William James
7/23/2015 7:53:00 PM
Gabe Garza wrote:
> Peter Seibel <peter@javamonkey.com> writes:
>
> > As part of a larger program I needed a function which given a number
> > (n) and a vector (row), returns the index into row of the n'th nil in
> > row. Here's the first (working) version I came up with:
> >
> > (defun nth-slot (n v)
> > "Find the nth nil cell in a vector v."
> > (loop with slots-seen = 0
> > for item across v
> > for idx from 0
> > counting (not item) into slots-seen
> > until (> slots-seen n)
> > finally (if (> slots-seen n) (return idx) (error "No slot"))))
>
> Here's how I'd write it (criticism welcome, of course!):
>
> (defun nth-slot (n vector)
> (loop for index from 0 below (length vector)
> if (null (aref vector index))
> do (decf n)
> (when (zerop n)
> (return index))))
Gauche Scheme:
(use srfi-43 :only (vector-index))
(define (nth-false-slot n vec)
(vector-index
(^x (zero? (dec! n (if x 0 1))))
vec))
(nth-false-slot 2 #(2 3 #f #f 5 6))
===>
3
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