CSC153 2004S, Class 33: Loops

Admin:
* Cool convo tomorrow
* No homework yet
* Lab-based day
* Add the following line to the end of your .bashrc (e.g., with "gedit .bashrc")
	export PATH="$PATH:/usr/java/j2sdk1.4.1_03/bin"
  and then open a new terminal window.  From now on, you can use javac and java without doing the extra stuff

Overview:
* Lab: J5.1-J5.3 and (maybe) J5.7

Observation: Many algorithms require repetition
* One good mechanism for repetition: Recursion
* Another less-good mechanism for repetition: Goto
* Some folks dislike recursion
* In the early 1960's, some computer scientists designed new ways of referring to repetition "looping structures"
* Three (more or less) basic looping structures
  * For loops: Fixed number of repetitions, usually with an associated counter variable
  * While loops: Indeterminate number of repetions, with the test before the fisrt repetition
  * Repeat loops (Do loops):  Interminate number of repetions > 0, with the test after the first repetion

In Java (and C, and C++, and most other C-based languages),

While loops:

  while (TEST) {
    BODY
  }

Meaning:
  1. Evaluate test
  2. If test holds, evaluate BODY and go to line 1.
  3. Otherwise, "exit the loop"; Go on to the subsequent statement

For loops:

  for(INITIALIZE; TEST; INCREMENT) {
    BODY
  }

Meaning

  INITIALIZE;
  while (TEST) {
    BODY;
    INCREMENT;
  }

Most common for loops

  for (VAR = INITIAL; VAR < TERMINATE; ++VAR) {
    BODY
  }

"Real" C programmers prefer that their for loops not have bodies
Programmers who want others to read and use their code try to write more
readable code.

I never remember the repeat-like loops

	do {
	  BODY
	} while (TEST)

Nearly equivalent to

	BODY
	while (TEST) {
	  BODY
	}

Applets in Java
* Need a "draw" method that draws on the screen.
* Need an accompanying HTML file to set up the size of the screen
* Run with appletviewer