▼ |  | Turtle Graphics

1 | Traditional Turtles
2 | Value Turtles

► | 2 | Value Turtles

2.1 | Examples

On this page:

turtles
turtles?
move
draw
erase
move-offset
draw-offset
erase-offset
turn
turn/ radians
merge
clean
2.1`` Examples
radial-turtles
spaced-turtles
neato
regular-poly
regular-polys
spokes
spyro-gyra

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### 2`` Value Turtles

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

The value turtles are a variation on traditional turtles. Rather than having just a single window where each operation changes the state of that window, in the graphics/value-turtles library, the entire turtles window is treated as a value. This means that each of the primitive operations accepts, in addition to the usual arguments, a turtles-window value; instead of returning nothing, each returns a turtles-window value.

> > procedure
> > (turtles |  | width |  |  |  |
> > |  | height |  |  |  |
> > | [ | init-x |  |  |  |
> > |  | init-y |  |  |  |
> > |  | init-angle]) |  | → |  | turtles?
> width : real?
> height : real?
> init-x : real? = (/ width 2)
> init-y : real? = (/ height 2)
> init-angle : real? = 0

Creates a new turtles window with the given width and height. The remaining arguments specify position of the initial turtle and the direction in radians (where 0 is to the right).

> procedure (turtles? v) → boolean?
> v : any/c

Determines if v is a turtles drawing.

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

Moves the turtle n pixels, returning a new turtles window.

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

Moves the turtle n pixels and draws a line along the path, returning a new turtles window.

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

Moves the turtle n pixels and erases a line along the path, returning a new turtles window.

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

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

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

Turns the turtle theta degrees counter-clockwise, returning a new turtles window.

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

Turns the turtle theta radians counter-clockwise, returning a new turtles window.

> procedure (merge turtles1 turtles2) → turtles?
> turtles1 : turtles?
> turtles2 : turtles?

The split and tprompt forms provided by graphics/turtles aren’t needed for graphics/value-turtles, since the turtles window is a value.

Instead, the merge accepts two turtles windows and combines the state of the two turtles windows into a single window. The new window contains all of the turtles of the previous two windows, but only the line drawings of the first turtles argument.

> procedure (clean turtles) → turtles?
> turtles : turtles?

Produces a turtles like turtles, but with only a single turtle, positioned in the center.

#### 2.1`` Examples

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

The graphics/turtle-examples library’s source is meant to be read, but it also exports the following examples.

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

Places 2 n turtles spaced evenly pointing radially outward.

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

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

> procedure (neato turtles) → turtles?
> turtles : turtles?

As the name says...

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

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

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

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

> procedure (spokes turtles) → turtles?
> turtles : turtles?

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

> procedure (spyro-gyra turtles) → turtles?
> turtles : turtles?

Draws a spyro-grya reminiscent shape.
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