Re: how to access a large datastructure efficiently?

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

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

> 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.

Case where your element is member of list, return it:

,----
| (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...

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