lag

112211 — Fri, 06 Oct 2023 17:27:45 GMT

data svn;
input id sv vt ;
informat sv vt date9.;
cards ;
101 01jan2020 31jan2020
101 15feb2020 28feb2020
101 11mar2020 31mar2020
102 01apr2020 30apr2020
102 18may2020 31may2020
102 22sep2020 30sep2020
;

From using above data i want below mentioned output :

ID SV VT lag tot

101 01jan2020 31jan2020 .

101 15feb2020 28feb2020 31JAN2020 15

101 11mar2020 31mar2020 28FEB2020 12

102 01apr2020 30apr2020 . .

102 18may2020 31may2020 30APR2020 18

102 22sep2020 30sep2020 31MAY2020 114

Re: lag

Kurt_Bremser — Fri, 06 Oct 2023 17:53:48 GMT

data want;
set svn;
by id;
lag = lag(vt);
if not first.id
then tot = sv - lag;
else lag = .;
format lag date9.;
run;

Re: lag

mkeintz — Sat, 07 Oct 2023 00:28:11 GMT

There's a good argument to be made that the LAG function is misleadingly named. Folks familiar with spreadsheets, for example, just think of the LAG function as a way to look back 1 row (or 2 rows for a 2-period lag, etc.). So you might naturally expect that the syntax below, using an IF ... THEN ... ELSE construct.

  if first.id then y=.;
  else y=lag(x);

This works for all other functions, but will yield unexpected results for the lag function (and the related dif function). You will discover that the second obs for all id's but the first id will get a Y value taken from the last obs of the preceding ID, rather than from the first obs of the same ID.

That's because the unfortunately named LAG function is not a lookback - it is a FIFO queue update (a queue of length 1 in this case). Think of the queue as a retained value (or retained values for lag2+). For most purposes, including this task, that queue of retained value(s) has to be updated for every observation even if the value in the queue is not going to be used. So many folks use this 4-statement structure:

lx = lag(x);
if first.id) then y=.;
else y=lx;
drop lx;

But you can instead do the above in one statement using the IFN (or IFC) function, as in

y=ifn(first.id,.,lag(x));

This works because the IFN always evaluates both the second and third arguments, regardless of whether the first argument is true or false. The lag-generated queue is therefore always updated, as is needed in this case.

topic Re: lag in SAS Programming

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Re: lag