Re: Scheme question: convert a range

[David Nalesnik]

Hi Simon,

On Sun, Nov 15, 2015 at 12:53 PM, Simon Albrecht <address@hidden> wrote:

Hello,

The subject certainly seems cryptic – it’s difficult to summarize, but an example will make it clear immediately.
I want to write a scheme procedure, which takes a pair like #'(3 . 7) and returns a list with all the numbers in the range: #'(3 4 5 6 7)
How is this done most easily?

Well, if the numbers are ascending you could do something like:

(define (expand-interval arg)

   (iota (- (1+ (cdr arg)) (car arg)) (car arg)))

(display (expand-interval '(3 . 7)))

If not, something like this would work:

(define (expand-interval arg)

   (let ((dir (if (> (car arg) (cdr arg)) -1 1)))

     (let loop ((elt (car arg)) (result '()))

       (if (= elt (+ dir (cdr arg)))

           (reverse result)

           (loop (+ dir elt) (cons elt result))))))

HTH,

David