CSCI 159: exam practice sample solutions

Question 1: Basic pointers

   int x, y;
   y = 32;
   x = 5;
   int *ptr1;
   int *ptr2;
   ptr1 = &x;
   (*ptr1) = 10;
   x++;
   cout << x << "," << y << endl;
   ptr2 = ptr1;
   ptr1 = &y;
   y = 45;
   cout << x << "," << y << endl;
   (*ptr1) = 99;
   (*ptr2) = (*ptr1) - 5;
   cout << x << "," << y << endl;

Sample answer:
   // here I'm showing the explanation as comments beside the lines of code

   ptr1 = &x;     // ptr1 now points to x
   (*ptr1) = 10;  // put 10 in x through ptr1
   x++;           // x now 11
   cout << x << "," << y << endl; // 11,32 (y hasn't changed yet)

   ptr2 = ptr1;   // both ptrs now point to x
   ptr1 = &y;     // ptr1 now points to y (ptr2 still points to x)
   y = 45;        // y now 45
   cout << x << "," << y << endl; // 11,45 (x hasn't changed since last cout)

   (*ptr1) = 99;   // change y to 99 through ptr1
   (*ptr2) = (*ptr1) - 5; // change x to 99-5, i.e. 94, through ptr2
   cout << x << "," << y << endl; // 94,99

Question 2: Arrays, new and delete[]

• gets an integer size from the user (no error checking required),
• allocates one array of doubles of that size and one array of ints of that size (using new for each of the allocations),
• fills the array of ints with values the user supplies (again no error checking required),
• fills the array of doubles with the square roots of the corresponding ints (e.g. if our arrays were mydoubles and myints then mydoubles[i] gets the square root of myints[i]),
• deletes the array of ints,
• prints the elements from the array of doubles,
• deletes the array of doubles

Sample answer:
int main()
{
   // get the size
   int size;
   cout << "Enter the array size";
   cin >> size;

   // allocate the arrays
   int *myints = new(std::nothrow) int[size];
   double *mydoubles = new(std::nothrow) double[size];
   // note: really I should also have specified checks for nullptr here

   // fill the int array
   for (int i = 0; i < size; i++) {
      cout << "Enter an integer";
      cin >> myints[i];
   }

   // fill the doubles array
   for (int i = 0; i < size; i++) {
      mydoubles[i] = sqrt(myints[i]);
   }

   // delete the ints array
   delete [] myints;

   // display the doubles
   for (int i = 0; i < size; i++) {
      cout << mydoubles[i] << endl;
   }

   // delete the ints array
   delete [] mydoubles;

}