Recursion in Lisp and making my own length function

Question

I am trying to make my own length/2 function(which allows you to find the length of a list) in lisp and am having an issue.

If I were to program this in java I would create a global index variable

that = 0

Then in the method I would do

if(list.equal(null))
{
    return index;
}
else
{
    index++;
    return functioname(tail of list) ;
}. 

Obviously this is not actual java syntax but I am just trying to relay the logic I want to apply in lisp.

My main issue is that ifs in lisp only allow you to do

if test expression
    then do something
else
    do something else

while I am trying to do

if test expression
    then do something
else
    do 2x something

Is there a way I can accomplish this in lisp or is there a better way to go about this problem?

Answer 1

Do it recursively:

len(list):
    if list is null
        return 0
    else
        remove first_item from list
        return 1 + len(list)


(define (length items)
  (if (null? items)
      0
      (+ 1
         (length (cdr items)))))


(length (list 1 2 3))

3

Or use set!:

(define count 0)

(define (length items)
  (when (not (null? items))
    (set! count (+ count 1))
    (length (cdr items))))

(length (list 1 2 3 4 5))

count

5