how does sas calculates the value of a case statement - mysql

Question

I don't understand how sas calculates the value for PATIENT , I've looked everywhere but there seems to be no documentation for how sas, calculates the formulas in the case parenthesis, I only know that when the value of
PATIENT is even the CASE expression has a Boolean result of 1 , 0 or 2, but I just don't see how to get the value out of the expression, could someone please explain me?
PROC SQL;
CREATE TABLE TESTMED AS
SELECT PATIENT,
CASE ((PATIENT/2 = INT(PATIENT/2)) +
(PATIENT = .))
WHEN 1 THEN 'Med A' WHEN 0 THEN
'Med B' ELSE 'Error' END AS DOSEGRP
LENGTH=5
FROM VITALS
ORDER BY PATIENT;
QUIT;

Answer 1

I can't see any error.
if patient could be . 0 1 2 this will be (first expression + second expression):
0 ---> {0=0}true(1) + {patient=.} false(0) = 1
1 ---> {0.5=0}false(0) + {patient=.} false(0) = 0
2 ---> {1=1}true(1) +{patient=.} false(0) = 1
. ---> {.=.} true(1) + {patient=.} true(1) = 2
If you want to see the result of that boolean expression you can simply use:
PROC SQL;
CREATE TABLE TESTMED AS
SELECT patient,
((PATIENT/2 = INT(PATIENT/2)) + (PATIENT = .)) AS result
LENGTH=5
FROM VITALS
ORDER BY PATIENT;
QUIT;
and you will see that the results are correct.
So your boolean expression could assume only 3 values:
(1=0+1)
(0=0+0)
(2=1+1)
When 0 you assign MEDA;
When 1 you assign MEDB;
When 2 you assign ERROR;
And ERROR is assigned for missing values.
1st expression:
{missing/2=missing} = {int(missing)=missing} [errors are reported as missing]
missing=missing ---> true ---> 1
2nd expression:
missing=missing ---> true ---> 1
result:
1+1=2 ---> ERROR

Answer 2

SAS evaluates boolean expressions to 1 (true) or 0 (false). So your expressions is the sum of two of these. So the possible answers are 0, 1 or 2.
The first expression, (PATIENT/2 = INT(PATIENT/2)), will be true when PATIENT is even or missing. The second expression, (PATIENT = .), will be true when PATIENT is missing. So a result of 2 means that PATIENT is missing a result of 1 means that PATIENT is even and a result of 0 means that patient is odd.
So you could rewrite the case statement as:
CASE WHEN MISSING(PATIENT) then 'Error'
WHEN MOD(PATIENT,2)=1 then 'Med A'
ELSE 'Med B'
END AS DOSEGRP