topic Re: How is it stored this boolean expression? in SAS Programming

How is it stored this boolean expression?

Bogdan — Mon, 01 Apr 2013 05:54:20 GMT

Which way is faster?

Suppose I want to write the expression (k=1)*e+(k=2)*f and suppose that e,f need to be calculated. Does SAS calculate e,f , then (k=1)*e+(k=2)*f or does it evaluate first the boolean expressions (k=1) and (k=2) and then the corresponding e if k=1 is true or f if k=2 is true.

If SAS calculates both e and f then it is more efficient to do a conditional

if k=1 then do e;

else do f;

Re: How is it stored this boolean expression?

data_null__ — Mon, 01 Apr 2013 10:11:47 GMT

Why don't you write some code to test your assumption? Do each a million times and observe the time difference. You will need options FULLSTIMER.

Re: How is it stored this boolean expression?

Ron_MacroMaven — Mon, 01 Apr 2013 17:18:15 GMT

Here are references to help you understand the order of evaluation of an expression:

Logic: and, not, or

SAS(R) 9.3 Language Reference: Concepts, Second Edition

arithmetic:

SAS Operators in Expressions

SAS(R) 9.3 Language Reference: Concepts, Second Edition

Ron Fehd knot logically lazy maven

Re: How is it stored this boolean expression?

Astounding — Mon, 01 Apr 2013 18:58:37 GMT

Yes, all the math will get performed. You might get some savings by switching as you suggest:

if k=1 then ...;

else if k=2 then ...;

In addition, this code works faster if k=1 most of the time. If k=2 most of the time, you could switch and test for k=2 first. But truthfully, the savings will be tiny, perhaps not even measurable. If you are interested in efficiency, you might post more of your code and get suggestions about how to speed it up.

Good luck.