Kenny Tilton wrote:

> Peter Seibel wrote:
> > 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"))))
>
> Please take that inscrutal glop to comp.lang.loop. <g>
>
> (defun nth-null-slot-index (n v &optional must-find-p &aux (nil-ct 0))
> "Return the index of the nth nil cell in vector v"
> (dotimes (x (length v) (when must-find-p (error "bzzzt")))
> (when (null (elt v x))
> (when (= n (incf nil-ct)) ;; [1]
> (return-from nth-null-slot-index x)))))
>
> (nth-null-slot-index 3 #(nil 1 2 nil 3 4 nil 5 nil nil))
> =>6
>

We all tend to forget how powerful DO is.

Scheme:

(define (nth-false-index n vec)
(do ((i 0 (+ 1 i))
(n n (if (vector-ref vec i) n (- n 1))))
((or (zero? n) (and (= i (vector-length vec)) (error 'bzzt)))
(- i 1))))

(nth-false-index 1 #(#f 1 2 #f 3 4 #f 5 #f #f))
===>
0

(nth-false-index 2 #(#f 1 2 #f 3 4 #f 5 #f #f))
===>
3

(nth-false-index 3 #(#f 1 2 #f 3 4 #f 5 #f #f))
===>
6

(nth-false-index 4 #(#f 1 2 #f 3 4 #f 5 #f #f))
===>
8

(nth-false-index 1 #(2 3 4 5 ))
===>
*** ERROR: bzzt

(nth-false-index 2 #(2 3 4 5 #f))
===>
*** ERROR: bzzt