Computer Science 2610
Fall 2000
Solutions to practice questions for quiz 2

The answers are in blue. Remarks on the solutions are in red.

  1. If function t is called, what is printed? Be very careful with this question. 
    	void r(int x, int y, int& z)
	{
	  cout << "r: x = " << x << " y = " << y 
               << " z = " << z << endl;
	  x = x + 1;
	  z = y + x;
	}
	 
	void t()
	{
	  int a, b, x;
	  a = 20;
	  b = 25;
	  x = 250;
	  r(a,b,x);
	  r(x,a,b);
	  cout << "t: a = " << a << " b = " << b 
               << " x = " << x << endl;
     }

    Answer: 

             r: x = 20 y = 25 z = 250
         r: x = 46 y = 20 z = 25
         t: a = 20 b = 67 x = 46

    The key to getting this right is to do the hand simulation carefully. Be sure to remember how to handle call-by-reference and call-by-value. Before the first call to r, the picture looks like this. 

             t
     ----------
       a = 20
       b = 25
       x = 250

    Now r(a,b,x) is called. The picture looks like this. 

             t                    r
     ----------           ---------
       a = 20              x = 20
       b = 25              y = 25
       x = 250 <-          z .
                 |___________|
    When function r runs, it prints line 
             r: x = 20 y = 25 z = 250
    and then performs the two assignments. The picture looks like this. 
             t                    r
     ----------           ---------
       a = 20              x = 21
       b = 25              y = 25
       x = 46  <-          z .
                 |___________|
    Now r returns, and t is about to call r again. The picture is like this. 
             t
     ----------
       a = 20
       b = 25
       x = 46
    Now t calls r(x,a,b). The picture looks like this just before r runs. 
             t                    r
     ----------           ---------
       a = 20              x = 46
       b = 25  <-          y = 20
       x = 46    |         z .
                 |___________|
    Now r runs. It prints line 
             r: x = 46 y = 20 z = 25
    Then r does the two assignments. The picture is like this. 
             t                    r
     ----------           ---------
       a = 20              x = 47
       b = 67  <-          y = 20
       x = 46    |         z .
                 |___________|
    Now r returns, leaving t ready to print its line. It prints 
             t: a = 20 b = 67 x = 46

  2. What is the value of mystery(4)? 
    	int mystery(int n)
	{
	  if(1 == n) return 1;
	  else return 3*mystery(n-1);
	}

    You can find out the value of mystery(4) by computing mystery(1), mystery(2) and mystery(3) first. 

       mystery(1) = 1
   mystery(2) = 3*mystery(1) = 3*1 = 3
   mystery(3) = 3*mystery(2) = 3*3 = 9
   mystery(4) = 3*mystery(3) = 3*9 = 27
    The key to getting this kind of question right is to stay organized, and not to be sloppy.

    Answer: 27

  3. What is the value of whatisthis(3)? 
            int whatisthis(int n)
        {
          if(n == 0) return 1;
          else return whatisthis(n-1) + whatisthis(n-1);
        }

    This is similar to the preceding question. Find out what the small values lead to first. Don't be sloppy. 

         whatisthis(0) = 1
     whatisthis(1) = whatisthis(0) + whatisthis(0) = 1 + 1 = 2
     whatisthis(2) = whatisthis(1) + whatisthis(1) = 2 + 2 = 4
     whatisthis(3) = whatisthis(2) + whatisthis(2) = 4 + 4 = 8

    Answer: 8

  4. You would like to open a file called "mydata.txt" for reading. Write a declaration that will create a object that can be used to read from that file, in a way similar to the way the object cin is used to read from the standard input. 

       ifstream in("mydata.txt");

  5. Suppose that variable n has been declared to have type int. You would like to read one integer from file "mydata.txt", using the object created for the preceding problem, and put that integer into variable n. Write a statement that will do that. 

       in >> n;

  6. You would like to open a file called "myout" for writing. Write a declaration that will create an object that can be used to write to that file, in a way similar to the way the object cout is used to write to the standard output. 

       ofstream out("myout");

  7. Suppose that variable n has been declared to have type int. You would like to write the value of n in file "myout", using the object created for the preceding problem. Write a statement that will do that. 

       out << n;
    Remark. There was a mistake in this question. It should have been as stated above.

  8. Consider the structure definition 
         struct Plant
     {
       int root;
       double leaf;
     };
    Write C++ statements that will create an object called bush of type Plant, and will set bush so that its root variable holds 12 and its leaf variable holds 90.0. 
        Plant bush;
    bush.root = 12;
    bush.leaf = 90.0;