AMAT 2120 F05: Comments to tracing question on the Final Exam

• One way to answer a tracing question is to write and compile the program on the system. In order to make this approach unfeasiable or unattractive, in this question you'll see a function with defined functionality but hidden implementation --- in other words, we tell you what it does, but do not provide the actual code of the function's body.


• Example to practice

  10    int shuffle(int x);
   
   /* 
   This function interchanges odd-numbered and even-numbered
   digits of x  (counting from the leftmost digit). If x has
   odd number of digits, the rightmost one remains intact.
   For example,
   
     x = 17    =>  result = 71
     x = 20    =>  result = 02 (that is, 2)
     x = 2005  =>  result = 0250 (that is, 250)
     x = 135   =>  result = 315
     x = 34576 =>  result = 43756
  ****/
  
  20  int main()
  21  {
  22    int y,z;
  23    y=2005;
  24    z=11;
  25    while (y>z) /* loop begins */
  26    {
  27  	if ((y%z)!=0)
  28  	{
  29  	  printf("Case 1. [Shuffling $%d! Incrementing #%d.]\n",y,z);
  30  	  y=shuffle(y);
  31  	  z++;
  32  	}
  33  	else 
  34  	{  
  35  	  printf("Case 2. (Reducing %d. Shuffling %d)\n",y,z);
  36  	  y= (y/z) - 1;
  37  	  z=shuffle(z)-1;
  38  	}  
  39    }/* loop ends */
  40    
  41    printf("Done. y=%d, z=%d\n", z,y);
  42    return(0);
  43  }
  

• In terms of trace length, the exam tracing question will be perhaps a little bit longer (the program will be of about same size). However, unlike in the above code where the numbers you obtain seem rather arbitrary (except for the initial value of y), the exam question will let you recognize that you're on the right track.