[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: alist and assq
From: |
Freeman Gilmore |
Subject: |
Re: alist and assq |
Date: |
Fri, 20 Mar 2020 10:39:03 -0400 |
On Fri, Mar 20, 2020 at 2:33 AM Gábor Boskovits <address@hidden> wrote:
>
> Hello,
>
> Freeman Gilmore <address@hidden> ezt írta (időpont: 2020. márc. 20., Pén
> 1:45):
>>
>> Hi:
>>
>> I am not a programmer. Have some understanding of scheme. I have a
>> question that I cannot find the answer to on the net. I joined this group
>> to ask if someone would help me find the answer.
>>
>> I am looking for a manual or whatever that will explain in detail how an
>> alist is stored in memory and how it works at this level. Also, how assq
>> and assq-ref work, what is past to the alist at the same level. Also, the
>> source code would be good.
>
> Storing an atomic value is implementation dependent, but usually implemented
> by a type tagged structure.
> (You can think about this as a tuple of a type, buffer, where buffer is a
> length,pointer there are several optimizations, but this is a possibility).
>
> A pair is a structure holding two atomics with accessors to them.
> (Possible implementation:
> Type:pair, buffer is twice as long as an atomic, two atomics are stored
> there. The accessors return the buffer buffer+sizeof(atomic) respectively.)
>
> For lists there is a special value, the empty list. (This can be implemented
> by setting the pointer to null)
>
> A list is stored as a pair where the first member is a value, the current
> first element, and the second is a list, the current tail.
>
> An alist is a list of pairs.
I know that in a pair the is a key that points to a value ant the pair
points to the next pair in the alist.
>
>
> Assq and assq-ref is just a find with a predicate on the first element of the
> pair eq to the value passed in as key. Actually assq-ref is (compose car
> assq).
I know (assq X (alist)) but it takes more code to do (assq X my-alist),
my-alist = (alist)
>
>
> What find does is that it recurses on the list. It is something like:
> (define (find pred list)
> (if (empty list)
> #f
> (if (pred (car list))
> (car list)
> (find pred (cdr list)))))
>
> I hope it helps.
What I am looking for is a step by step through the complete procedure,
my-alist = #’((K1 . V1) (K2 . V2) (K3 . V2)))
#(assp ‘K2 my-lis) => (K2 . V2),
At the source code level starting at assq. Do not care how the K’s
and V’s were place in memory or how K2 was input. The source code
would help but I would probably need some help stepping through it
unless it is well documented.
Thank you, ƒg
>
>
> This is just a conceptual level, usually a modern implementation has a lot of
> optimalizations on top of that.
>
> An alist is a list of pairs.
>>
>> Thank you, ƒg
>
> Best regards,
> g_bor