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Re: Element-Relative Sequence Insertion
From: |
Pascal J. Bourguignon |
Subject: |
Re: Element-Relative Sequence Insertion |
Date: |
Thu, 11 Jun 2009 15:01:18 +0200 |
User-agent: |
Gnus/5.101 (Gnus v5.10.10) Emacs/22.2 (gnu/linux) |
Nordlöw <per.nordlow@gmail.com> writes:
> On 11 Juni, 14:38, p...@informatimago.com (Pascal J. Bourguignon)
> wrote:
>> Nordlöw <per.nord...@gmail.com> writes:
>> > How do I insert an element (object) relative to another in a sequence
>> > (or list in my case)? Example:
>>
>> > (insert-after 'X '(a b c) 'b) => '(a b X c)
>> > (insert-before 'X '(a b c) b) => '(a X b c)
>>
>> This is not what you want. You don't what these function to return a
>> 2-element lists, the first of which is the symbol quote. Unless
>> you're doing meta-programming you never want to have such a result.
>>
>> You wan this:
>>
>> (insert-after 'X '(a b c) 'b) --> (a b X c)
>> (insert-before 'X '(a b c) 'b) --> (a X b c)
>>
>> Also, you may want to mention whether your insert-after and
>> insert-before function are destructive or non-destructive. Since you
>> show as only specification examples using literal data, we will assume
>> you want them to be non-destructive.
>>
>> So you do that, just by doing it. There's nothing simplier.
>>
>> (qrsha vafreg-nsgre (arj yvfg byq)
>> (pbaq ((ahyy yvfg) '())
>> ((rdy byq (pne yvfg)) (pbaf (pne yvfg) (pbaf arj (pqe yvfg))))
>> (g (pbaf (pne yvfg) (vafreg-nsgre arj (pqe yvfg) byq)))))
>>
>> (qrsha vafreg-orsber (arj yvfg byq)
>> (pbaq ((ahyy yvfg) '())
>> ((rdy byq (pne yvfg)) (pbaf arj yvfg))
>> (g (pbaf (pne yvfg) (vafreg-orsber arj (pqe yvfg) byq)))))
>>
>> --
>> __Pascal Bourguignon__
>
> But how do I implement insert-after() and insert-before()? I guess the
> code in the end doesn't do anything meaningful :)
It does, but you have to decrypt it first.
Could you implement first something simplier than insert-after or
insert-before?
For example, could you implement a function map, which builds a new
list with the result of calling a function to each element of a list:
(defun map (function list) ...)
(map (lambda (x) (1+ x)) '(1 2 3)) --> (2 3 4)
(map (lambda (x) (1+ x)) '()) --> ()
If you can do that, then you should be able to implement
insert-before and insert-after adding only one line.
--
__Pascal Bourguignon__