Lag function Basics

[POOJA_J]

I came across this MCQ. 

The correct ans is B and even I read the logic behind this. But can anyone plz explain the same in more simple words. I didn't understand the logic explained in the image below. 

[PaigeMiller · Accepted solution]

From now on, please post text and code as text and not as screen captures. From now on, please provide data as working SAS data step code.

Perhaps these words work better for you: https://communities.sas.com/t5/SAS-Programming/Unexpected-results-with-lag-function/m-p/820751#M3240...

Basically, if you call LAG in an IF statement, and that branch of the IF statement doesn't execute, then the lagged value doesn't get created and thus the lagged value can't be used by the next observation.

[POOJA_J]

Thanks for explaining.
I will take care not to post screen shots next time.

[ballardw]

LAG, and its companion function DIF, are queued functions. That means that when you use Lag inside of an IF, or related conditional code structure, that the "previous value" returned by the function is that the previous time that the IF was true.

So generally the preferred usage is to execute the Lag before any IF so you have the value as needed conditionally and typically drop the lagged variable unless needed later.

[Tom]

The real answer is (C) use the DIF() function instead.

tempdiff=dif(temp);