Transcript Karel J Robot Chapter 5 PowerPoint
Karel J. Robot
Chapter 5 Conditionally Executing Instructions 1
That was then…
In the preceding chapters, a Robot’s exact initial situation was known at the start of the task. When we wrote our programs, this information allowed Karel to find beepers and avoid running into walls.
However, these programs worked only in their specific initial situation. If a robot tried to execute one of these programs in a slightly different initial situation, the robot would almost certainly perform an error shutoff.
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This is now.
What a robot needs is the ability to survey its local environment and then decide from that information what to do next.
Now I have a brain!
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if instruction
They provide robots with their decision ability. They allow a robot to test its environment and, depending on the result of the test, decide which instruction to execute next.
The if instructions enable us to write much more general programs for our robots that accomplish the same task in a variety of similar, but different, initial situations.
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The General Form Of An if
if (
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How does it work?
1.
2.
3.
Robot checks to see if
Note about
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New Class of Robot
public class Robot extends UrRobot { public boolean frontIsClear(); public boolean nextToABeeper(); Use these public boolean nextToARobot(); method public boolean facingNorth(); names as the
Class Robot
Inherits all instructions from UrRobot We will use the Robot class as our base robot from now on The new methods of the class are
predicates
Preceded by the
boolean
return type These methods
return
either
true
or
false
(functions such as
move() void
or
turnLeft()
have a return type, because they return no information to the robot; merely cause the robot to behave in a particular way) 8
Example
if (nextToABeeper()) { pickBeeper(); } turnLeft(); 1 9
Examples
if (frontIsClear() ) { move(); // no danger of hitting wall } if ( anyBeepersInBeeperBag() ) { putBeeper(); // no danger of error } 10
Negative conditions?
Suppose we want a negative form of a predicate?
We can precede a predicate with the negative operator
!
if (! frontIsClear()) { turnLeft(); } move();
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Writing New Predicates
Since the predicates return a boolean value (true or false), we need to use the return instruction.
Return instructions are only legal in predicates. They can’t be used in ordinary (void) methods, nor in the main task block.
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Example
public class CheckerRobot extends Robot { // insert constructor here public boolean frontIsBlocked() { return ! frontIsClear(); } public boolean notNextToABeeper() { return ! nextToABeeper(); } } 13
The General Form Of An if/else
if (
How does it work?
1.
2.
3.
Robot checks to see if
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You try
Write a new predicate
leftIsBlocked
that determines whether there is a wall exactly one-half block away on a robot’s left. Be sure that when it terminates, the robot is on the same corner and facing in the same direction.
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Extending Robot
public class BiggerBrains extends Robot { // constructor here public boolean beeperIsToLeft() {…} try writing these predicates public void faceEast() {…} public boolean twoBeepersOnCornerOrMore() {…} } 17
public boolean beeperIsToLeft () { 18
public boolean beeperIsToLeft () { turnLeft(); move(); if ( nextToABeeper() ) { MUST put world back in initial situation that it was in BEFORE the function was called turnLeft(); turnLeft(); move(); turnLeft(); return true; } turnLeft(); turnLeft(); move(); turnLeft(); return false; } 19
public void faceEast () 20
public void faceEast () { if (facingWest()) { turnLeft(); turnLeft(); } else if (facingNorth()) { turnRight(); } else if (facingSouth()) {turnLeft(); } } public void faceEast () { if (!facingEast()) turnLeft(); if (!facingEast()) turnLeft(); if(!facingEast()) turnLeft(); } 21
public boolean twoBeepersOnCornerOrMore () { 22
public boolean twoBeepersOnCornerOrMore () { if (!nextToABeeper()) return false; else { pickBeeper(); if (nextToABeeper()) { putBeeper(); } else return true; { putBeeper(); return false; } } } 23
Simplify – bottom factoring
if ( facingSouth() ) { turnLeft(); move(); } else { turnRight(); move(); } 24
Simplify – top factoring
if ( beeperOnLeft() ) { move(); turnLeft(); } else { move(); turnRight(); } 25
Problems p.25 #2-5, 7, 8
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Sparse Harvester
The field to be harvested has been hit by a storm, so that not every corner has a beeper. Let us use conditionals to modify Harvester to solve this problem. Note: This is an advantage of using object oriented programming. The previously written Harvest class can be reused. All we need to do is create a new version of the harvestCorner method in a new subclass of Harvester. (see Chapter 3 p. 19) 27
Hurdle Jumper
We want to program the robot to run a mile long hurdle race, where vertical wall sections represent hurdles. The hurdles are only one block high and are randomly placed between any two corners in the race course. One of the many possible race courses for this task is in Figure 5-2 on page 12. Require the robot to jump if, and only if, faced with a hurdle.
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SteepleChaser
#6.
Program a robot to run a mile-long steeplechase. The steeplechase course is similar to the hurdle race, but here barriers can be one, two, or three blocks high.
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MazeWalker
#9.
Write an instruction followWallRight for the MazeWalker class, assuming that whenever a robot executes this instruction there is a wall directly to the right. Figure 4-6 shows four of the different position changes that the robot must be able to make. This instruction is the cornerstone for a program that directs a robot to escape from a maze (next chapter) 30
Carpet Layer
#11.
A robot has been hired to carpet some “small rooms” along a one-mile section of its world. A small room is a corner that has a wall segment immediately to the west, north, and east. The door is to the south. Karel is to put a single beeper in only the small rooms and no other corners. Figure 4-8 shows one set of initial and final situations. You may assume that Karel has exactly eight beepers in its beeper-bag.
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Boolean Operators
Pascal (
and
,
or
,
not
) C++/ Java (
&&
,
||
,
!
)
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AND operator
p t t false otherwise “Truth table” q p and q t f f f operates on two boolean values (ie, frontIsClear, etc) will evaluate to true if
both
of the values are true.
t f 33
AND operator
p t t false otherwise “Truth table” q p and q f t t f f f operates on two boolean values (ie, frontIsClear, etc) will evaluate to true if
both
of the values are true.
t f f f 34
OR operator
operates on two boolean values (ie, frontIsClear, etc) will evaluate to true if
at least one
are true.
of the values f t p t f false otherwise “Truth table” t q p or q f f t 35
OR operator
operates on two boolean values (ie, frontIsClear, etc) will evaluate to true if
at least one
are true.
of the values f t p t f false otherwise “Truth table” t q p or q t t f f t t f 36
isBeepOnLeftANDisBeepOnRight()
Write a predicate which returns true if there is at least one beeper on both sides, false otherwise.
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isBeepOnLeftANDisBeepOnRight()
Write a predicate which returns true if there is at least one beeper on both sides, false otherwise.
public boolean isBeepOnLeftANDisBeepOnRight() { if (isBeepOnLeft()) if (isBeepOnRight()) { return true; } return false; } 38
isBeepOnLeftORisBeepOnRight()
Write a predicate which returns true if there is at least one beeper on both sides, false otherwise.
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isBeepOnLeftORisBeepOnRight()
Write a predicate which returns true if there is at least one beeper on both sides, false otherwise.
public boolean isBeepOnLeftORisBeepOnRight() { if (isBeepOnLeft()) { return true; } if (isBeepOnRight()) { return true; } return false; } 40
#13
Write a predicate that will return true if and only if the robot executing it is both next to a beeper AND its left is blocked.
Write a predicate that will return true if the robot executing it is either next to a beeper OR its left is blocked.
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#14
Write a predicate that will return true if and only if the robot executing it has exactly two beepers in its beeper-bag.
Write a predicate that will return true if and only if the robot executing it is on a corner with at most two beepers.
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