Building Your Own Turtle
Functions
For making really cool pictures!
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5 Cool Things…
1.
2.
3.
4.
5.
Variables and Arithmetic
Branches for variety
Using Functions
Building your own Functions (Today)
Loops to repeat (Next week)
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Review:
1) Variables and Arithmetic
var n = 4;
var y = 2;
var x = n + y;
y = n * 2;
document.write(“x=” + x + “, y=” + y);
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2) Using Functions
Identify the Function
calls and the arguments
Math Functions:
x = Math.sqrt(81);
y = Math.sqrt(x);
document.write(“x=” + x + “, y=” + y);
Turtle Functions:
forward(20);
left(90);
forward(20);
right(135);
backward(40);
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Which command(s) uses “relative”
positioning, and which “absolute”
forward -- move turtle forward some number of
pixels
left – turn left some number of degrees
moveTo -- move to an x,y coordinate
turnTo – turn to a particular degree heading
home – send turtle back to center of screen,
facing up
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Today—3) Building your own functions
Lets you “abstract” a collection of moves
For example, these lines make a square:
forward(20);
forward(20);
forward(20);
forward(20);
right(90);
right(90);
right(90);
right(90);
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If you want to draw a lot of squares,
put this at the top of your script…
function square( )
{
forward(40); right(90);
forward(40); right(90);
forward(40); right(90);
forward(40); right(90);
}
This is a Function
Definition
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Now you can ‘call’ your square
function
square();
-------------------left(30); square();
left(180); square();
moveTo(-300, -100);
left(30); square();
left(180); square();
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Functions help manage complexity
You can do interesting patterns without a lot
of repetition of code
They save time and effort
Functions can use other functions
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What if you want different sizes of
squares?
n is called a parameter
function square( n )
{
forward(n); right(90);
forward(n); right(90);
forward(n); right(90);
forward(n); right(90);
}
It’s a variable that receives
the size of the square
(given as the argument in
the function call)
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Now when you call the square function,
just say how big you want it to be
square(100);
square(50);
square(25);
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You can generate random sized
squares…
var size, angle;
size=rand(50,100); square(size);
angle=rand(0,180); turnTo(angle);
size=rand(50,100); square(size);
rand(low, high)
gives random number between low and high
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Every time you run the code from last
slide, it gives a different result
It’s interesting to observe the variations and similarities
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Capture your images in a “screen shot”
Press Alt and PrtScr at same time
Open Paint
Edit/Paste
You can chop out image using select tool
Dotted line box
Then paste into Microsoft Word
Or save as a .jpg file (project 2)
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That’s it! Have fun in lab
Next week…branches and loops
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