cs230 Lecture Notes
Week 2, Monday

Homework 0 due today.  Face-to-face grading in lab tomorrow.

Before lab, read Chapter 8 and hw01, and the documentation
of the BigInteger class.

For next time, read Chapters 8 and 9 and prepare for a quiz.


Quiz 1 Solution
---------------

Write a class method named areFactors that takes
an integer n and an array of integers, and that returns
true if the numbers in the array are all factors of n
(which is to say that n is divisible by all of them).

Algorithm: traverse the array.  If we find an element
that doesn't divide n, we can return false immediately.
If we get all the way through the array without finding
a clinker, then we can return true.

public static boolean areFactors (int n, int[] array) {
   for (int i=0; i<array.length; i++) {
       if (!isDivisible (n, array[i]) {
           return false;
       }
   }
   return true;
}

Notes:

1) use the idiom for traversing an array

2) use the method isDivisible

3) make sure that "return true" is OUTSIDE the loop

4) there is nothing wrong with exiting from the middle of a loop.

It is more efficient to exit immediately, and more importantly,
it is more likely to be correct.


Performance analysis
--------------------

Two versions of power.


Encapsulation and generalization
--------------------------------

An alternate form of program development.

Instead of writing methods by writing a skeleton first...

1) write the body of the method in main, and get it working
   on a specific test case

2) move it into a method and invoke the method (encapsulate)

3) generalize the method by replacing values with parameters

Example:  last

String bob = "banana";
System.out.println (bob.charAt (bob.length-1));


Put it in a method (encapsulate):

public static void last () {
    String bob = "banana";
    System.out.println (bob.charAt (bob.length-1));
}

Change the print statement to a return statement:

public static void last () {
    String bob = "banana";
    return bob.charAt (bob.length-1);
}

Replace the specific value with a parameter (generalize):

public static void last (String bob) {
    return bob.charAt (bob.length-1);
}

Remember: parameters get their values when the method
is invoked.  You should (almost) NEVER make an assignment
to a parameter...

public static void last (String bob) {
    String bob = "banana";               // VERY VERY VERY BAD!!!
    return bob.charAt (bob.length-1);
}


Advance information for hw01
----------------------------

Two parts: using a built-in class (BigInteger)

and creating a user-defined class (Rational)

I've given you an example of a user-defined class (Complex)

For part 1

1) test the BigInteger methods in isolation and then
   move them into methods

2) keep track of types ... what's an integer and what's
   a BigInteger?

3) some BigInteger methods are class methods, others are
   object methods -- keep the syntax straight

For part 2

1) use Complex as a template... Rational is very similar

2) follow the instructions in the homework.  They guide you
   through a good program development plan for this kind of
   new class.

3) big idea of hw01: defining a new class creates a new
   object type.