University of Virginia, Department of Computer Science
CS201J: Engineering Software, Fall 2003

Notes: Thursday 20 November 2003
Schedule

Exam 2

Exam 2 will be handed out on Tuesday, 2 December and due on Friday, 5 December. The exam will be take home, open book and notes, and similar in format to Exam 1.

Exam 2 will cover all material in the class up to and including today's lecture and problem sets 1-6. It will emphasize material covered since Exam 1, but may include questions on any course material. You should not be surprised if it has questions about subtyping, concurrency, design, type safety, Java byte codes and garbage collection. You should also not be surprised to find questions that cover material from the first half of the class also: writing good specifications, reasoning about data abstractions and design.

Tuesday's class will be a review session if and only if enough students send in good questions by Monday.

Pointers in C
Why is it useful to be able to manipulate memory addresses directly?







Why is it dangerous to be able to manipulate memory addresses directly? 







int match (char *s, char *t) {
  char *os = s;
  while (*s++ == *t++);
  return s;
}

Clue

Friday's section will review subtyping and concurrency. One problem for tomorrow's section is below. Please look it over before section.

In this game of 4 players, the participant who collects 5 clues wins and ends the game. Look at the following code and:

  1. Describe where the race condition is.
  2. Explain how to avoid this race condition. 
class ParticipantThread implements Runnable {
   private Participant participant;
   public ParticipantThread(Participant p) { participant = p; }
   public void run() { participant.getClue(); }
}

class Clues {
   private int numclues;
   public Clues (int p_numclues) { numclues = p_numclues; }
   public void removeClue () { numclues--; }
   public boolesn hasCluesLeft () { return numclues > 0; }
}

public class Participant {
   private int num_clues; // number of clues this participant has collected
   static private Clues clues;

   public Participant () { num_clues = 0; }

   public void getClue() {
      if (clues.hasCluesLeft()) {	
         num_clues++;		
	 clues.removeClue(); // decreases the total number of clues in the game
      }
   }

   public static void main(String[] args) {
      // the four players
      Participant p1;
      Participant p2;
      Participant p3;
      Participant p4;
      clues = new Clues (16); // Start with 16 clues
      Thread p1thread = new Thread(new ParticipantThread(p1));
      Thread p2thread = new Thread(new ParticipantThread(p2));
      Thread p3thread = new Thread(new ParticipantThread(p3));
      Thread p4thread = new Thread(new ParticipantThread(p4));
 
      p1thread.start();
      p2thread.start();
      p3thread.start();
      p4thread.start();
   }
}
The International Obfuscated C Code Contest is a contest to write an incomprehensible C program. Why is there no International Obfuscated Java Contest? 

#include 
int l;int main(int o,char **O,
int I){char c,*D=O[1];if(o>0){
for(l=0;D[l              ];D[l
++]-=10){D   [l++]-=120;D[l]-=
110;while   (!main(0,O,l))D[l]
+=   20;   putchar((D[l]+1032)
/20   )   ;}putchar(10);}else{
c=o+     (D[I]+82)%10-(I>l/2)*
(D[I-l+I]+72)/10-9;D[I]+=I<0?0
:!(o=main(c/10,O,I-1))*((c+999
)%10-(D[I]+92)%10);}return o;}
Raymond Cheong's short program from IOCCC 2002 for calculating square roots

CS201J University of Virginia
Department of Computer Science
CS 201J: Engineering Software
 		Sponsored by the
National Science Foundation 		cs201j-staff@cs.virginia.edu