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std::generate_n

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

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.

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.

Return value

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

Complexity

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

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.

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;
}

Example

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

#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

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