Question

From R5RS under 4.2.4 Iteration –

(let ((x '(1 3 5 7 9)))
  (do ((x x (cdr x))
       (sum 0 (+ sum (car x))))
      ((null? x) sum))) 

What is the usage of – cdr and car .

And what happens with this x in the lines –

(do ((x x (cdr x))
           (sum 0 (+ sum (car x))))
          ((null? x) sum))

Answer 1

• car = first (as in first element of list)
• cdr = rest (as in everything but the first element of list)

The loop adds up the items in the list.