the SICP book solution error help, number 1.7

Question

Hi I am reading the SICP, I am stuck in the Exercise 1.7:

here is my code:

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

;;(avg 1 2)


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

;;(square 2)


(define (improve guess x)
  (avg guess (/ x guess)))

;;(improve 1 2)


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

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

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

(my-sqrt 100)

and the DrRacket gives an error:

function call: expected a function after the open parenthesis, but received 50.005

what's that mean and how to fix the problem?

Answer 1

There are a couple of mistakes in the code, including misplaced parentheses. For starters, good-enough? as defined for the exercise 1.7, should look like this:

(define (good-enough? guess-old guess-new)
  (< (/ (abs (- guess-old guess-new)) guess)
     (/ 0.001 guess)))

And sqrt-iter, as written in your code, will lead to an infinite recursion. Try this instead:

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

(define (my-sqrt x)
  (sqrt-iter 0.0 1.0 x))