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From: Robert Feyerharm on 3 Feb 2010 11:47
Has anyone else struggled with the lag function in SAS? The lag function
often doesn't work in an intuitive manner in conditional statements, or if
missing values are involved.

Anyways, I wrote a short macro code to handle lag calculations which I'd
like to share. It works fairly well, although I may be reinventing the
wheel. A colleague was working with a large dataset containing missing
values, and wanted to impute the last known value into the missing values
for a particular field. For example, given the following test dataset, the
macro fills in the missing values of x with the last known value of x
(1,2, or 3):

data test;
input x y;
datalines;
1 1
.. 2
.. 3
.. 4
.. 5
2 6
.. 7
.. 8
3 9
.. 10
.. 11
.. 12
.. 13
.. 14
;
run;

/* Let num = no. of records in target dataset. */
%let num=14;

%macro lagvar(num);
%do i = 1 %to #
data test;
set test;
z=lag(x);
if x=. then x=z;
run;
%end;

data test;
set test;
drop z;
run;
%mend;

%lagvar(&num)

I'm curious if there is a shorter program that can handle the same
operations without resorting to a macro?

Robert Feyerharm
Oklahoma State Department of Health
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