Multiply two lists in Racket

Question

I am using Racket to multiply to list with the same length. So far I have tried:

(define (multiply-list a b)
  (if ([(empty? a) (empty)])
      else (cons(*car(a)car(b)))
      cdr(a) cdr(b)))                  

I am having trouble understanding the syntax of Racket. I want to update the list to it's cdr . but I cannot get it right a and b are the lists.

Answer 1

I believe you were aiming for something like this:

(define (multiply-list a b)
  (if (empty? a)
      empty
      (cons (* (car a) (car b))
            (multiply-list (cdr a) (cdr b)))))

Explanation:

• In Scheme, we have to be very, very careful where we put a pair of () . In your code, some parentheses are unnecessary and others are misplaced. A good IDE will help you put them in the right place
• For instance, the pair of [] surrounding the condition are wrong, and so is this: (empty) because empty is not a function, we surround something with () when we want to call it like a function
• And we don't call a function like this: car(a) . The correct way is: (car a)
• When we use if , the alternative part of the expression must not be preceded by else , maybe you're confusing an if expression with a cond expression.
• And last but not least: don't forget to call the recursion!