Scheme: Proabably a typo in SICP. 1.3.2. Am I right?

Question

In SICP, (1.3.2. page: 62), there is a solution for finding pi-sum using lambda. The solution is:

(define (pi-sum a b)
  (sum (lambda (x) (/ 1.0 (* x (+ x 2))))
       a
       (lambda (x) (+ x 4))
       b))

However, I feel there should be a bracket enclosing ((lambda (x) (+ x 4) b). The program as such produces an error saying sum is expecting a number but getting a procedure.

Modifying the above code is giving no error.

(define (pi-sum a b)
  (sum ((lambda (x) (/ 1.0 (* x (+ x 2))))
       a)
       ((lambda (x) (+ x 4))
       b)))

Please correct me if my understanding is wrong. I assume the book cannot be wrong.

https://stackoverflow.com/questions/49604094/sicp-1-2-4-typo-in-fast-exp — Sreekumar R, Sep 16 '20 at 03:01

score 4 · Accepted Answer · answered Sep 16 '20 at 02:00

The pi-sum procedure in the book is making use of the higher-order procedure sum, defined earlier in 1.3.1. The sum procedure takes a and b as arguments which describe the bounds of the summation, and it takes term and next as arguments which describe how to create a term from a, and how to get the next a from the current a. Both term and next need to be procedures. Here is the definition of sum from the book:

(define (sum term a next b)
  (if (> a b)
      0
      (+ (term a)
         (sum term (next a) next b))))

If you add parentheses in the definition of pi-sum, you should get an exception because sum requires four arguments, but now only two are passed. I am not sure why you are getting an error like "sum is expecting a number but getting a procedure", but I suspect that you have a different definition for sum than the one intended by the book.

Sorry. It's my mistake. Instead of understanding the `sum` procedure, I defined my own sum procedure `(define sum +)`. Thanks. — Sreekumar R, Sep 16 '20 at 03:05

Scheme: Proabably a typo in SICP. 1.3.2. Am I right?

1 Answers1