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.
2
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!
3
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 ( <test>)
{
<instruction list>
// otherwise known as the
// “then” instruction
}
Note the indentation, the curly braces, and no semicolon
after (<test>).
There will be semicolons in the instruction list.
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How does it work?
1.
2.
3.
Robot checks to see if <test> is true or false
based on the robot’s current situation
If the <test> is true, the robot executes the
<instruction list>
If the <test> is false, the robot skips the
<instruction list>.
Note about <test>: The state of the robot doesn’t
change, but the sender of the message will
obtain information about the state of the robot.
6
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 <test>
public boolean facingSouth(); condition
public boolean facingEast();
public boolean facingWest();
public boolean anyBeepersInBeeperBag();
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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() or turnLeft() have a
void 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();
11
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.
12
Example
public class CheckerRobot extends Robot
{ // insert constructor here
public boolean frontIsBlocked()
{ return ! frontIsClear();
}
public boolean notNextToABeeper()
{ return ! nextToABeeper();
}
}
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The General Form Of An if/else
if ( <test>)
{
<instruction list 1>
}
else
{
<instruction list 2>
}
Used when you
have to execute
one of two
alternatives
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How does it work?
Robot checks to see if <test> is true or
false based on the robot’s current situation
2. If the <test> is true, the robot executes the
<instruction list 1>
3. If the <test> is false, the robot executes
the <instruction list 2>.
1.
15
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.
16
Extending Robot
public class BiggerBrains extends Robot
{ // constructor here
public boolean beeperIsToLeft()
{…}
try writing these predicates
public void faceEast()
{…}
public boolean twoBeepersOnCornerOrMore()
{…}
}
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public boolean beeperIsToLeft()
{
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public boolean beeperIsToLeft()
{
MUST put world
back in initial
turnLeft();
situation that it
move();
was in BEFORE
the function was
if ( nextToABeeper() )
called
{
turnLeft(); turnLeft(); move(); turnLeft();
return true;
}
turnLeft(); turnLeft(); move(); turnLeft();
return false;
}
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public void faceEast()
20
public void faceEast() public void faceEast()
{ if (facingWest())
{ if (!facingEast())
{ turnLeft();
turnLeft();
turnLeft();
if (!facingEast())
}
turnLeft();
else if (facingNorth())
if(!facingEast())
{ turnRight();
turnLeft();
}
}
else if (facingSouth())
{turnLeft();
}
}
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public boolean
twoBeepersOnCornerOrMore()
{
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public boolean twoBeepersOnCornerOrMore()
{ if (!nextToABeeper())
return false;
else
{ pickBeeper();
if (nextToABeeper())
{ putBeeper();
return true;
}
else
{ putBeeper();
return false;
}
}
}
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Simplify – bottom factoring
if ( facingSouth() )
{
turnLeft();
move();
}
else
{
turnRight();
move();
}
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Simplify – top factoring
if ( beeperOnLeft() )
{
move();
turnLeft();
}
else
{
move();
turnRight();
}
25
Problems p.25 #2-5, 7, 8
26
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)
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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)
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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 (&&, ||, !)
32
AND operator
p
t
t
f
f
operates on two boolean values (ie, frontIsClear,
etc)
will evaluate to true if both of the values are true.
false otherwise
“Truth table”
q
p and q
t
f
t
f
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AND operator
p
t
t
f
f
operates on two boolean values (ie, frontIsClear,
etc)
will evaluate to true if both of the values are true.
false otherwise
“Truth table”
q
p and q
t
t
f
f
t
f
f
f
34
OR operator
p
t
t
f
f
operates on two boolean values (ie, frontIsClear,
etc)
will evaluate to true if at least one of the values
are true.
false otherwise
“Truth table”
q
p or q
t
f
t
f
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OR operator
p
t
t
f
f
operates on two boolean values (ie, frontIsClear,
etc)
will evaluate to true if at least one of the values
are true.
false otherwise
“Truth table”
q
p or q
t
t
f
t
t
t
f
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;
}
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isBeepOnLeftORisBeepOnRight()
Write a predicate which returns true if there
is at least one beeper on both sides, false
otherwise.
39
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;
}
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#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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