allocating and populating a large array

Question

I want to allocate and initialize a very large array of unsigned long ints. The array is a simple 1-D array, and I want to populate it with the value of the first N primes. I already have a program that will spit these out into a .cpp file for inclusion. I'm just not sure of the syntax to initialize the array. I know that

unsigned long int *known = new unsigned long int[N];

will create the array on the heap, but I'm not sure how to initialize the members.

Answer 1

You basically just loop over the array and assign the correct value to every element, like so:

for(size_t idx = 0; idx < N; ++idx)
{
  *(known + idx) = retrieve_value(idx);
}

Obviously with the retrieve_value function returning the appropriate value that needs to be assigned to known[idx].

You can also make use of the pointer/array equivalence and write the code using array syntax:

for(size_t idx = 0; idx < N; ++idx)
{
  known[idx] = retrieve_value(idx);
}

That said, unless there is a good reason for new'ing the array this way I would strongly recommend using one of the standard containers like std::vector or std::array in this case as it'll avoid the memory management headaches that are mostly unnecessary these days. If N is known at compile time, std::array is likely to have less overhead, otherwise using std::vector with an appropriate reserve() call should do the job fine, too. I would really only suggest dealing with raw memory if you absolutely have to squeeze the last byte out of the available memory.

Answer 2

I would suggest you to create an array initializer class which will accept a pointer to array and number of elements to be created. Use a global variable of this class. Upon creation the constructor will populate the array.

    class ArrayInitializer{
       ArrayInitializer(unsigned long* ptr, size_t Size);
       ...
       ...
    };

Now create a global variable

    //include your header here,
    ArrayInitializer g_arrayInit(known,N);

Global variables are created as soon as program execution begins. Also you can you this class to clean up the memory allocated using 'new'