DIF function order

[okaka23]

data four;
set two;
by x;
diff=dif(total);
if first.x then diff=.;
run;

Why i cannot put this way? any difference ? thanks

data four;
set two;
by x;

if first.x then diff=.;
diff=dif(total);
run;

[Reeza]

Order of operations. If you set it missing and then calculate it, it has a value. 

If you calculate it, know it’s wrong then set to missing it’s fine, then it stays missing. 

DIF like LAG is a queue function

https://www.lexjansen.com/phuse/2011/cc/CC08.pdf

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@okaka23 wrote:

data four;
set two;
by x;
diff=dif(total);
if first.x then diff=.;
run;

Why i cannot put this way? any difference ? thanks

data four;
set two;
by x;

if first.x then diff=.;
diff=dif(total);
run;

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