| Tutorial 4: Solutions (October 5, 2007)(and (symbol? 'hello)
(= (- 5 1) (* 2 3))
(/ "a string" "another string"))
⇒(and true
(= (- 5 1) (* 2 3))
(/ "a string" "another string"))
⇒(and (= (- 5 1) (* 2 3))
(/ "a string" "another string"))
⇒(and (= 4 (* 2 3))
(/ "a string" "another string"))
⇒(and (= 4 6)
(/ "a string" "another string"))
⇒(and false
(/ "a string" "another string"))
⇒false
(define a (+ 2 3))
(define (foo2 x)
(cond [(or (> x 1)
(< x -1))
(sqr x)]
[(zero? x) 1]))
(foo2 a)
(foo2 (/ a a))
⇒(define a 5)
(define (foo2 x)
(cond [(or (> x 1)
(< x -1))
(sqr x)]
[(zero? x) 1]))
(foo2 a)
(foo2 (/ a a))
⇒...
(foo2 5)
(foo2 (/ a a))
⇒...
(cond [(or (> 5 1)
(< 5 -1))
(sqr 5)]
[(zero? x) 1])
(foo2 (/ a a))
⇒...
(cond [(or true
(< 5 -1))
(sqr 5)]
[(zero? x) 1])
(foo2 (/ a a))
⇒...
(cond [true
(sqr 5)]
[(zero? x) 1])
(foo2 (/ a a))
⇒...
(sqr 5)
(foo2 (/ a a))
⇒...
25
(foo2 (/ a a))
⇒...
25
(foo2 (/ 5 a))
⇒...
25
(foo2 (/ 5 5))
⇒...
25
(foo2 1)
⇒...
25
(cond [(or (> 1 1)
(< 1 -1))
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [(or false
(< 1 -1))
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [(or (< 1 -1))
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [(or false)
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [(or)
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [false
(sqr 1)]
[(zero? 1) 1])
⇒...
25
(cond [(zero? 1) 1])
⇒...
25
(cond [false 1])
⇒...
25
(cond)
⇒Semantics error (no substitution rule for (cond)) (define (foo3 5)
(+ 1 5))
(/ (foo3 5)
0)
⇒Syntax error: 5 is not a valid variable name Note: The division by zero is not even considered because the syntax error is seen first. (define-struct name (first middle last))
(define (foo4 nme)
(name-middle (+ nme 1)))
(name-last (make-name "James" "A" "Garfield"))
⇒(define-struct name (first middle last))
(define (foo4 nme)
(name-middle (+ nme 1)))
"Garfield"
Note: The function foo4 will generate a semantics error on every function call, but since it is never called, there is no error here. (define (foo5 x)
(cond [(= 1 x) 2]
[else
(* 2
(foo5 (sub1 x)))]))
(foo5 3)
(foo5 -2)
⇒...
(cond [(= 1 3) 2]
[else
(* 2
(foo5 (sub1 3)))])
(foo5 -2)
⇒...
(cond [false 2]
[else
(* 2
(foo5 (sub1 3)))])
(foo5 -2)
⇒...
(cond [else
(* 2
(foo5 (sub1 3)))])
(foo5 -2)
⇒...
(* 2
(foo5 (sub1 3)))
(foo5 -2)
⇒...
(* 2
(foo5 2))
(foo5 -2)
⇒...
(* 2
(cond [(= 1 2) 2]
[else
(* 2
(foo5 (sub1 2)))]))
(foo5 -2)
⇒...
(* 2
(cond [false 2]
[else
(* 2
(foo5 (sub1 2)))]))
(foo5 -2)
⇒...
(* 2
(cond [else
(* 2
(foo5 (sub1 2)))]))
(foo5 -2)
⇒...
(* 2
(* 2
(foo5 (sub1 2))))
(foo5 -2)
⇒...
(* 2
(* 2
(foo5 1)))
(foo5 -2)
⇒...
(* 2
(* 2
(foo5 1)))
(foo5 -2)
⇒...
(* 2
(* 2
(cond [(= 1 1) 2]
[else
(* 2
(foo5 (sub1 1)))])))
(foo5 -2)
⇒...
(* 2
(* 2
(cond [true 2]
[else
(* 2
(foo5 (sub1 1)))])))
(foo5 -2)
⇒...
(* 2
(* 2
2))
(foo5 -2)
⇒...
(* 2
4)
(foo5 -2)
⇒...
8
(foo5 -2)
⇒Semantics error: infinite loop
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