Posted: September 13th, 2017

Java

Paper, Order, or Assignment Requirements

Assignment 3
Turtle Programs
We are going to implement part of a program to implement turtle graphics. See the wikipedia
article on turtle graphics (http://en.wikipedia.org/wiki/Turtle_graphics) if you are unfamiliar with
turtle graphics. We will support the turtle moving, turning, and raising/lowering a pen to draw.
We will not support changing the color of the pen. The goal is to create a small language that
kids can use to program the turtle. We will not implement the graphics part of the program. We
are just interested in the section of the program that interprets the language. The language is
given below.
SDSU Mini Turtle Language
The turtle language consists of the following commands.
penUp
When the pen is up and the turtle moves nothing is drawn.
penDown
When the pen is down and the turtle moves a black line is drawn
move X
Move the turtle X units in the current direction. X is a positive integer.
turn X
Turn the direction of the turtle X degrees.
repeat n
statement1
statement2
…
statementJ
end
n is an integer. The statements inside the repeat block are any legal statement in
the language. the repeat statement has the obvious semantics.
$K = Y
Y is an integer. The variable $K can be used in move, turn and repeat commands.
When a program starts the turtle is in the middle of the screen headed horizontally right (zero
degrees) and the pen is up. Here are two sample turtle programs which draw a square.
$length = 10
penDown
move $length
turn 90
move $length
turn 90
move $length
turn 90
move $length
Nearly the same program using the repeat statement.
$length = 10
penDown
repeat 4
move $length
turn 90
end
Turtle Class
The full program will have a Turtle class that draws on the screen. The Turtle class has fields
to hold the current direction (in degrees) of the turtle and the current location (X & Y coordinates).
The class has the following methods.
void move(int distance)
Move the turtle distance units in the current direction and draw on the screen if
the pen is down.
void turn(int degrees)
Add “degrees” to the current heading of the turtle.
void penUp()
Lift the pen up.
void penDown()
Put the pen down.
boolean isPenUp()
Return true if pen is up, false if the pen is down.
int direction()
Returns the current direction of the turtle.
Point location()
Returns the current location of the turtle.
Our goal to is to be able to read and execute a turtle program. The focus is not on the graphics
part. So we will create a mock Turtle class that has the same fields and methods as the turtle
class but will not draw on the screen. All the methods perform the same operation as the real
Turtle class except for the move method. This will allow us to test our code without having to
draw on the screen.
void move(int distance)
Move the turtle distance units in the current direction and updates the field(s) indicating
the current location of the turtle. The method does not draw on the screen.
Geometry & Computing New Location
The turtle starts at the location (0, 0), which if we were going to draw on the screen would be in
the center of the screen. The turtle also starts with a direction of zero degrees with the pen
down. A direction of 0 degrees is to the right parallel to the X-axis. A direction of 90 degrees is
straight up parallel to the Y-axis. A direction of -270 degrees is also the same as a direction of
90 degrees.
Now if the turtle is at (10,20), heading in a direction of 30 degrees and travels 15 units how do
we compute where it ends up? First we have to convert the degrees to radians via the formula:
radians = degrees * pi / 180
Then we can get the deltaX and deltaY using:
deltaX = cosine(radians) * distance
deltaY = sine(radians) distance
So in our example we get
x = 10
y = 20
degrees = 30
distance = 15
radians = pi*degrees/180 => 0.5236
deltaY = sin(radians) * distance => 7.5
deltaX = cos(radians) * distance => 12.9904
newX = x + deltaX => 22.9904
newY = y + deltaY => 27.5
So the turtle will end up at (22.99, 27.5)
The Assignment
1. Using the interpreter pattern we want to be able to read a text file containing a turtle program
and represent the program as abstract syntax trees which we can execute. So if our
file contained the following program after Reading and executing the program the turtle
would end up at (22.99, 27.5).
move 10
turn 90
move 20
turn -60
move 15
2. The problem with using the abstract syntax trees to execute the program is that the program
cannot be stopped or reversed. It is often useful to step through a program. So use
the visitor pattern so we can send a visitor to the parts of the program. After which the visitor
can return a list of commands. Each command will perform one operation in the program.
Using the commands we can then execute the program step by step. The commands
also need to allow us to undo an operation. This will allow us to undo and redo parts of our
program.
3. Create a second visitor that will compute the total distance traveled by the turtle. So for example
given the program below the visitor would compute the distance of 20 even though
the turtle would end up where it started.
move 10
turn 180
move 10
Grading
Item Points
Working Code 10
Unit Tests 10
Proper implementation of Patterns 20 per Pattern (60 points total)
Quality of Code 10