From: Stefan Huchler <stefan.huchler@mail.de>
To: help-gnu-emacs@gnu.org
Subject: Re: beginnerquestion (nconc)
Date: Fri, 17 Mar 2017 17:59:20 +0100 [thread overview]
Message-ID: <87k27nzvo7.fsf@mail.de> (raw)
In-Reply-To: CAP_d_8XMuGGtuOukcb6UUvf9KpejjjP_Q1DJJr-S1mqftBpu=g@mail.gmail.com
Yuri Khan <yuri.v.khan@gmail.com> writes:
> You are probably annoyed because you are familiar with list
> implementations in other languages where appending an element is a
> cheap operation. For example, with a doubly linked list, appending at
> either end is constant time.
>
> However, Lisp uses singly linked lists. Appending at the start is a
> matter of allocating a single cons cell and setting its cdr to point
> to the old head of the list. However, appending at the end requires
> traversing the whole list to find its last cell, and then adding a new
> cell there. That’s what nconc does. So if your list is 1000 items
> long, populating it from beginning to end takes roughly 500000
> operations. Populating from the end to beginning and then reversing
> will only take on the order of 3000 operations.
Hello Yuri,
thanks at least I see know WHY it is designed that way, performence.
But if I have to reverse, wouldnt it be easier that there is some sort
of:
(yreverse sequence)
that alters the sequence directly, or overwrites it.
instead of:
(setq sequence (nreverse sequence))
would make it less ugly.
next prev parent reply other threads:[~2017-03-17 16:59 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-17 5:58 beginnerquestion (nconc) Stefan Huchler
2017-03-17 7:33 ` Thien-Thi Nguyen
2017-03-17 8:52 ` Thien-Thi Nguyen
2017-03-17 14:19 ` Stefan Huchler
2017-03-17 14:48 ` tomas
2017-03-17 14:42 ` Yuri Khan
2017-03-17 16:59 ` Stefan Huchler [this message]
2017-03-17 17:20 ` Drew Adams
2017-03-17 17:32 ` Drew Adams
2017-03-18 2:15 ` Stefan Monnier
2017-03-21 16:47 ` Stefan Huchler
2017-03-21 16:59 ` Stefan Huchler
2017-03-21 20:14 ` John Mastro
2017-03-22 0:32 ` Stefan Huchler
2017-03-22 15:02 ` Michael Heerdegen
2017-03-22 19:00 ` John Mastro
2017-03-21 11:05 ` Michael Heerdegen
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