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std::generate_n

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
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 OutputIt, class Size, class Generator >
OutputIt generate_n( OutputIt 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 does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[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.

[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, std::terminate is called.
  • 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.

#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

assigns a value to a number of elements
(function template) [edit]
saves the result of a function in a range
(function template) [edit]
parallelized version of std::generate_n
(function template) [edit]