CODEWITHSUNDEEP
schemelisp
Tianshu Xu
Tianshu Xu

Reputation: 3

difference in "if" and "cond" in Scheme

I'm learning Berkeley CS61A using Scheme. pigl function is used to introduce recursion and the sample code is as follows

(define (pigl wd)
  (if (pl-done? wd) 
    (word wd 'ay)
    (pigl (word (bf wd) (first wd))) ) )

I tried to change if tocond expression, the modified code is as follows

(define (pigl2 wd)
  (cond ((pl-done? wd) (word wd 'ay))
      (else pigl2 (word (bf wd) (first wd))) ) )

Based on my understanding of the interpretation of if and cond in SICP, I think these two procedures should be equivalent. However pigl2 didn't work that it gave 'ab for input 'ba and the correct answer is abay.

I don’t know if I misunderstood the evaluation rules of if and cond or if I made any other stupid mistakes. Please help me, thanks!

Upvotes: 0

Views: 113

Answers (1)

Óscar López
Óscar López

Reputation: 235984

This is your code, commented to highlight the problem:

(define (pigl2 wd)
  (cond ((pl-done? wd) (word wd 'ay))
      (else
       pigl2 ; this line does nothing at all
       (word (bf wd) (first wd))))) ; return result of calling `word`

You forgot to actually call pigl2: it's missing the surrounding brackets! This should fix the problem:

(define (pigl2 wd)
  (cond ((pl-done? wd) (word wd 'ay))
        (else (pigl2 (word (bf wd) (first wd))))))

Upvotes: 2

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