1. Karel – Chapter 6 Instructions That Repeat
Note: Original slides provided by and modified for Mr. Smith’s AP Computer Science A class

2. Iteration Graphic Organizer
Control structures
Methods
Sequential
Iteration
Conditional
Types of control structures
Types of loops
Java statement
Syntax
Examples:

3. Iteration (Loops) Loops repeat a set of instructions
Two types of loops:
Definite loops ( for loops ) perform instructions an explicit number of times
Indefinite loops ( while loops ) perform instructions an indefinite number of times (until a certain test fails)

4. Definite vs. Indefinite Loop?
A new class you’re writing has a method with the following signature:
public void move (int numMoves)
Should it be implemented with a definite loop or an indefinite loop? Justify your answer.
Could it be implemented with the other type of loop? If so, should we? Is it a good idea or bad idea?
- should use a definite loop (for)
- we know in advance exactly how many times (numMoves number of times) that we should move - therefore, definite
- yes, we could use an indefinite loop - the loops are always interchangable - that doesn’t mean we should use whatever
we feel like - I can, for example, use a sledge hammer to pound in a thumb tack - but it’s not the right tool. recognize
patterns in cs and solve using a standard technique/tool.
ok - i’ve heard the argument that always using a for loop is good because your initialization/increment/test are all located
in one location (namely, between the parentheses). fine. i see merit. but, for now - in the beginning - i teach patterns.
later, i change my mind and do things like:
for (Iterator iter=someArrayList.iterator(); iter.hasNext(); )

5. Iteration Graphic Organizer
Control structures
Methods
Sequential
Iteration
Conditional
Types of control structures
Definite Loops
Types of loops
Indefinite Loops
for
Java statement
while
Syntax
Examples:

6. for Loops General syntax:
for ( <initialize variable>; <test>; <increment> ) { <instruction set> }

7. for Loops (cont.)
<initialize variable> sets a variable/identifier to a certain value (variable is usually count, i, j)
<test> is a test that is evaluated each time the body is about to be executed (when false, the loop is exited)
<increment> changes the loop control variable (usually i++ or i--). The incrementing is done at the bottom of the loop.
i++ means “add 1 to i”.
i-- means “subtract 1 from i”.

8. for Loops (cont.)
Example: for (int i=1; i<=3; i++) { move(); putBeeper(); }
This causes the robot to move and place a beeper down on 3 different corners
Note: Write this on the board so that we can refer to it on next slide

9. for Loops (cont.) Flow of for loops: Initialization statement executes
If test is true, proceed to next step; if test is false, go to step 6
Instructions in body of loop are executed
Increment statement executes
Return to step 2
Program proceeds to statements after loop

10. Iteration Graphic Organizer
Control structures
Methods
Sequential
Iteration
Conditional
Types of control structures
Definite Loops
Types of loops
Indefinite Loops
for
Java statement
while
initialize variable
test
increment
Syntax
Examples:
for ( int i=1; i<=3; i++ ) {
move();
putBeeper(); }

11. Move a specified number of times
Example: public void move (int numMoves) {
for (int i=1; ___________; i++)
super.move(); }
This causes the robot to move a specified number of times
for (int i=1; i<=numMoves; i++)

12. Try this yourself
Write a for loop to pick up a beeper (if there is one) on the four street corners directly in front of the robot:
for (int i=1; i<=4; i++) { move(); if ( nextToABeeper() )
pickBeeper(); }

13. BeeperSquare Exercise

14. BeeperSquare Exercise
Create a BeeperSquare robot class that extends SmartBot or Robot (you may want to use some of its methods)
Create a method named drawSquare() that uses at least one “for” loop (two “for” loops would be better) that will create a square with 6 beepers on each side. Use a single robot to do this. The end result should be a square containing 20 beepers (only one beeper on each street corner).
Create a client program that will test this new class. Try constructing two robots and drawing 2 separate squares in the world (overlapping the two).

15. while Loop Situation
Team up with your neighbor for a couple minutes and discuss a situation that would necessitate that you use a while (indefinite) loop to solve the problem
Note: We might use one or two of these as examples later on when we are discussing while loops.

16. while Loops
General form: while ( <test> ) { <instruction list> }
Loop continues until test is false

17. while Loops (cont.) Example: while (frontIsClear()) { move(); }
Causes the robot to move continuously until there is a wall in front of it

18. while Loops (cont.) Flow of while loops:
If test is true, proceed to step 2; if test is false, go to step 4
Instructions in body of loop are executed
Go to step 1
Statements after loop are executed

19. Iteration Graphic Organizer
Control structures
Methods
Sequential
Iteration
Conditional
Types of control structures
Definite Loops
Types of loops
Indefinite Loops
for
Java statement
while
initialize variable
test
increment
Syntax
test
Examples:
for ( int i=1; i<=3; i++ ) {
move();
putBeeper(); }
while ( frontIsClear() ) {
move(); }

20. Building a While Loop The following is a more systematic way of creating while loops. This will be extremely helpful to you at various times in the course and on the AP exam. Example: the robot moves until it comes to a beeper and then picks it up
Step 1 – Identify what is true when the loop is finished (this is always easier than determining what is false while it is still executing)
Step 2 – Use the opposite form of step 1’s result as the <test> for the loop. You just need to negate the entire thing. (note: Now and again you might find that “DeMorgan-izing” your result is helpful when answering AP-like questions.)
Step 3 – make progress toward the goal (completion of the loop) within the loop
nextToABeeper()
while ( ! nextToABeeper() )
move();

21. Building a while Loop (cont’d)
Step 4 – Do whatever is required before and/or after the loop to ensure that we solve the given problem
Putting it all together using this 4-step approach:
while ( ! nextToABeeper() ) { move(); } pickBeeper();
Another Example: Pick all beepers from a corner without knowing how many there are.
pickBeeper();

22. while Loop Exercise
Back on an earlier slide, you drew up a situation that would best be solved using a while loop - now write the code to solve your problem
If you did not come up with a problem, use the following situation:
The robot should lay down a single beeper on each street corner in a straight line in the direction it is facing until it comes to a wall. In this case it should stop and turn off.
To test this, create a robot on street corner (4, 8) which is facing west, and has 10 beepers in its beeper bag.

23. DeMorgan’s Law
De Morgan's laws are rules relating the logical operators “and" and “or" in terms of each other via negation.
In another form:
NOT (a AND b) = (NOT a) OR (NOT b)
NOT (a OR b) = (NOT a) AND (NOT b)
In java, this corresponds to:
! (a && b) is the same as !a || !b
! (a || b) is the same as !a && !b

24. DeMorgan’s Law Useful in Step 2 for negating the end situation
We want to loop until a wall is in front AND a beeper is on the current corner
! frontIsClear() && nextToABeeper()
Use DeMorgan’s Law to negate this. Negate the two predicates and replace && with || :
while ( frontIsClear || !nextToABeeper() )

25. Let’s build a while loop
Move a robot until it is not on a corner with any other robots and there is a wall on the left, then pick up the beeper that will be waiting there
(the “hard” part is getting the condition correct without using a trial-and-error method – you don’t get to run any programs during our tests nor on the AP exam – also, you’ll code your labs faster if you have a way to ensure correct conditions EVERY time – without the crutch of running your code)
You may use && and || for this exercise. Use the 4-step process now…
while (???) {
??? }

26. You try the while condition
What is true when the loop is finished?
!nextToARobot() && !leftIsClear()
Use the opposite form of step 1’s result as the <test> for the loop
while (nextToARobot() || leftIsClear())
Make progress toward the goal within loop
move();
Do whatever is required before and/or after the loop to ensure that we solve the given problem
pickBeeper();

27. You try the while condition
You may use && and || for this
Write a loop that moves a robot forward until it is either next to a robot, next to a beeper, or there is no wall on its right
while ( ??? ) {
??? }

28. You try the while condition
What is true when the loop is finished?
nextToARobot() || nextToABeeper() || rightIsClear()
Use the opposite form of step 1’s result as the <test> for the loop
while (!nextToARobot() && !nextToABeeper() && !rightIsClear())
Make progress toward the goal within loop
move();
Do whatever is required before and/or after the loop to ensure that we solve the given problem
Nothing required

29. Fence Post Problem
This is a situation where we must perform some (but not all) of the statements in the loop either before or after the loop to solve the given problem. This is because the instructions in the loop do not entirely solve the problem for either the first time through the loop or the last time through the loop.
This is called the Fence Post Problem because if we order 3 fence sections, we actually need 4 fence posts.
What if we have five beepers in a row. The robot starts on the same corner as the first beeper. It needs to pick up all 5 beepers and it needs to end up at the corner of the last beeper. How would we go about doing this?
for (int i=1; i<=4; i++) {
pickBeeper();
move(); }

30. Fence Post Problem move() move() move() move() pickBeeper()

31. Infinite Loops
In a for or while loop, it is possible for the test to never be false
When this happens, the loop continues infinitely
Depending on the compiler, application, and other circumstances, an error may occur or the app may crash with a memory error