Question

Here is the question:

Write a function (first-n-elements lst n), which returns a list containing only the first n elements of lst. For example, (first-n-elements '(1 2 3 4 5 6) 3) should return '(1 2 3). Your function should handle the case where n is greater than the length of the list (in which case it would return the entire list), and where n is 0 (should return '()).

My answer is:

(define (first-n-elements lst n)
  (cond((null? lst) '())    
       ((= n 0) lst))   
       ((> n 0) (cons (+ (car lst) 1) (first-n-elements) (cdr lst) (- n 1))))

I know it’s wrong, please help

Answer 1

Part of the question says that if n is 0, it should return '(). But what does your function do when n is 0?

Also, think about the recursive case here (> n 0). You’re currently returning the cons of four arguments:

• (+ (car lst) 1)
• (first-n-elements)
• (cdr lst)
• (- n 1)

But cons only takes two arguments. You have all the parts there, but they’re not quite put together right. What do you want to be consing together?

Also: why are you adding 1 to (car lst)? That’s not going to work if lst has something other than numbers in it.