Date:         Sun, 21 Mar 1999 15:41:11 +0000
Reply-To:     John Whittington <medisci@POWERNET.COM>
Sender:       "SAS(r) Discussion" <SAS-L@UGA.CC.UGA.EDU>
From:         John Whittington <medisci@POWERNET.COM>
Subject:      Re: Trouble using lags to assign values?
Comments: To: "LINCK, JIM" <LINCK@SSB.ROCHESTER.EDU>
Content-Type: text/plain; charset="us-ascii"

At 00:49 21/03/99 -0500, LINCK, JIM wrote:
 
>Is there some limitation to lags that they can't be used to assign values in
>a data step?  For example, suppose I have a list like:
>ID  Date Var1
>1   1965  2
>1   1966  .
>1   1967  .
>etc.
>where . are missing values.  All I want to do is assign 1966's . a 2 and
>1967's . a 2.  Thought I could do it as simply as:
>if ID=lag(ID) then do;
>  if Var1=. then Var1=lag(Var1);
>end;
>No dice.  Doesn't work.  ....
 
You have hit the classic 'catch' of conditionally calling a LAG function in
SAS.  However, if all you want to do is 'carry forward' real values into
subsequent missing ones, you don't need LAG functions at all - you can
simply do it by retaining the last non-missing value.  Something like:
 
data whatever (drop = lastval) ;
    retain lastval ;
    <code as required>
    if var1 = . then var1 = lastval ; else lastval = var1 ;
run ;
 
Regards
 
John
 
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