From Newsgroup: comp.lang.scheme

B. Pym wrote:

Write a Function 'total' that takes an orderd list ie.
((itemA quantityA costA)(itemB quantityB costB)....)
but returns a list giving the total cost plus the overall cost.

Eg:

LISP>(total'((book 2 10)(pen 3 2)(notepad 1 12)))
((BOOK 20)(PEN 6)(NOTEPAD 12)(TOTAL 38))

Thank you for your time

Here are 3 quite different solutions... one recursive, one iterative,
and one obscure. I won't do ALL your homework, so you will have to
figure out yourself how they work, which shouldn't be very hard. Disclaimer: I'm quite new to Lisp myself, so there may be better and
more elegant solutions.

;;; 1
(defun total (lst)
(labels ((total-rec (sublst subtot)
(if sublst
(let* ((sub (car sublst))
(sum (* (cadr sub) (caddr sub))))
(cons `(,(car sub) ,sum)
(total-rec (cdr sublst) (+ subtot sum))))
`((total ,subtot)))))
(total-rec lst 0)))

;;; 2
(defun total (lst)
(let ((total 0)
(ret))
(dolist (elt lst (nreverse (push `(total ,total) ret)))
(let ((subtotal (* (cadr elt) (caddr elt))))
(push `(,(car elt) ,subtotal) ret)
(incf total subtotal)))))

;;; 3
(defun total (lst)
(append
(mapcar #'(lambda (e) (list (car e) (* (cadr e) (caddr e)))) lst)
`((total ,(apply #'+ (mapcar #'(lambda (elt) (* (cadr elt) (caddr elt))) lst))))))

Gauche Scheme

(define (proc purchases)
(define grand-total 0)
(define (calc item quan price)
(let1 total (* quan price)
(inc! grand-total total)
(list item total)))
(append
(map: calc purchases)
(list (list 'total grand-total))))

(proc '((book 2 10) (pen 3 2) (notepad 1 12)))
===>
((book 20) (pen 6) (notepad 12) (total 38))


Given:

(define (map: func seq)
(map (lambda(x) (apply func x)) seq))
--
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