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Re: Reverse pop, return value


From: Barry Margolin
Subject: Re: Reverse pop, return value
Date: Tue, 17 Nov 2009 16:34:53 -0500
User-agent: MT-NewsWatcher/3.5.3b3 (Intel Mac OS X)

In article 
<address@hidden>,
 LanX <address@hidden> wrote:

> Thanx Lennart!
> 
> Thats what I was looking for! 8)
> 
> ---------------
> (defun rpop (LIST)
>        "reverse pop"
>        (prog1
>             (car (last LIST))
>             (nbutlast LIST 1)
>        )
>        )
> --------------
> 
> bye
>   rolf

As I mentioned in the "remove last element in a list" thread, this won't 
work when the list gets down to 1 element.  nbutlast works by changing 
the cdr of the 2nd-to-last cons cell in the list to point to nil.  But 
if there's only 1 element, there's no 2nd-to-last cons cell to modify.  
You need to assign to the original variable, and this can only be done 
using a macro (that's why push and pop are macros).

-- 
Barry Margolin, address@hidden
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***


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