CS115 lecture notes, Spring 1999
Week 6, Wednesday

Good morning and welcome to the first (and hopefully last)

     CS115 Silent Lecture

I have laryngitis, so I am trying to minimize the amount
of talking I do today.  We are going to have the quiz as
planned, then I will write the solutions on the board.
I would like you to take the rest of the class time
to read these lecture notes and Chapter 6.

I don't know yet what I am going to do about lab this
afternoon.  It depends on what my voice is like.  In the
meantime, I will also hand out the assignment now so you can
prepare for lab.  Actually, I might do that more in the
future, since I think it is often good to do some work
on paper before sitting down in front of the computer.

Thanks very much for accomodating me.  I hope things will
be back to normal soon.


Multiple assignment
-------------------

int fred = 5;
fred = 7;

1) Old value replaces new.

2) You can assign a value as many times as you want,
   but you can only declare once.

int fred = 5;
int fred = 7;    // ERROR

3) You can change the value of parameters by assigning
   values to them, but it is almost never a good idea.

   Of course, changing the value of a parameter has no
   effect on the caller (who provided the original values,
   but has no interest in the final values).


Iteration
---------

Doing things repeatedly in programs is so common that there
is a program construct for it.

It is not necessary, since we already know how to iterate using
recursion, but it is sometimes easier to write iterative programs.


The while statement
-------------------

1) the syntax is identical to a simple if statement

   while (condition) {
     body
   }

2) The condition has to be a boolean expression

3) The body can contain any kind of statement, including
   declarations, assignments, method invocations, if statements,
   and even while statements

4) The semantics are almost like the English:

   While the condition is true, keep repeating the body
   over and over until the condition becomes false.

5) Iterative structures are called loops.  Each time the
   loop is executed is called an iteration.


Example
-------

  public static void countup (int n) {
    int i = 0;
    while (i <= n) {
      System.out.println (i);
      i = i+1;
    }
    System.out.println ("Blastoff!");
  }

In this case i is called a loop variable, because it is the
variable that keeps track of the iterations and determines
when the loop terminates.


Infinite loops
--------------

Somehow or other, the body of the loop should do something
that eventually makes the condition false.

If the condition is always true, the loop will run forever,
which is usually not what you want.

This requirement is similar to what I said about recursive
methods, which is that you have to make sure you reach the
base case eventually.

In some cases, we can't tell whether the method will terminate.

  public static void sequence (int n) {
    while (n != 1) {
      System.out.println (n);
      if (n%2 == 0) {           // n is even
        n = n / 2;
      } else {                  // n is odd
        n = n*3 + 1;
      }
    }
  }

But most of the time you can.