4 Racket Turtle examples

4.1 Drawing a square

First you need to define a list of turtle commands for drawing a square. We will name it square1 . Actual drawing of the turtle image is done using draw function, the turtle command list is given as an argument to draw.

(define square1

(list (forward 100)

(turn-left 90)

(forward 100)

(turn-left 90)

(forward 100)

(turn-left 90)

(forward 100)))

4.2 Drawing a square using repeat

Since the command list for drawing a square has a repeating pattern, it is better to define it separately and use it repeatedly. We will name it side and we will repeat it 4 times using repeat.

4.3 Drawing two squares in a same picture

We can use a previously defined square and draw two of those in the same picture. We will define a list of commands move to move to a new location and changes the pen color before drawing the second square.

(define move

(list (pen-up)

(turn-right 90)

(forward 100)

(pen-down)

(change-color "red")))

(define two-squares

(list square1

move

square1))

4.4 Drawing a square using a function

To be able to draw squares with changing side lengths, we need to define a function, which we will name changing-square. We need also a helper function changing-side to draw sides with changing lengths. The variable x is the side length. Finally we call the new function changing-square with argument 30 (x=30).

(define (changing-side x)

(list (forward x)

(turn-left 90)))

(define (changing-square x)

(repeat 4 (changing-side x)))

4.5 Drawing a square using coordinates

You can draw a square also by ordering the turtle to go via some points in the coordinate plane using go-to commands. To get easier coordinates we change the place of the origin with set-origin command.

(define coordinate-square

(list (set-origin)

(go-to 100 0)

(go-to 100 100)

(go-to 0 100)

(go-to 0 0)))

4.6 Mirroring a square

We can draw two square so that the second one is created by mirroring the first one using a pivot point. The pivot point is the location where the mirroring was turned on (mirron-x-on, mirron-y-on), in this example it is the starting point.

(define mirroring-square

(list (mirror-x-on) (mirror-y-on)

square1))

4.7 Drawing a square using a stamper

We can activate the stamper functionality using stamper-on command. Now the turtle will "stamp" the given image after each movement. Here we use a red circle as the stamp. You could also let the pen draw the line also (here is it taken up).

(define STAMP (circle 5 "solid" "red"))

(define stamper-square

(list (stamper-on STAMP)

(pen-up)

square1))

4.8 Changing pen style and size

You can change the style of the line using change-pen-style command and the width of the line using change-pen-size command.

Note! This doesn’t work in WeScheme.

(define special-pen-square

(list (change-pen-size 5)

(change-pen-style "dot")

square1))

4.9 Drawing a line in a grid

First we draw a background grid using set-bg-grid. We move to the origin using (go-to 0 0) and activate the stamper with a blue circle as the stamp. To get the stamps in the right coordinates we need to command the turtle using go-to multiple times. In this example the stamper is inactivated after four points .

(define line-with-grid

(list (set-bg-grid 20 20 "pink")

(pen-up)

(go-to 0 0)

(stamper-on (circle 5 "solid" "blue"))

(pen-down)

(go-to 40 40)

(go-to 80 80)

(go-to 120 120)

(go-to 160 160)

(stamper-off)

(go-to 500 500)))

4.10 Drawing a line with multiple stamps

We move to origin using go-to-origin command and draw a line using a list of stamps.

(define STAMPS

(list (circle 10 "solid" "red")

(star 10 "solid" "blue")

(circle 10 "solid" "green")

(star 10 "solid" "yellow")

(circle 10 "solid" "black")))

(define line-with-stamps

(list (pen-up)

(go-to-origin)

(turn-right 45)

(stamper-on STAMPS)

(repeat 8 (forward 50))))

4.11 Changing background color, background image and animation size

You can change the background color using change-bg-color color and you can put an additional image on top of it using set-bg-image. In the example we want to draw a picture with different dimensions so draw-custom is used (setting the drawing speed to zero will not affect the drawing speed).

(define square-over-bg

(list (change-bg-color "black")

(set-bg-image (circle 100 "solid" "gold"))

square1))

4.12 A line with changing colors

In this example we set the pen color to be a list of colors. Turtle will use each color one after another. The animation speed has been set to be slower (the last argument of draw-custom). The turtle is also hidden during the first part of the animation.

(define COLORS

(list "red" "blue" "green" "yellow" "purple"))

(define color-line

(list (pen-up)

(go-to 0 0)

(turn-right 45)

(pen-down)

(change-color COLORS)

(change-pen-size 10)

(hide-turtle)

(repeat 8 (forward 40))

(show-turtle)

(repeat 8 (forward 40))))