CS115 lecture notes, Spring 1999
Week 12, Monday

I will have an alternate version of Chapter 12 available
later today or tomorrow.


For loops and internal variables
--------------------------------

Be aware that if you declare a variable inside a loop, it
only exists inside the loop:

  public static int sum (int[] a) {
    for (int i = 0, int total = 0; i<a.length; i++) {
      total += a[i];
    }
    return total;   // VARIABLE UNDECLARED
  }


Encoding and encryption
-----------------------

To encode means to create a mapping between an internal
representation and something in the outside world.

Should not be confused with encryption, which is a way
of transforming a message so that it cannot be transformed
back without the use of a secret key.

Java comes with a number of standard mappings.

They are not secret.

For example, if you saw the bit pattern 01100001, you
would not know how to interpret it, unless I told you it
was an integer, in which case you would know it was 97.

If I told you it was a character, you would interpret it
as the letter 'a'.

If I told you it was a double (and it was preceded by
56 zeroes), you would interpret it as an extremely small
number (roughly 1e-300).

The point of these examples is that the mapping is not
written down inside the computer anywhere.

It is implied by the operations we perform on these values.

When we do addition, we interpret the values as integers
or floating-point numbers and do different things accordingly.

When we print values, we translate them into human-readable
form according to the mapping.

Card objects
------------

The same is true of card objects.

We chose a mapping between integers and suits, and another
mapping between integers and ranks.

In both cases, we had lots of choices, and we were able to
choose the mapping that was most convenient.

There is no place in the program where you explicitly state
what the mappings are.

They are implied by the operations you provide.  Most obviously,
printCard uses the mapping to transform the internal representation
(two integers) into a human-readable form.

One way to facilitate this translation is with an array of
Strings.

    String[] suits = new String [4];

    suits[0] = "Clubs";
    suits[1] = "Diamonds";
    suits[2] = "Hearts";
    suits[3] = "Spades";

Or, more concisely:

   String[] suits = { "Clubs", "Diamonds", "Hearts", "Spades" };

Then you can print the suit of a card by using the rank
as an index into the array:

   String suitWord = suits[card.suit];


Using data as indices
---------------------

This example shows something common, which is the use of
a datum (the suit of the card) as an index.

The same thing happens when you make histograms (as in
Section 10.11)



Sorting
-------

Assume that you have methods called indexOfMaxInRange
and swapElement.  How can you sort the elements of an
array?


Searching
---------

Assume that you have an array of integers that is sorted 
from smallest to largest.

You want to write the indexOf method.  How do you do it?

Linear search?

Bisection search?