;; iterative version of sum
(define sum
  (lambda (num-list)
    (define iter
      (lambda (lst result)
        (if (null? lst)
            result
            (iter (cdr lst) (+ (car lst) result)))))
    (iter num-list 0)))

(define infinity (/ 1.0 0.0))

;; iterative version of minimum
(define minimum
  (lambda (num-list)
    (cond ((null? num-list)
           infinity)
          ((null? (cdr num-list))
           (car num-list))
          ((< (car num-list) (cadr num-list))
           (minimum (cons (car num-list) (cddr num-list))))
          (else (minimum (cdr num-list))))))