What is wrong with my scheme code?
Question
The function I wrote for SICP 2.20 is:
(define (same-parity x . y)
(if (null? (car y)
'()
(if (= (even? (car y)) (even? x))
(cons (car y) (same-parity (cons x (cdr y))))
(same-parity (cons x (cdr y))))))
And then I try to call it with
(same-parity 1 2 3 4 5 6 7)
The error I get is: "The object #t, passed as the first argument to integer-equal? is not the correct type."
I thought that equal works with #t and #f...
An example of code I found online is the following, I ran it and it works. But, what am I doing wrong?
(define (same-parity a . rest)
(define (filter rest)
(cond ((null? rest) '())
((= (remainder a 2) (remainder (car rest) 2))
(cons (car rest) (filter (cdr rest))))
(else
(filter (cdr rest)))))
(filter (cons a rest)))
Solution
The = procedure accepts numbers. But even? returns a boolean not a number.
Use equal? instead of =.
equal? works with booleans.
For instance at the REPL:
> (even? 2)
#t
> (= (even? 2) (even? 2))
=: expects type <number> as 1st argument, given: #t; other arguments were: #t
> (equal? (even? 2) (even? 2))
#t
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