Scheme - Appending to the end of a list

Question

I'm very sorry for such a simple question. It's so trivial, I haven't been able to find someone online with this problem. So I'd appreciate some help.

I want to write a very simple function that takes a list and an item, and appends that item to the end of the list.

The function I wrote recursively goes to the end of the list and returns the new item. It's so frustrating that this isn't working. It's the simplest function I've ever seen

(define (my-append lst item)
  (if (null? lst)
    item
    (cons (car lst) (my-append (cdr lst) item))))

(display (my-append (list 1 2 3 4) 5))

This displays

(1 2 3 4 . 5) 

I don't know why that dot is there, and it's extremely frustrating. I haven't come across it in any previous SO questions.

I just want to see

(1 2 3 4 5)

I would really really appreciate some help, because I'm extremely frustrated with this. If it helps, I'm running this code using an online compiler https://repl.it/languages/scheme

Answer 1

You just need to end the recursion with a list, not an item. Instead of this:

(if (null? lst)
    item

Do this:

(if (null? lst)
    (list item)

To clarify - a list in Scheme must end in the empty list '(). If your recursion ends with an item, you'll get something like this in the end:

(cons 4 5)
=> '(4 . 5)

That's a cons pair. A proper list ends in the empty list:

(cons 4 (cons 5 '()))
=> '(4 5)

Which is the same as:

(cons 4 (list 5))
=> '(4 5)

By the way, this is the idiomatic way to append an item at the end:

(define (my-append lst item)
  (append lst (list item)))