Question

Currently I am trying to fill an array of size num with random values. To do this, I need to create two functions:

1: Write a function (*createdata) that allocates a dynamic array of num double values, initialising the values to 0.0.

2: Write a different function (gendata) that will populate an array of double values with random values generated using the rand() function.

Here is my attempt at writing how the functions operate (outside main() ):

double *createdata(int num)
  {
         int i = 0;

         double *ptr;

         ptr = (double *)malloc(sizeof(double)*num);     

         if(ptr != NULL)
         {
           for(i = 0; i < num; i++)
           {
                 ptr[i] = 0.0;       
           }
         } 
  }  

double gendata(int num)
  {

         createdata(num); 

         int j = 0;

         for(j = 0; j < num; j++)
           {
                 ptr[j] = rand();       
           }               
  }

However, I know that there is something certainly wrong with the above.

What I would like is that once I have called both functions in main, I will have generated an array of size num that is filled with random numbers.

Answer 1

You have to pass the allocated pointer to the gendata function, because in it’s current form, ptr is unknown. Does that even compile?

Example:

double *createdata(int num)
{
    int i = 0;
    double *ptr;

    ptr = (double *)malloc(sizeof(double)*num);

    if(ptr != NULL)
    {
        for(i = 0; i < num; i++)
        {
            ptr[i] = 0.0;
        }
    }
    return ptr;
}

And:

double gendata(int num)
{
    double *ptr = createdata(num);
    int j = 0;

    if(ptr != NULL)
    {
        for(j = 0; j < num; j++)
        {
            ptr[j] = rand();
        }
    }
}

Also note I’m checking for return NULL again from malloc.

Other hints others might say here:

• Don’t cast the return of malloc.
• Don’t forget to free the pointer after you use it entirely.
• You’re not returning anything in your function gendata, but you declared it as double. Use void instead if you’re not going to return anything.

But you’re probably gonna need to return the pointer from it anyways, so that you can use it later in other functions, such as main.

EDIT: So, this would be like this, as an example:

double *gendata(int num)
{
    double *ptr = createdata(num);
    int j = 0;

    if(ptr != NULL)
    {
        for(j = 0; j < num; j++)
        {
            ptr[j] = rand();
        }
    }
    return ptr;
}

And in your main:

int main(void)
{
    double *data;
    data = gendata(100);
    printf("%g\n", data[5]);

    // When you're done, call free
    free(data);
    return 0;
}