Re: how to access a large datastructure efficiently?

[Thierry Volpiatto <thierry.volpiatto@gmail.com>]

Hi,

Christian Wittern <cwittern@gmail.com> writes:

> Hi there,
>
> Here is the problem I am trying to solve:
>
> 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]
>
> Now when I am trying to access with a key that is not in the list, I 
> want to have the one with the closest smaller key returned, so for 6 
> and 7 this would be 1, but for 8 and 9 this would be 8.
>
> Since the list will have thousands of elements, I do not want to simply 
> loop through it but am looking for better ways to do this in Emacs lisp.  
> Any ideas how to achieve this?

,----
| (defun closest-elm-in-seq (n seq)
|   (let ((pair (loop with elm = n with last-elm
|                  for i in seq
|                  if (and last-elm (< last-elm elm) (> i elm)) return (list last-elm i)
|                  do (setq last-elm i))))
|     (if (< (- n (car pair)) (- (cadr pair) n))
|         (car pair) (cadr pair))))
`----

That return the closest, but not the smaller closest, but it should be
easy to adapt.

-- 
Thierry Volpiatto
Gpg key: http://pgp.mit.edu/