1.10 Managing Control Arrows: "arrow.rkt"

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1.1 | Manipulating Images: "image.rkt"
1.2 | Simulations and Animations: "world.rkt"
1.3 | Converting Temperatures: "convert.rkt"
1.4 | Guessing Numbers: "guess.rkt"
1.5 | Master Minding: "master.rkt"
1.6 | Playing Master Mind: "master-play.rkt"
1.7 | Simple Drawing: "draw.rkt"
1.8 | Hangman: "hangman.rkt"
1.9 | Playing Hangman: "hangman-play.rkt"
1.10 | Managing Control Arrows: "arrow.rkt"
1.11 | Manipulating Simple HTML Documents: "docs.rkt"
1.12 | Working with Files and Directories: "dir.rkt"
1.13 | Graphing Functions: "graphing.rkt"
1.14 | Simple Graphical User Interfaces: "gui.rkt"
1.15 | An Arrow GUI: "arrow-gui.rkt"
1.16 | Controlling an Elevator: "elevator.rkt"
1.17 | Lookup GUI: "lkup-gui.rkt"
1.18 | Guess GUI: "guess-gui.rkt"
1.19 | Queens: "show-queen.rkt"
1.20 | Matrix Functions: "matrix.rkt"

On this page:

control-left-right
control-up-down
control
LEFT-ARROW
RIGHT-ARROW
UP-ARROW
DOWN-ARROW

HtDP
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### 1.10 Managing Control Arrows: "arrow.rkt"🔗ℹ

(require htdp/arrow) | package: htdp-lib

The teachpack implements a controller for moving shapes across a canvass.

> procedure (control-left-right shape n move draw) → true
> shape : Shape
> n : number?
> move : (-> number? Shape Shape)
> draw : (-> Shape true)

Moves shape n pixels left (negative) or right (positive).

> procedure (control-up-down shape n move draw) → true
> shape : Shape
> n : number?
> move : (-> number? Shape Shape)
> draw : (-> Shape true)

Moves shape n pixels up (negative) or down (positive).

> procedure (control shape n move-lr move-ud draw) → true
> shape : Shape
> n : number?
> move-lr : (-> number? Shape Shape)
> move-ud : (-> number? Shape Shape)
> draw : (-> Shape true)

Moves shape N pixels left or right and up or down, respectively.

The teachpack also provides four images:

> value LEFT-ARROW : image?

an arrow pointing left.

> value RIGHT-ARROW : image?

an arrow pointing right.

> value UP-ARROW : image?

an arrow pointing up.

> value DOWN-ARROW : image?

an arrow pointing down.

Example:

> ; A shape is a structure:
> ; (make-posn num num)
> ; RAD : the radius of the simple disk moving across a canvas
> (define RAD 10)
> ; move : number shape -> shape or false
> ; to move a shape by delta according to translate
> ; effect: to redraw it
> (define (move delta sh)
> (cond
> [(and (clear-solid-disk sh RAD)
> (draw-solid-disk (translate sh delta) RAD))
> (translate sh delta)]
> [else false]))
> ; translate : shape number -> shape
> ; to translate a shape by delta in the x direction
> (define (translate sh delta)
> (make-posn (+ (posn-x sh) delta) (posn-y sh)))
> ; draw-it : shape -> true
> ; to draw a shape on the canvas: a disk with radius
> (define (draw-it sh)
> (draw-solid-disk sh RAD))
> ; Run:
> ; this creates the canvas
> (start 100 50)
> ; this creates the controller GUI
> (control-left-right (make-posn 10 20) 10 move draw-it)

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