Scheme - ex1.6 of the SICP book

Author

ex1.6 of the SICP book

Matthew Zhou

2007-03-19, 8:12 am

Any one also reading the book, Structure and Interpretation of
Computer Programs may help. I cannot work out the answer of ex1.6

I translated this problem to the code below, what's wrong with it?

(define (new-if predicate then-clause else-clause)
(cond (predicate then-clause)
(esle else-clause)))

(define (sqrt-iter guess x)
(new-if (good-enough? guess x)
guess
(sqrt-iter (improve guess x)
x)))

(define (improve guess x)
(average guess (/ x guess)))

(define (average x y)
(/ (+ x y) 2))

(define (good-enough? guess x)
(< (abs (- (square guess) x)) 0.1))

(define (square x)
(* x x))

(define (abs x)
(if (> x 0) x
(- x)))

(define (sqrt x)
(sqrt-iter 1.0 x))

As I run this program, the interpreter gave me the message below:
;Aborting!: maximum recursion depth exceeded

Jussi Piitulainen

2007-03-19, 8:12 am

Matthew Zhou writes:

> Any one also reading the book, Structure and Interpretation of
> Computer Programs may help. I cannot work out the answer of ex1.6
>
> I translated this problem to the code below, what's wrong with it?
>
> (define (new-if predicate then-clause else-clause)
> (cond (predicate then-clause)
....

It's a matter of timing: sometimes, like in this exercise, it really
is too late to decide whether you want to compute something or not;
you have done it already! The argument names are misleading.

> As I run this program, the interpreter gave me the message below:
> ;Aborting!: maximum recursion depth exceeded

That is an expected result. The exercise asks for an explanation. How
does a procedure call, like (acos (- 1)), work? How does a conditional
expression, like (if (> 0 0) (/ 0) 0), work?

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