本次美国代写主要为Racket相关的限时测试

Question Format

Objective #N [ do not title questions, it makes it even easier for students to collude ]

[potential code / diagram / figure that will be referenced in the question]

Satisfactory:

[problem description that outlines what the student must answer to be marked as

satisfactory. Note that small mistakes (e.g., typos such as 1-2 misplaced

parentheses where the TA can understand what the student meant without spending

>60seconds) are okay if they do not compromise the essence of the answer.]

Excellent:

[an add-on to the satisfactory problem, usually a bit harder, that the student must

perform to get this question right. Note that they can only get the excellent mark if

they both get a good solution and also have a nearly-100% correct answer to the

satisfactory part]

Objective #1: “Identify expressions, forms, and callsites (in

Racket)”

Question 1Sample

Satisfactory:

Both A-B required for satisfactory

How many callsites does the following piece of Racket code contain:

(define (foo) (bar (baz)))

(define (bar x) (baz))

(define (baz) (+ 1 (bar x)))

Explain why the following Racket expression prints 3 rather than 1.

(let ([x 3]) (let ([x 1] [y x]) y))

Excellent:

A student believes that one of the guards to their if forms is incorrect, so they

deﬁne the following helper function:

(define (if guard e-true e-false)

(displayln (format “guard is ~a” guard))

(if guard e-true e-false))

Aside from printing to the console, are there any other ways in which deﬁning

this helper function may change the behavior of the program? Give a precise

explanation (with example code, if possible) of how this may happen.

Comments:Question 1A

Satisfactory:

Both A-B required for satisfactory

(A)How many callsites does the following Racket expression contain?

(g (f ((lambda (x) x) 2))1)2 3

(B) Consider the following Racket expression using let*:

(let ([x (let* ([x 1] [y x]) y)]) x)

How could this expression be written without let* (using only let). For your

solution to receive credit, it must still include bindings for each of x and y within

the top-level let (i.e., your answer may not simply be `1`).

(let ([x (let ([x 1]) (let ([y x])) y)]) x)

(let* ([x 1]

[y …])

…)

(let ([x 1]) (let ([y …]) …)…)

Excellent:

In class, we have discussed that or is not a Racket function, but rather it is a

form. In general, for arbitrary expressions e-ﬁrst and e-second the following two

Racket snippets are not identical (in the sense that you could replace one with

the other).

(deﬁne (loop) (loop))

(if #t #t (loop)) ;; #t

(if #t #t (loop)) ;; #t

((lambda (x y) (if x x y)) #t (loop)) ;; function call to (lambda (x y) …)

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