&& policy, OutputIt first, Size count, Generator g ); | (2) | (since C++17)

1) Assigns values, generated by given function object `g`, to the first `count` elements in the range beginning at `first`, if `count>0`. Does nothing otherwise.
2) Same as (1), but executed according to `policy`. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

### [edit] Parameters

first | - | the beginning of the range of elements to generate
count | - | number of the elements to generate
policy | - | the execution policy to use. See execution policy for details.
g | - | generator function object that will be called. The signature of the function should be equivalent to the following:
Ret fun(); |  |
The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret. ​
Type requirements
- `OutputIt` must meet the requirements of `OutputIterator`.
- `ForwardIt` must meet the requirements of `ForwardIterator`.

### [edit] Return value

(none) | (until C++11)
Iterator one past the last element assigned if `count>0`, `first` otherwise. | (since C++11)

### [edit] Complexity

Exactly `count` invocations of `g()` and assignments, for `count>0`.

### [edit] Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the three standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### [edit] Possible implementation

    template< class OutputIt, class Size, class Generator >
    OutputIt generate_n( OutputIt first, Size count, Generator g )
    {
        for( Size i = 0; i < count; i++ ) {
            *first++ = g();
        }
        return first;
    }

### [edit] Example

The following code fills an array of integers with random numbers.

Run this code

    #include <cstddef>
    #include <cstdlib>
    #include <iostream>
    #include <iterator>
    #include <algorithm>

    int main()
    {
        const std::size_t N = 5;
        int ar[N];
        std::generate_n(ar, N, std::rand); // Using the C function rand()

        std::cout << "ar: ";
        std::copy(ar, ar+N, std::ostream_iterator<int>(std::cout, " "));
        std::cout << "\n";
    }

Possible output:

    ar: 52894 15984720 41513563 41346135 51451456

### [edit] See also

fill_n | copy-assigns the given value to N elements in a range (function template) [edit]
generate | assigns the results of successive function calls to every element in a range (function template) [edit]

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