CSC151 2015F, Review Session 06 - 2015-10-08 ============================================ _Overview_ * We look at general quiz issues. * We do some sample quiz questions. * We gather additional questions and arrange them. * I attempt answers. * More Q&A, as appropriate. About the Quiz -------------- Topics * `image-compute` * `let` and `let*` * New list operations (`cons`, `car`, `cdr`) * Conditionals (`if`, `when`, `cond`) * Anonymous procedures (using lambda, `section`, `o`, `l-s`, `r-s`) Some things to know about * Similarities and differences between `let` and `let*`. * Similarities and differences between lists and list-like things. * Similarities and differences between the three conditional operations. * Different ways to make anonymous procedures: `section`, `l-s`, `r-s`, `o`, `lambda`. Types of Questions * What is the value of this expression? * Write code to ____. * Document the following procedure. * Summarize tests for the following procedure. (You will not have to write them as Rackunit tests.) * Random silly extra credit question Sample Quiz Questions --------------------- *There are enough of these that you should check the eboard.* ### Values of Expressions _What is the value of `(reverse (append (iota 5) (iota 5)))`?_ _What is the value of `(reverse (append (iota 5) 5))`?_ _What is the value of `(cadr (reverse (iota 5)))`?_ _What is the value of `(cdar (reverse (iota 5)))`?_ _Sketch the image that results from the following code._ (let ([red (irgb 255 0 0)] [blue (irgb 0 0 255)] [white (irgb 255 255 255)]) (image-show (image-compute (lambda (col row) (cond [(< col 100) red] [(< row 100) blue] [else white])) 200 200))) _What is the value of `(map (l-s - 2) (iota 5))`?_ _What is the value of `(map (r-s - 2) (iota 5))`?_ _What is the value of `(map (o (l-s - 2) (r-s * 3)) (cons 4 (iota 5)))`?_ _What is the value of `(map (lambda (x) (* 3 (+ 2 x))) (iota 5))`?_ _What is the value of `(let ([a 10] [b 5]) (list a b))`?_ _What is the value of `(let ([a 10] [b (square a)]) (list a b))`?_ _What is the value of `(let* ([a 10] [b (square a)]) (list a b))`?_ ### Code to Write _Write a procedure to get the last value in a list._ _Without using lambda, write a procedure to remove the last value in a list (or, more precisely, to return a list that contains all but the last value of its parameter list)._ > (remove-last (list 1 2 3)) '(1 2) > (remove-last (reverse (iota 5))) '(4 3 2 1) _Using `image-compute`, write an expression to create a 100x100 image with a red circle of radius 30 centered at (40,50)._ _The following code makes way too many lists. Rewrite it using a single call to `map`._ (map hshift-drawing (make-list 20 5) (map vshift-drawing (map * (make-list 20 10) (iota 20)) (make-list 20 drawing-unit-circle))) ### Documentation _Document the following procedure using the six P's._ (define rac (o car reverse)) _Document the following procedure using the six P's._ (define something (lambda (img x y) (when (< x y) (image-select-rectangle! img REPLACE 0 0 x y) (context-set-fgcolor! (irgb (random 256) (random 256) (random 256))) (image-fill! img) (image-select-nothing! img)))) ### Tests _List the tests you would conduct for the following procedure._ ;;; Procedure: ;;; trim-ends ;;; Parameters: ;;; lst, a list ;;; Purpose: ;;; Removes the last and first elements, if they exist. ;;; Produces: ;;; trimmed, a list ;;; Postconditions: ;;; If lst is empty, returns an empty list ;;; If lst has only one element, returns an empty list ;;; Otherwise ;;; We have removed two elements ;;; (length trimmed) = (length lst) - 2 ;;; The second to the penultimate are in the same relative position ;;; For all i < (length trimmed) ;;; (list-ref trimmed i) = (list-ref lst (+ i 1)) Questions --------- _Can you briefly go over `l-s` and `r-s`?_ > `(l-s fun left)` == `(section fun left <>)` == `(lambda (right) (fun left right))` > `(r-s fun right)` == `(section fun <> right)` == `(lambda (left) (fun left right))` _When does `cdar` work?_ > `(cdar val)` == `(cdr (car val))`. So this only works if the first element of val is itself a list. > It would work with `(cdar (list (iota 5) (iota 4)))` > `(cdar (list (iota 5) (iota 4)))` => `(cdr (car (list (iota 5) (iota 4))))` => `(cdr (iota 5))` => `(1 2 3 4)` Welcome to DrRacket, version 6.1 [3m]. Language: racket [custom]; memory limit: 128 MB. > (list (iota 5) (iota 4)) '((0 1 2 3 4) (0 1 2 3)) > (car (list (iota 5) (iota 4))) '(0 1 2 3 4) > (cdr (list (iota 5) (iota 4))) '((0 1 2 3)) > (iota 4) '(0 1 2 3) > (cdr (car (list (iota 5) (iota 4)))) '(1 2 3 4) > (cadr (iota 5)) 1 > (cdar (iota 5)) . . cdar: contract violation expected: (cons/c pair? any/c) given: '(0 1 2 3 4) > (cdr (car (iota 5))) . . cdr: contract violation expected: pair? given: 0 _Why would anyone use cdar?_ > Example: I might have a list of students, and each student is a list of the form `(name class grade-on-exam1 ...)`. `(cdar students)` gives me the name of the first student. `(cdar (sort-by-grade students))` gives me the name of the top student. _Why is `(cdr (list (iota 5) (iota 4)))` equal to `(list (iota 4))` rather than just `(iota 4)`?_ > What is `(cdr (list 5 4))`? It's `(list 4)` or `'(4)`. `cdr` always gives back a list (if given a list); it just removes the first element. _Should we use the single-quotation mark (aka tick) to make lists?_ > No! It leads to confusion down the line. _Do we use `cdr` to traverse down lists?_ > Yes. Great idea. _Do we have to know `cadr`?_ > Yes. _What is `(cdr (list 1))`?_ > `null` _Can you do the sketching one?_ (let ([red (irgb 255 0 0)] [blue (irgb 0 0 255)] [white (irgb 255 255 255)]) (image-show (image-compute (lambda (col row) (cond [(< col 100) red] [(< row 100) blue] [else white])) 200 200))) > It will be a drawing of size 200x200 > The left half will be red. > The top right corner will be blue > The bottom right corner will be white _Can you give some phrases never to say to your professor?_ > "I don't listen in class." _Write a procedure to get the last value in a list._ > Tools: `reverse`, `car`, `(list-ref lst pos)` (gives you the value at the given position, counting at 0) (define last (lambda (lst) (car (reverse lst)))) (define last (lambda (lst) (list-ref lst (- (length lst) 1)))) > (define letters (list 'a 'b 'c 'd 'e)) > (list-ref letters 3) 'd > (list-ref letters 4) 'e > (length letters) 5 > (list-ref letters 5) . . list-ref: index too large for list index: 5 in: '(a b c d e) (define last (o car reverse)) _Without using lambda, write a procedure to remove the last value in a list (or, more precisely, to return a list that contains all but the last value of its parameter list)._ (define remove-last (o reverse cdr reverse)) _Using `image-compute`, write an expression to create a 100x100 image with a red circle of radius 30 centered at (40,50) and white everywhere else._ > Start with (image-compute (lambda (col row) ??? 100 100)) > We can use the distance formula "square root of ((y2-y1)^2 + (x2-x1)^2)" `(sqrt (+ (square (- 50 row)) (square (- 40 col))))` (image-compute (lambda (col row) (if (< (sqrt (+ (square (- 50 row)) (square (- 40 col)))) 30) (irgb 255 0 0) (irgb 255 255 255))) 100 100) _Rewrite the following._ (map hshift-drawing (make-list 20 5) (map vshift-drawing (map * (make-list 20 10) (iota 20)) (make-list 20 drawing-unit-circle))) > It appears that the purpose of this is to take twenty copies of the unit circle, vertically shift each by a separate multiples of 10, and horizontally shift it by 5. (map (lambda (i) (hshift-drawing 5 (vshift-drawing (* 10 i) drawing-unit-circle))) (iota 20)) > Can you make my brain hurt? (map (o (section hshift-drawing 5 <>) (section vshift-drawing <> drawing-unit-circle) (section * 10 <>)) (iota 20)) ### Documentation