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From: chucko@ptolemy.arc.nasa.gov (Chuck Fry)
Newsgroups: comp.lang.lisp
Subject: Re: another question, arrays
Message-ID: <13228@ptolemy-ri.arc.nasa.gov>
Date: 31 May 91 00:43:33 GMT
References: <43929@netnews.upenn.edu>
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In article <43929@netnews.upenn.edu> vinson@linc.cis.upenn.edu (Jack Vinson) writes:
>Me again,  I am also playing with arrays for the first time in lisp and am
>wondering what kind of interesting functions there are for messing with them.
>In particular I will be frequently updating one array by moving elements down
>into the array.  ie if the array is (1.2 1.1 1.2 1.3 1.1 1.0) and new data is
>1.2 I want to put the new info at the 'beginning' of the array and shift
>everything down, so I would get (1.2 1.2 1.1 1.2 1.3 1.1) with the last element
>knocked off.  The way I know how to do this is with a do-loop that copies
>element i into i+1, starting from the highest element.

CLtL provides a facility for dealing with vectors (simple arrays) as
stacks.  If you don't mind the nth element of the array being most
recent instead of the 0th, you can use VECTOR-PUSH or
VECTOR-PUSH-EXTEND on an array with a fill-pointer.
VECTOR-PUSH-EXTEND adjusts the array if the initial array size is
exceeded, providing you created the array with :ADJUSTABLE T to begin
with.

Then VECTOR-POP can destructively access the most recent element, or
 (AREF stack (1- (FILL-POINTER stack)))
will do the same thing non-destructively.

If the most recent must be the 0th element, you're pretty much stuck
with the DO-loop approach.

    Chuck Fry  Chucko@Charon.ARC.NASA.GOV  ...ames!ptolemy!chucko
  Disclaimer: No one but me is responsible for this misinformation.

"Sun is in an ideal situation.  It has convinced the market that open
technology is the best.  Yet, the company narrows its own interpretation
of 'open' as its market share and technology advantages expands."
 -- "The Fifth Architecture", Mark Hall, in SunWorld, April 1991