So I'm going through the first chapter of How To Design Programs 2nd Edition. I believe I made pretty good progress. But there's a "suggestion" to add another graphic to the grid. Every time I try I get an error. At this point, I'm stuck. Below is the code and the error.
Note: the ROCKET image is in the Chapter 1. I just copy and pasted it into the IDE.
Note: The "suggestion" is: How would change the program so that the rocket lands on a flat rock bed that is 10 pixels higher than the bottom of the scene? Don’t forget to change the scenery, too.
Here's code that works.
(define BOARDWIDTH 200)
(define BOARDHEIGHT 200)
(define STARTPOSITION 50)
(define BOARDBKGR "blue")
(define GAMEBOARD (empty-scene BOARDWIDTH BOARDHEIGHT BOARDBKGR))
(define ROCKET .)
(define UFO (overlay (circle 10 "solid" "red")
(rectangle 40 4 "solid" "green")))
(define FLATBED (rectangle 60 10 "outline" "black"))
(define (SPACESHIP option)
(cond
[(= option 1) ROCKET]
[(= option 2) UFO]))
(define SHOWNSHIP (SPACESHIP 1))
(define V 20) ;Velocity
(define A 1) ;Acceleration
(define (distance t) ;t = Time
(- (* V t) (* 1/2 A (sqr t))))
(define SPACESHIP-BOTTOM (- BOARDHEIGHT (/ (image-height SHOWNSHIP) 2)))
(define (render-shownship x y)
(place-image SHOWNSHIP x y GAMEBOARD))
(define (create-rocket-scene.v7 t)
(cond
[(<= (distance t) SPACESHIP-BOTTOM)
(render-shownship STARTPOSITION (distance t))]
[(> (distance t) SPACESHIP-BOTTOM)
(render-shownship STARTPOSITION SPACESHIP-BOTTOM)]))
Here's the code that doesn't work:
(define BOARDWIDTH 200)
(define BOARDHEIGHT 200)
(define STARTPOSITION 50)
(define BOARDBKGR "blue")
(define GAMEBOARD (empty-scene BOARDWIDTH BOARDHEIGHT BOARDBKGR))
(define ROCKET .)
(define UFO (overlay (circle 10 "solid" "red")
(rectangle 40 4 "solid" "green")))
(define FLATBED (rectangle 60 10 "outline" "black"))
(define (SPACESHIP option)
(cond
[(= option 1) ROCKET]
[(= option 2) UFO]))
(define SHOWNSHIP (SPACESHIP 1))
(define V 20) ;Velocity
(define A 1) ;Acceleration
(define (distance t) ;t = Time
(- (* V t) (* 1/2 A (sqr t))))
(define SPACESHIP-BOTTOM (- BOARDHEIGHT (/ (image-height SHOWNSHIP) 2)))
(define (render-shownship x y)
(place-image SHOWNSHIP x y GAMEBOARD)
(place-image FLATBED STARTPOSITION 195 GAMEBOARD)) ;offender
(define (create-rocket-scene.v7 t)
(cond
[(<= (distance t) SPACESHIP-BOTTOM)
(render-shownship STARTPOSITION (distance t))]
[(> (distance t) SPACESHIP-BOTTOM)
(render-shownship STARTPOSITION SPACESHIP-BOTTOM)]))
And the error I get is:
define: expected only one expression for the function body, but found 1 extra part
place-image always takes 4 arguments - the image to be placed, x and y coordinates, and the scene (background) on which to place the image. The problem in your code is that the expression (place-image FLATBED STARTPOSITION 195) is providing only 3 inputs to place-image.
So, back up a little and consider: what does the first expression produce? (place-image SHOWNSHIP x y GAMEBOARD) produces a game board scene with a ship on it, correct? Now on top of that scene you further want to place the FLATBED. So instead of sequencing the place-image function calls, instead consider composing them - i.e. what do you think the missing piece is in (place-image FLATBED STARTPOSITION 195 ____)? upon what scene do you want to place the FLATBED? (Hint: we just answered that above). What expression produces that scene? (hint: you already have that expression).
If you understand the idea, you see that to place multiple images on a scene, you compose or nest the function calls (instead of sequencing them as you are attempting):
(place-image img1 x1 y1 (place-image img2 x2 y2 ...))