Transcript Karel – Chapter 6 Instructions That Repeat Note: Original slides provided by

Karel – Chapter 6
Instructions That Repeat
Note: Original slides provided by www.apComputerScience.com
and modified for Mr. Smith’s AP Computer Science A class
1
Iteration
Graphic Organizer
Control structures
Methods
Sequential
Iteration
Conditional
Types of control structures
Types of loops
Java statement
Syntax
Examples:
2
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)
3
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?
4
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:
5
for Loops
General syntax:
for ( <initialize variable>; <test>; <increment> )
{
<instruction set>
}
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, 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”.
7
for Loops (cont.)
• Example:
for (int i=1; i<=3; i++)
{
Note: Write this on the board so that
we can refer to it on next slide
move();
putBeeper();
}
• This causes the robot to move and place
a beeper down on 3 different corners
8
for Loops (cont.)
Flow of for loops:
1. Initialization statement executes
2. If test is true, proceed to next step; if test
is false, go to step 6
3. Instructions in body of loop are executed
4. Increment statement executes
5. Return to step 2
6. Program proceeds to statements after loop
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Iteration
Graphic Organizer
Control structures
Methods
initialize variable
Examples:
Sequential
Iteration
Conditional
Types of control structures
Definite Loops
Types of loops
Indefinite Loops
for
Java statement
while
test
for ( int i=1; i<=3; i++ )
{
move();
putBeeper();
}
increment
Syntax
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Move a specified number of times
• Example:
public void move (int numMoves)
{
for (int i=1; ___________;
i<=numMoves; i++)
{
super.move();
}
}
• This causes the robot to move a specified
number of times
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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();
}
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BeeperSquare Exercise
13
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).
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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.
15
while Loops
• General form:
while ( <test> )
{
<instruction list>
}
• Loop continues until test is false
16
while Loops (cont.)
• Example:
while (frontIsClear())
{
move();
}
• Causes the robot to move continuously
until there is a wall in front of it
17
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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Iteration
Graphic Organizer
Control structures
Methods
initialize variable
Examples:
Sequential
Iteration
Conditional
Types of control structures
Definite Loops
Types of loops
Indefinite Loops
for
Java statement
while
test
for ( int i=1; i<=3; i++ )
{
move();
putBeeper();
}
increment
Syntax
test
while ( frontIsClear() )
{
move();
}
19
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)
nextToABeeper()
• 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.)
while ( ! nextToABeeper() )
• Step 3 – make progress toward the goal (completion of the
loop) within the loop
move();
20
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
pickBeeper();
• 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.
21
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.
22
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
23
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() )
24
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 (???)
{
???
}
25
You try the while condition
1) What is true when the loop is finished?
!nextToARobot() && !leftIsClear()
2) Use the opposite form of step 1’s result as
the <test> for the loop
while (nextToARobot() || leftIsClear())
3) Make progress toward the goal within loop
move();
4) Do whatever is required before and/or
after the loop to ensure that we solve the
given problem
pickBeeper();
26
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 ( ??? )
{
???
}
27
You try the while condition
1) What is true when the loop is finished?
nextToARobot() || nextToABeeper() || rightIsClear()
2) Use the opposite form of step 1’s result as the
<test> for the loop
while (!nextToARobot() && !nextToABeeper() &&
!rightIsClear())
3) Make progress toward the goal within loop
move();
4) Do whatever is required before and/or after
the loop to ensure that we solve the given
problem
Nothing required
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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();
}
pickBeeper();
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pickBeeper()
move()
pickBeeper()
move()
pickBeeper()
move()
pickBeeper()
move()
pickBeeper()
Fence Post Problem
30
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
31