Rainer Joswig wrote:

How would you take a sequence of numbers in groups of 2 and print out
the numbers and their product?

Here is my solution, but I'm sure some li5p h4x0r has a much more 1337 solution :)

(setf nums (vector 21 47
23 43
24 29
22 28
22 53
31 36
22 56
31 72))

Remember that variables are introduced with DEFPARAMETER (or DEFVAR)
and not with SETF.

Also remember that all top-level variables should be by convention
written like this: *nums*

(loop for i from 0 below (car (array-dimensions nums)) by 2
do (progn
(let ((a (aref nums i))
(b (aref nums (1+ i))))
(format t "~A * ~A = ~A~%" a b (* a b)))))

ARRAY-DIMENSION is not the best operation for a vector in
general, since the vector could be one with a fill-pointer.
LENGTH will give you the real used length of a vector.

This solution is basically the same as yours but a bit
differently written:

(loop with size = (length *nums*)
for i from 0 below size by 2
for j from 1 below size by 2
for a = (aref *nums* i) and b = (aref *nums* j)
do (format t "~A * ~A = ~A~%" a b (* a b)))

(use srfi-43) ;; vector->list for Gauche
or
(require srfi/43) ;; vector->list for Racket

(require srfi/48) ;; format for Racket

(define nums
(vector 21 47
23 43
24 29
22 28
22 53
31 36
22 56
31 72))

(do ((i 0 (+ i 2)))
((= i (vector-length nums)))
(let ((both (vector->list nums i (+ i 2))))
(apply format #t "~a * ~a = ~a~%" `(,@both ,(apply * both)))))

21 * 47 = 987
23 * 43 = 989
24 * 29 = 696
22 * 28 = 616
22 * 53 = 1166
31 * 36 = 1116
22 * 56 = 1232
31 * 72 = 2232

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XPost: comp.lang.scheme

Rainer Joswig wrote:

This solution is basically the same as yours but a bit
differently written:

(loop with size = (length *nums*)
for i from 0 below size by 2
for j from 1 below size by 2
for a = (aref *nums* i) and b = (aref *nums* j)
do (format t "~A * ~A = ~A~%" a b (* a b)))

Gauche Scheme

(define nums (vector 21 47
23 43
24 29
22 28
22 53
31 36
22 56
31 72))

(with-gen (n nums)
(until (n-null?)
(let ((a (n)) (b (n)))
(format #t "~a * ~a = ~a\n" a b (* a b)))))

21 * 47 = 987
23 * 43 = 989
24 * 29 = 696
22 * 28 = 616
22 * 53 = 1166
31 * 36 = 1116
22 * 56 = 1232
31 * 72 = 2232

Given:

;; Make a vector generator.
(define (gmk vec)
(let ((v vec)
(i 0))
(values
(lambda()
(if (= i (vector-length v)) #f (begin0 (~ v i) (inc! i))))
;; Tells if generator is exhausted.
(lambda() (= i (vector-length v))))))

(define-macro with-gen
(lambda (args . body)
(define duos
(let go ()
(if (null? args) '() (cons (list (pop! args)(pop! args)) (go)))))
(define (proc s v)
`((,s ,(symbol-append s '-null?)) (gmk ,v)))
`(let-values
(,@(map (lambda (xs) (apply proc xs)) duos))
,@body)))


--
[T]he problem is that lispniks are as cultish as any other devout group and basically fall down frothing at the mouth if they see [heterodoxy].
--- Kenny Tilton
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham

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