SICP Video Lecture 2

Question

I have a problem with this example

(define (+ x y)
  (if (= x 0)
      y
      (+ (-1+ x) (1+ y))))

What is the problem with -1+ and 1+, when i evaluate it i get this result

• DrScheme: -1+: this function is not defined
• racket : reference to undefined identifier: -1+

but i write this instead and it works

(define (add x y)
  (if (= x 0)
      y
      (+ (- x 1) (+ y 1))))

Answer 1

For Racket:

• Use add1 instead of 1+
• Use sub1 instead of -1+ or 1-

The trouble is, none of those names are standard, so you can't reliably use them across all Scheme implementations. :-)

Answer 2

You can fix this by adding SICP support to DrRacket.

http://www.neilvandyke.org/racket-sicp/

Anymore trouble let me know.