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Re: how to access a large datastructure efficiently?
From: |
Thierry Volpiatto |
Subject: |
Re: how to access a large datastructure efficiently? |
Date: |
Thu, 04 Mar 2010 08:25:52 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.1.93 (gnu/linux) |
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/
- how to access a large datastructure efficiently?, Christian Wittern, 2010/03/03
- Re: how to access a large datastructure efficiently?, Christian Wittern, 2010/03/04
- Re: how to access a large datastructure efficiently?, Thierry Volpiatto, 2010/03/04
- Re: how to access a large datastructure efficiently?, Thien-Thi Nguyen, 2010/03/04
- Re: how to access a large datastructure efficiently?, Andreas Politz, 2010/03/04