Re: how to access a large datastructure efficiently?

[Christian Wittern]

Thierry Volpiatto <thierry.volpiatto <at> gmail.com> writes:

> >> I have a large list of items which I want to access.  The items are in 
> >> sequential order, but many are missing in between, like:
> >>
> >> (1 8 17 23 25 34 45 47 50)  [in reality, there is a value associated 
> >> with this, but I took it out for simplicity]
> 
> ,----
> | (defun closest-elm-in-seq (n seq)
> |   (let ((pair (loop with elm = n with last-elm
> |                  for i in seq
> |                  if (eq i elm) return (list i)
> |                  else if (and last-elm (< last-elm elm) (> i elm)) return
(list last-elm i)
> |                  do (setq last-elm i))))
> |     (if (> (length pair) 1)
> |         (if (< (- n (car pair)) (- (cadr pair) n))
> |             (car pair) (cadr pair))
> |         (car pair))))
> `----
> For the smallest just return the car...
> 

This seems to do what I need, thanks!  Now I have to see how that 
performs on the real data.  I was hoping there would be a method 
that did not involve loops, but some kind of binary search.  Would it be
possible to use a hash-table here?

Christian