Transcript Iteration (Loops) - AP Computer Science

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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Ch. 6
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???
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Ch. 6
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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Ch. 6
for Loops
• General form:
for ( <initialize variable>;
<test>;
<increment> )
{
<instruction set>
}
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Ch. 6
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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Ch. 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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Ch. 6
for Loops (cont.)
Flow of for loops:
1. Initialization statement executes
2. If test is true, proceed to step 3; if test is
false, go to step 6
3.
4.
5.
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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Ch. 6
while Loops
• General form:
while ( <test> )
{
<instruction list>
}
• Loop continues until test is false
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Ch. 6
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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Ch. 6
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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Ch. 6
while Loops (cont.)
Flow of while loops
1. If test is true, proceed to step 2; if test is false,
go to step 4
2. Instructions in body of loop are executed
3. Go to step 1
4. Statements after loop are executed
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Ch. 6
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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Ch. 6
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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Ch. 6
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 4step process now…
while (???) {
???
}
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Ch. 6
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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Ch. 6
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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Ch. 6