1 / 33

Lecture 12

Lecture 12. Data directed programming Message passing dotted-tail notation & apply Section 2.4, pages 169-187 2.5.1,2.5.2 pages 187-197 (but with a different example). Multiple Representations of Abstract Data. Example: geometrical figures.

don
Download Presentation

Lecture 12

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 12 Data directed programmingMessage passingdotted-tail notation & applySection 2.4, pages 169-1872.5.1,2.5.2 pages 187-197(but with a different example) מבוא מורחב - שיעור 12

  2. Multiple Representations of Abstract Data Example: geometrical figures • We develop a package for handling geometrical figures • Each figure has a unique data representation • Want to support Generic Operations on figures, such as: • Compute figure area • Compute figure circumference • Print figure parameters, etc. מבוא מורחב - שיעור 12

  3. Geometrical figures: interface Constructors: (make-rectangle width height) (make-circle radius) (make-square length) Rectangle implementation.. Circle implementation.. Square implementation.. Generic operations: (area figure) (circumference figure) (fig-display figure).. We will now see 3 alternative implementations… מבוא מורחב - שיעור 12

  4. Implementation #1: tagged data • Main idea: add a tag (symbol) to every figure instance • Generic procedures dispatch on parameter type ('circle 2) ('square 1) ('rectangle 1 . 2) מבוא מורחב - שיעור 12

  5. Attaching tags. (define (attach-tag type-tag contents) (cons type-tag contents)) (define (type-tag datum) (if (pair? datum) (car datum) (error "Bad tagged datum -- TYPE-TAG" datum))) (define (contents datum) (if (pair? datum) (cdr datum) (error "Bad tagged datum -- CONTENTS" datum))) מבוא מורחב - שיעור 12

  6. Tagged data: rectangle implementation (define (make-rectangle width height) (attach-tag 'rectangle (cons width height))) (define (width rect) (car rect)) (define (height rect) (cdr rect)) (define (circumference-rect rect) (* 2 (+ (width rect) (height rect)))) (define (area-rect rect) (* (width rect) (height rect)))(define (fig-display-rect rect) (my-display "Rectangle: width, ” (width rect) ", height: " (height rect))) assume my-display takes any number of arguments and prints them. מבוא מורחב - שיעור 12

  7. Tagged data: circle implementation (define (make-circle radius) (attach-tag 'circle (list radius))) (define PI 3.1415926) (define (radius circle) (car circle)) (define (circumference-circle circle) (* 2 PI (radius circle))) (define (area-circle circle) (let ((r (radius circle))) (* PI r r))) (define (fig-display-circle circle) (my-display “Circle: radius, ” (radius circle))) מבוא מורחב - שיעור 12

  8. Dispatch on type Tagged data: generic operations (define (circumference fig) (cond ((eq? 'rectangle (type-tag fig)) (circumference-rect (contents fig))) ((eq? 'circle (type-tag fig)) (circumference-circle (contents fig) . .)) (define (area fig) (cond ((eq? 'rectange (type-tag fig)) (area-rect (contents fig))) ((eq? 'circle (type-tag fig)) (area-circle (contents fig) . .)) (define (fig-display fig) (cond ((eq? 'rectange (type-tag fig)) (fig-display-rectangle (contents fig))) ((eq? 'circle (type-tag fig)) (fig-display-circle (contents fig) . .)) מבוא מורחב - שיעור 12

  9. The Difficulties The system is not additive/modular • The generic procedures must know about all types. • If we want to add a new type we need to • add it to each of the operations, • be careful with name clashes. (define (area fig) (cond ((eq? 'rectange (type-tag fig)) (area-rect (contents fig))) ((eq? 'circle (type-tag fig)) (area-circle (contents fig) . .)) מבוא מורחב - שיעור 12

  10. types Circle Rectangle Square circumference-circle area-circle fig-display-circle circumference-rect area-rect fig-display-rect circumference area fig-display circumference-square area-square fig-display-square Generic operations Implementation #2: data directed programming • Main idea: work with a table. • Keep a pointer to the right procedure to call in the table, keyed by the operation/type combination מבוא מורחב - שיעור 12

  11. Data-directed programming (Cont) Assume we have a global two dimensional table and the following operations on it: Adds val to the table, under keys key1, key2 (put key1 key2 val) Retrieves the value found in the table under keys key1, key2 OR #f if the value is not found (get key1 key2)  val/#f מבוא מורחב - שיעור 12

  12. Rectangle implementation (define (install-rectangle-package) ;; Implementation (define (width rect) (car rect)) (define (height rect) (cdr rect))(define (circumference rect) (* 2 (+ (width rect) (height rect)))) (define (area rect) (* (width rect) (height rect))) (define (fig-display rect) (my-display "Rectangle: width ,” (width rect) “, height " (height rect))) (define (make-rect width height) (attach-tag 'rectangle (cons width height))) ;; Interface to the rest of the system (put 'circumference 'rectangle circumference) (put 'area 'rectangle area) (put 'fig-display 'rectangle fig-display) (put 'make-fig 'rectangle make-rect) 'done) מבוא מורחב - שיעור 12

  13. Circle implementation (define (install-circle-package) ;; Implementation (define PI 3.1415926) (define (radius circle) (car circle))(define (circumference circle) (* 2 PI (radius circle))) (define (area circle) (let ((r (radius circle)) (* PI r r))) (define (fig-display circle) (my-display “Circle: radius,” (radius rect))) (define (make-circle radius) (attach-tag 'circle (list radius))) ;; Interface to the rest of the system (put 'circumference 'circle circumference) (put 'area 'circle area) (put 'fig-display 'circle fig-display) (put 'make-fig 'circle make-circle) 'done) מבוא מורחב - שיעור 12

  14. Generic procedures (define (circumference fig) (apply-generic 'circumference fig)) (define (area fig) (apply-generic 'area fig)) (define (fig-display fig) (apply-generic 'fig-display fig)) • Apply generic: • Locates the right procedure in the table • Calls it • Passes the figure-data as a parameter מבוא מורחב - שיעור 12

  15. Get the type of the figure Locate the procedure that implements op on that type If there is such a procedure (get did not return #f) Call it and pass the figure-data as a parameter Apply-generic (define (apply-generic op arg) (let ((type (type-tag arg))) (let ((proc (get op type))) (if proc (proc (contents arg)) (error "No operation for this type -- APPLY-GENERIC" op type-tag))))) מבוא מורחב - שיעור 12

  16. Summary: Data Directed programming Data Directed programming: The data (argument) triggers the right operation based on the data type. • Data Directed programming is more modular: • To add a representation, we only need to write • a package for the new representation without • changing generic procedures. (Execute the install • procedure once). • Changes are local. • No name clashes. • install-circle-package • install-rectangle-package • .. מבוא מורחב - שיעור 12

  17. Implementation #3: Message Passing • Main idea: the figure is represented by a procedure • The procedure dispatches on the operation type Data Directed: Intelligent operations that work with different data types. Message Passing: Intelligent data types that support different operations. In message passing the idea is that the data gets a message that tells it which operation to invoke, and returns the result. מבוא מורחב - שיעור 12

  18. Data Directed: in each operation we dispatch on type. types Message Passing: in each type we dispatch on operation. Circle Rectangle Square circumference-circle area-circle fig-display-circle circumference-rect area-rect fig-display-rect circumference-square area-square fig-display-square Message Passing (cont’) circumference area fig-display Generic operations מבוא מורחב - שיעור 12

  19. Message passing style – rectangle figure The constructor has it all. (define (make-rectangle width height) (lambda (op) (cond ((eq? op 'area) (* height width)) ((eq? op 'circumference) (* 2 (+ width height))) ((eq? op 'fig-display) (my-display "Rectangle: width, ” width) “height, " height)) (else (error "Unknown op – MAKE-RECTANGLE" op))))) Another way to write this: (define (make-rectangle width height) (define (dispatch op) (cond ... ;; as above )) dispatch) מבוא מורחב - שיעור 12

  20. Message passing style – circle figure The constructor has it all. (define (make-circle radius) (define PI 3.1415926) (lambda (op) (cond ((eq? op 'area) (* PI radius radius)) ((eq? op 'circumference) (* 2 PI radius)) ((eq? op 'fig-display) (my-display “Circle: radius ” radius)) (else (error "Unknown op – MAKE-CIRCLE" op))))) מבוא מורחב - שיעור 12

  21. Generic operations and new apply-generic (define (circumference fig) (apply-generic 'circumference fig)) (define (area fig) (apply-generic 'area fig)) (define (fig-display fig) (apply-generic 'make-fig fig)) (define (apply-generic op arg) (arg op)) מבוא מורחב - שיעור 12

  22. Summary: Message Passing • Additive • Avoids name-clashes problems • No need for a table • More difficult to extend for multi-parameter operations Message passing is the basis for Object Oriented Programming and we will discuss it again later. מבוא מורחב - שיעור 12

  23. At least two parameters must be passed This dot signifies that proc accepts a variable number of parameters Writing procedures with a variable number of parameters (dotted-tail notation) All other parameters will be passed in a list named ‘z’ (define (proc x y . z) <body>) מבוא מורחב - שיעור 12

  24. x  ? y  ? z  ? 1 • (proc 1 2) 2 Dotted-tail notation: examples x  ? y  ? z  ? 1 • (define (proc x y . z) <body>) • (proc 1 2 3 4 5 6) 2 (3 4 5 6) null ERROR • (proc 1) מבוא מורחב - שיעור 12

  25. z  ? • (proc) z  ? • (proc 1 (list 2 3)) Dotted-tail notation: another example • (define (proc . z) <body>) • (proc 1 2 3 4 5 6) z  ? (1 2 3 4 5 6) null (1 (2 3)) מבוא מורחב - שיעור 12

  26. Dotted-tail notation: another example We want to implement a procedure (count…) that gets a variable number of parameters and returns their number (count 1 7 8 9 1)  5 (count 0)  1 (count)  0 (define (count . s) _________________) (length s) מבוא מורחב - שיעור 12

  27. Dotted-tail notation: yet another example (define (lt x . y) (length (filter (lambda (z) (< z x)) y))) What does this procedure do ? (lt 1 7 8 9 1 0 -1)  2 (lt 1000 1 10 0)  3 (lt 1)  0 (lt)  ERROR lt returns the number of parameters that are smaller than the first parameter. מבוא מורחב - שיעור 12

  28. The builtin procedure Apply • Suppose we want to apply some procedure proc • but the parameters are available as a list (apply proc lst) will give the same result as (proc v1 v2 … vn) assuming lst is (v1 v2 … vn) מבוא מורחב - שיעור 12

  29. Apply examples (apply * (list 1 2 3))  6 (apply count (list 1 2 3))  3 (apply lt (list 10 1 2 3 11 9))  4  (1 2 3 4) (apply append '((1) (2) (3) (4))) (apply expt '(2 4))  16  (1 2 3 4 5) (apply list '(1 2 3 4 5)) מבוא מורחב - שיעור 12

  30. (define (general-map proc list1 . other-lists) (if (null? list1) '() (let ((lists (cons list1 other-lists))) (let ((firsts (simple-map car lists)) (rests (simple-map cdr lists))) (cons (apply proc firsts) (apply general-map (cons proc rests))))))) We use the names simple-map and general-map for clarity. The general procedure is actually called map. General map (define (f x y z) (+ x (* 2 y) (* 5 z))) (general-map f '(1 2 3) '(2 3 4) '(2 4 7))  (15 28 46) מבוא מורחב - שיעור 12

  31. Another apply example Exercise: Implement a procedure, add-abs, that receives 0 or more numbers, and returns the sum of their absolute values. Examples:(add-abs)  0 (add-abs 1 -2 -3 4)  10 (define (add-abs . l) (if (null? l) 0 (+ (abs (car l)) _________________________))) Does not work!! (add-abs (cdr l)) מבוא מורחב - שיעור 12

  32. Another apply example Exercise: Implement a procedure, add-abs, that receives 0 or more numbers, and returns the sum of their absolute values. Examples:(add-abs)  0 (add-abs 1 -2 -3 4)  10 (define (add-abs . l) (if (null? l) 0 (+ (abs (car l)) _________________________))) This is ok! (apply add-abs (cdr l)) מבוא מורחב - שיעור 12

  33. Another apply example (cont) Exercise: Implement a procedure, add-abs, that receives 0 or more numbers, and returns the sum of their absolute values. Another solution (also using apply). (define (add-abs . l) (apply ________________________)) + (map abs l) מבוא מורחב - שיעור 12

More Related