Re: [racket-users] Moving an image with the keyboard
Patrick, I get the sense that this is your first experience with programming and programming languages. If so, Realm is not the best way to get started. As it says in the Preface, it's for people with a first course on programming under their belt (either How to Design Programs /HtDPor some standard Java course). The Bootstrap curriculum is an adaptation of the HtDP ideas, which isn't about programming but designing programs systematically. It lays out general principles of doing that, starting with simple programs and slowly moving up the chain to complex forms of data and programs. At each stage the design principles are expanded to cope with a large space of design alternatives. One step in this design process is to devise a data representation (name a set of values) and state signatures (say in terms of these data representations what functions take in and return). BUT, the languages we use at this stage do NOT check whether what you state is correct. This is by design -- because if we do, we just increase the number of error messages beginners get and because many college students now go out on internships and program in such languages (JavaScript, Python, Ruby, etc). Bootstrap makes sure that all of this relates back to Mathematics as it is taught at around the middle school level. That was the original idea behind Racket and our educational dimension. Big-bang describes a world with plain mathematical terms: the states it keeps track of, how to map a state to another one when something happens (a clock tick, a key press) and how to render the state as an image (which is a value just like a number). There is another clause you can add to big-bang programs (click on big-bang to open its documentation) that describes what kind of values big-bang is supposed to track: (big-bang (ufo 70 60) (on-key move-UFO) (to-draw render-ufo) (check-with ufo?)) If you add this, run and press - the program will signal an error that tells you what went wrong. Try it out, and otherwise follow Jordan's advice or switch back to HtDP/2e: file:///Users/matthias/0Unison/0Web/HtDP2e/Draft/index.html -- Matthias On Jun 27, 2015, at 8:50 AM, Patrick Ester wrote: Jordan, Thanks for the suggestion. Quick question, why aren't type signatures required? I've dabbled with a language called Elm (elm-lang.org) in which type signatures are mandatory, the code won't compile without them. Not that you control this, but Realm of Racket doesn't say anything about type signatures. I know that the Bootstrap curriculum, however, stresses contracts. Coincidentally, I'll be attending a Bootstrap conference next month. Anyways, everything works now, and I appreciate your help. On Friday, June 26, 2015 at 8:10:08 PM UTC-5, Jordan Johnson wrote: Hi Patrick, The problem you’re up against is that in the code you’ve attached, we don’t have a clear picture of what the signatures for your functions are, nor of the “world” upon which big-bang operates. Standard practice in HtDP-based courses dictates that every big-bang program should have a data definition for the world in comments, andevery function should have a signature and brief description of its purpose in comments (as well as one or more examples/tests to illustrate its use). Because you write (big-bang (ufo 70 60) ...), you clearly intend for your “world” to be a ufo struct. So in order for your readers to not have to guess at what you mean, you need a comment to accompany your struct definition: ;; A World is a (ufo Number Number): (struct ufo (xpos ypos)) ;; where xpos is ... ;; and ypos is ... And say what xpos and ypos represent. Thereafter, when you write function signatures, you can use World to describe the kind of data that’s passing in or out, e.g., ;; SIGNATURE: render-ufo : World - Image ;; PURPOSE: draws the UFO at the given location If you look at the Help Desk docs for 2htdp/universe, one fine thing it gives you in the description of big-bang is the signature required for every one of the handler functions. For example: big-bang’s first parameter must be a World, and in [to-draw f] the signature of f must be ;; f : World - Image Once you’ve worked out what the signatures should be for all of your handler functions, take a look at your code and start writing signatures for the functions you’ve written. This will lead you to figure out what’s going wrong. Cheers, jmj On Jun 26, 2015, at 4:42 PM, Patrick Ester patric...@gmail.com wrote: Dear All, I am not sure if this is the right place since I am working from the Realm of Racket book. If not, please let me know where I can take my question. I finished up chapter five, and wanted to modify the UFO example to move in relation to the arrow keys. Here is the code that I wrote: #lang racket (require 2htdp/universe 2htdp/image) (define WIDTH 600) (define
Re: [racket-users] Moving an image with the keyboard
Jordan, Thanks for the suggestion. Quick question, why aren't type signatures required? I've dabbled with a language called Elm (elm-lang.org) in which type signatures are mandatory, the code won't compile without them. Not that you control this, but Realm of Racket doesn't say anything about type signatures. I know that the Bootstrap curriculum, however, stresses contracts. Coincidentally, I'll be attending a Bootstrap conference next month. Anyways, everything works now, and I appreciate your help. On Friday, June 26, 2015 at 8:10:08 PM UTC-5, Jordan Johnson wrote: Hi Patrick, The problem you’re up against is that in the code you’ve attached, we don’t have a clear picture of what the signatures for your functions are, nor of the “world” upon which big-bang operates. Standard practice in HtDP-based courses dictates that every big-bang program should have a data definition for the world in comments, andevery function should have a signature and brief description of its purpose in comments (as well as one or more examples/tests to illustrate its use). Because you write (big-bang (ufo 70 60) ...), you clearly intend for your “world” to be a ufo struct. So in order for your readers to not have to guess at what you mean, you need a comment to accompany your struct definition: ;; A World is a (ufo Number Number): (struct ufo (xpos ypos)) ;; where xpos is ... ;; and ypos is ... And say what xpos and ypos represent. Thereafter, when you write function signatures, you can use World to describe the kind of data that’s passing in or out, e.g., ;; SIGNATURE: render-ufo : World - Image ;; PURPOSE: draws the UFO at the given location If you look at the Help Desk docs for 2htdp/universe, one fine thing it gives you in the description of big-bang is the signature required for every one of the handler functions. For example: big-bang’s first parameter must be a World, and in [to-draw f] the signature of f must be ;; f : World - Image Once you’ve worked out what the signatures should be for all of your handler functions, take a look at your code and start writing signatures for the functions you’ve written. This will lead you to figure out what’s going wrong. Cheers, jmj On Jun 26, 2015, at 4:42 PM, Patrick Ester patric...@gmail.com wrote: Dear All, I am not sure if this is the right place since I am working from the Realm of Racket book. If not, please let me know where I can take my question. I finished up chapter five, and wanted to modify the UFO example to move in relation to the arrow keys. Here is the code that I wrote: #lang racket (require 2htdp/universe 2htdp/image) (define WIDTH 600) (define HEIGHT 600) ;;The image won't copy and paste, but you get the idea. (define IMAGE-of-ufo .) (struct ufo (xpos ypos)) (define (move-up w) (add1 (ufo-xpos w))) (define (move-down w) (sub1 (ufo-xpos w))) (define (move-right w) (add1 (ufo-ypos w))) (define (move-left w) (sub1 (ufo-ypos w))) (define (render-ufo w) (place-image (scale 0.25 IMAGE-of-ufo) (ufo-xpos w) (ufo-ypos w) (empty-scene WIDTH HEIGHT))) (define (move-UFO w key) (cond [(key=? key up) (move-up w)] [(key=? key down) (move-down w)] [(key=? key left) (move-left w)] [(key=? key right) (move-right w)] [else w])) (big-bang (ufo 70 60) (on-key move-UFO) (to-draw render-ufo)) When I click the Run button, the output initially looks good. However, when I press an arrow key, I get an error. Attached is a screenshot with what I see in the definitions pane. The error message in the interactions pane is: ufo-xpos: contract violation expected: ufo? given: 61 I do not get why the contract is broken. The function place-image takes an image, number, number, and a scene. What mistake am I making? Thanks in advance. Patrick -- You received this message because you are subscribed to the Google Groups Racket Users group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users...@googlegroups.com. For more options, visit https://groups.google.com/d/optout. Screen Shot 2015-06-26 at 6.26.21 PM.png -- You received this message because you are subscribed to the Google Groups Racket Users group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: [racket-users] Moving an image with the keyboard
Hi Patrick, The problem you’re up against is that in the code you’ve attached, we don’t have a clear picture of what the signatures for your functions are, nor of the “world” upon which big-bang operates. Standard practice in HtDP-based courses dictates that every big-bang program should have a data definition for the world in comments, and every function should have a signature and brief description of its purpose in comments (as well as one or more examples/tests to illustrate its use). Because you write (big-bang (ufo 70 60) ...), you clearly intend for your “world” to be a ufo struct. So in order for your readers to not have to guess at what you mean, you need a comment to accompany your struct definition: ;; A World is a (ufo Number Number): (struct ufo (xpos ypos)) ;; where xpos is ... ;; and ypos is ... And say what xpos and ypos represent. Thereafter, when you write function signatures, you can use World to describe the kind of data that’s passing in or out, e.g., ;; SIGNATURE: render-ufo : World - Image ;; PURPOSE: draws the UFO at the given location If you look at the Help Desk docs for 2htdp/universe, one fine thing it gives you in the description of big-bang is the signature required for every one of the handler functions. For example: big-bang’s first parameter must be a World, and in [to-draw f] the signature of f must be ;; f : World - Image Once you’ve worked out what the signatures should be for all of your handler functions, take a look at your code and start writing signatures for the functions you’ve written. This will lead you to figure out what’s going wrong. Cheers, jmj On Jun 26, 2015, at 4:42 PM, Patrick Ester patrick.es...@gmail.com wrote: Dear All, I am not sure if this is the right place since I am working from the Realm of Racket book. If not, please let me know where I can take my question. I finished up chapter five, and wanted to modify the UFO example to move in relation to the arrow keys. Here is the code that I wrote: #lang racket (require 2htdp/universe 2htdp/image) (define WIDTH 600) (define HEIGHT 600) ;;The image won't copy and paste, but you get the idea. (define IMAGE-of-ufo .) (struct ufo (xpos ypos)) (define (move-up w) (add1 (ufo-xpos w))) (define (move-down w) (sub1 (ufo-xpos w))) (define (move-right w) (add1 (ufo-ypos w))) (define (move-left w) (sub1 (ufo-ypos w))) (define (render-ufo w) (place-image (scale 0.25 IMAGE-of-ufo) (ufo-xpos w) (ufo-ypos w) (empty-scene WIDTH HEIGHT))) (define (move-UFO w key) (cond [(key=? key up) (move-up w)] [(key=? key down) (move-down w)] [(key=? key left) (move-left w)] [(key=? key right) (move-right w)] [else w])) (big-bang (ufo 70 60) (on-key move-UFO) (to-draw render-ufo)) When I click the Run button, the output initially looks good. However, when I press an arrow key, I get an error. Attached is a screenshot with what I see in the definitions pane. The error message in the interactions pane is: ufo-xpos: contract violation expected: ufo? given: 61 I do not get why the contract is broken. The function place-image takes an image, number, number, and a scene. What mistake am I making? Thanks in advance. Patrick -- You received this message because you are subscribed to the Google Groups Racket Users group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout. Screen Shot 2015-06-26 at 6.26.21 PM.png -- You received this message because you are subscribed to the Google Groups Racket Users group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
[racket-users] Moving an image with the keyboard
Dear All, I am not sure if this is the right place since I am working from the Realm of Racket book. If not, please let me know where I can take my question. I finished up chapter five, and wanted to modify the UFO example to move in relation to the arrow keys. Here is the code that I wrote: #lang racket (require 2htdp/universe 2htdp/image) (define WIDTH 600) (define HEIGHT 600) ;;The image won't copy and paste, but you get the idea. (define IMAGE-of-ufo .) (struct ufo (xpos ypos)) (define (move-up w) (add1 (ufo-xpos w))) (define (move-down w) (sub1 (ufo-xpos w))) (define (move-right w) (add1 (ufo-ypos w))) (define (move-left w) (sub1 (ufo-ypos w))) (define (render-ufo w) (place-image (scale 0.25 IMAGE-of-ufo) (ufo-xpos w) (ufo-ypos w) (empty-scene WIDTH HEIGHT))) (define (move-UFO w key) (cond [(key=? key up) (move-up w)] [(key=? key down) (move-down w)] [(key=? key left) (move-left w)] [(key=? key right) (move-right w)] [else w])) (big-bang (ufo 70 60) (on-key move-UFO) (to-draw render-ufo)) When I click the Run button, the output initially looks good. However, when I press an arrow key, I get an error. Attached is a screenshot with what I see in the definitions pane. The error message in the interactions pane is: ufo-xpos: contract violation expected: ufo? given: 61 I do not get why the contract is broken. The function place-image takes an image, number, number, and a scene. What mistake am I making? Thanks in advance. Patrick -- You received this message because you are subscribed to the Google Groups Racket Users group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
