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Re: how to access a large datastructure efficiently?
From: |
Andreas Röhler |
Subject: |
Re: how to access a large datastructure efficiently? |
Date: |
Thu, 04 Mar 2010 09:13:48 +0100 |
User-agent: |
Thunderbird 2.0.0.19 (X11/20081227) |
Thierry Volpiatto wrote:
> 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...
>
if n is member of the seq, maybe equal-operator too
(<= last-elm elm)
is correct?
Thanks BTW, very interesting
Andreas
- 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