1. Iteration (Loops) Loops repeat a set of instructions
Two types of loops:
Definite loops ( for ) perform instructions explicit number of times
Indefinite loops ( while ) perform instructions an indefinite number of times (until a certain test fails)
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2. Questions for You
a new class you’re writing has a method with the following signature:
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/could we/good idea/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(); )
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3. Questions for You
take 5 minutes and draw a situation on paper that would necessitate that you use a while loop to solve the problem
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4. for Loops
General form: for ( <initialize variable>; <test>; <increment> ) { <instruction set> }
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5. 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, loop is exited)
<increment> changes the loop control variable (usually x++ or x--)
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6. for Loops (cont.) Example: for (int x=1; x<=5; x++) { karel.move();}
This causes karel to move 5 times
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7. for Loops (cont.) Flow of for loops: Initialization statement executes
If test is true, proceed to step 3; 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
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8. while Loops
General form: while ( <test> ) { <instruction list> }
Loop continues until test is false
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9. Questions for You
back on slide 3 you drew up a situation that would best be solved using a while loop - now write the code to solve your problem
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10. while Loops (cont.)
Example: while (karel.frontIsClear ()) { karel.move (); }
Causes karel to move continuously until there is a wall in front of it
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11. 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
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12. Building a While Loop Formalizing (making easier) the ad-hoc way you’ve been writing while loops to this point. This will be extremely helpful to you at various times in the course and on the AP exam.
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.)
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13. Building a While Loop (cont’d)
Step 3 – make progress toward the goal (completion of the loop) within the loop
Step 4 – Do whatever is required before and/or after the loop to ensure that we solve the given problem
Example: Pick all beepers from a corner without knowing how many there are.
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14. 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 (???) {
??? }
Assume existence of method leftIsBlocked()
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15. Your 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 ( ??? ) {
??? }
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16. 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
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