1 Traditional Turtles

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1 | Traditional Turtles
2 | Value Turtles

► | 1 | Traditional Turtles

1.1 | Examples

On this page:

turtles
move
draw
erase
move-offset
draw-offset
erase-offset
turn
turn/ radians
clear
home
split
split*
tprompt
save-turtle-bitmap
turtle-window-size
1.1 Examples
regular-poly
regular-polys
radial-turtles
spaced-turtles
spokes
gapped-lines
spyro-gyra
neato
graphics-bexam
sierp-size
sierp
sierp-nosplit
koch-size
koch-split
koch-draw
lorenz
lorenz1
peano
peano-position-turtle
peano-size
fern-size
fern1
fern2

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## 1 Traditional Turtles🔗ℹ

(require graphics/turtles) | package: htdp-lib

To use any of the turtle drawing functions, you first need to initialize the turtles by calling (turtles #t).

> procedure (turtles on?) → void?
> on? : any/c
> (turtles) → void?

Shows and hides the turtles window based on on?. If on? is not supplied, the state is toggled.

> procedure (move n) → void?
> n : real?

Moves the turtle n pixels without drawing.

> procedure (draw n) → void?
> n : real?

Moves the turtle n pixels and draws a line on the path.

> procedure (erase n) → void?
> n : real?

Moves the turtle n pixels and erase along the path.

> procedure (move-offset h v) → void?
> h : real?
> v : real?
> procedure (draw-offset h v) → void?
> h : real?
> v : real?
> procedure (erase-offset h v) → void?
> h : real?
> v : real?

Like move, draw, and erase, but using a horizontal and vertical offset from the turtle’s current position.

> procedure (turn theta) → void?
> theta : real?

Turns the turtle theta degrees counter-clockwise.

> procedure (turn/radians theta) → void?
> theta : real?

Turns the turtle theta radians counter-clockwise.

> procedure (clear) → void?

Erases the turtles window.

> procedure (home) → void?

Leaves only one turtle, in the start position.

> syntax (split expr ...)

Spawns a new turtle where the turtle is currently located. In order to distinguish the two turtles, only the new one evaluates expr. For example, if you start with a fresh turtle-window and evaluate:

> (split (turn/radians (/ pi 2)))

you will have two turtles, pointing at right angles to each other. Continue with

> (draw 100)

You will see two lines. Now, if you evaluate those two expression again, you will have four turtles, etc.

> syntax (split* expr ...)

Like (split expr ...), except that one turtle is created for each expr.

For example, to create two turtles, one pointing at π/2 and one at π/3, evaluate

> (split* (turn/radians (/ pi 3)) (turn/radians (/ pi 2)))

> syntax (tprompt expr ...)

Limits the splitting of the turtles. Before expr is evaluated, the state of the turtles (how many, their positions and headings) is “checkpointed.” Then expr is evaluated, and then the state of the turtles is restored, but all drawing that may have occurred during execution of expr remains.

For example

> (tprompt (draw 100))

moves a turtle forward 100 pixel while drawing a line, and then moves the turtle be immediately back to its original position. Similarly,

> (tprompt (split (turn/radians (/ pi 2))))

splits the turtle into two, rotates one 90 degrees, and then collapses back to a single turtle.

The fern functions below demonstrate more advanced use of tprompt.

> procedure (save-turtle-bitmap name kind) → void?
> name : (or/c path-string? output-port?)
> kind : (or/c 'png 'jpeg 'xbm 'xpm 'bmp)

Saves the current state of the turtles window in an image file.

> value turtle-window-size : exact-positive-integer?

The size of the turtles window.

### 1.1 Examples🔗ℹ

(require graphics/turtle-examples) | package: htdp-lib

The graphics/turtle-examples library’s source is meant to be read, but it also exports the following examples. To display these examples, first initialize the turtle window with (turtles #t).

> procedure (regular-poly sides radius) → void?
> sides : exact-nonnegative-integer?
> radius : real?

Draws a regular poly centered at the turtle with sides sides and with radius radius.

> procedure (regular-polys n s) → void?
> n : exact-nonnegative-integer?
> s : any/c

Draws n regular polys each with n sides centered at the turtle.

> procedure (radial-turtles n) → void?
> n : exact-nonnegative-integer?

Places 2n turtles spaced evenly pointing radially outward.

> procedure (spaced-turtles n) → void?
> n : exact-nonnegative-integer?

Places 2n turtles evenly spaced in a line and pointing in the same direction as the original turtle.

> procedure (spokes) → void?

Draws some spokes, using radial-turtles and spaced-turtles.

> procedure (gapped-lines) → void?

Draw a bunch of parallel line segments, using spaced-turtles.

> procedure (spyro-gyra) → void?

Draws a spyrograph-reminiscent shape.

> procedure (neato) → void?

As the name says...

> procedure (graphics-bexam) → void?

Draws a fractal that came up on an exam given at Rice in 1997 or so.

> value sierp-size : real?

A constant that is a good size for the sierp procedures.

> procedure (sierp sierp-size) → void?
> sierp-size : real?
> procedure (sierp-nosplit sierp-size) → void?
> sierp-size : real?

Draws the Sierpinski triangle in two different ways, the first using split heavily. After running the first one, try executing (draw 10).

> value koch-size : real?

A constant that is a good size for the koch procedures.

> procedure (koch-split koch-size) → void?
> koch-size : real?
> procedure (koch-draw koch-size) → void?
> koch-size : real?

Draws the same Koch snowflake in two different ways.

> procedure (lorenz a b c) → void?
> a : real?
> b : real?
> c : real?

Watch the Lorenz attractor (a.k.a. butterfly attractor) initial values a, b, and c.

> procedure (lorenz1) → void?

Calls lorenz with good initial values.

> procedure (peano peano-size) → void?
> peano-size : real?

Draws the Peano space-filling curve.

> procedure (peano-position-turtle) → void?

Moves the turtle to a good place to prepare for a call to peano.

> value peano-size : exact-nonnegative-integer?

One size to use with peano.

> value fern-size : exact-nonnegative-integer?

A good size for the fern1 and fern2 functions.

> procedure (fern1 fern-size) → void?
> fern-size : exact-nonnegative-integer?
> procedure (fern2 fern-size) → void?
> fern-size : exact-nonnegative-integer?

Draws a fern fractal.

For fern1, you will probably want to point the turtle up before running this one, with something like:

> (turn/radians (- (/ pi 2)))

For fern2, you may need to backup a little.

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