Date:         Mon, 24 Nov 2003 17:46:47 +0000
Reply-To:     John Whittington <John.W@MEDISCIENCE.CO.UK>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         John Whittington <John.W@MEDISCIENCE.CO.UK>
Subject:      Re: how to use lag
Comments: To: Richard Read Allen <peakstat@WISPERTEL.NET>
In-Reply-To:  <E1AOL9M-0003ja-00@coumxnn01.netbenefit.co.uk>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 11:09 24/11/03 -0700, Richard Read Allen wrote:

>Helen, Below is a simple way to accomplish what you want using retain
>rather than lag. One caution about the lag function is that it's best not
>to use it in
>a conditional statement as you have done (twice). It does funky things when
>used this way.
>
>[snip]
>
>data two;
>   set one;
>   retain x;
>   if y>. then x=y;
>run;

Richard, I'm afraid there are all sorts of reasons why this code would not
accomplish what Helen wants.

Firstly, it does not respect the ID boundaries, as Helen wants - i.e. it
would attempt to 'carry forward' data from one ID to the next.  Secondly,
the logic of the code is simply not correct.  As written, the value of x
will ALWAYS be missing, and will not achieve the desired 'carry
forward'.  However, it is very easy to achieve Helen's desired
functionality using RETAIN rather than lag functions, something like:

data two (drop = last_y) ;
   retain last_y ;
   set one;
   by id;
   if first.id then x=y;
   else if y=. then x=last_y;
     else x=y;
   last_y = y ;
run ;

Kind Regards,


John

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