What does `.Z` mean in SAS?

Question

Apologies for such an entirely uninformed question, but I don't know any SAS and just need to know what one line of code does, so I hope someone can help. I have a loop over an array of variables, and an if clause that is based on a comparison to .Z, but this variable is defined nowhere, so I'm guessing this is some sort of SAS syntax trick. Here's the loop:

ARRAY PTYPE{*} X4216 X4316 X4416 X4816 X4916 X5016;

DO I=1 TO DIM(PTYPE);
    IF (PTYPE{I}<=.Z) THEN PUT &ID= PTYPE{I}=;
END;

So on the first iteration, the loop would check whether the value in X4216 is smaller than .Z, and then...? ID is another varuable in the dataset, but I have no idea what's happening on the right hand side of that if clause. I've briefly consulted the SAS documentation to figure out that ampersands refer to macros, but my knowledge of SAS is to limited to understand what's happening.

Can anyone enlighten me?

Answer 1

.Z is a special missing value. In SAS a missing value (what you might call a NULL value) is indicated by a period. There are also 27 other special missing values that are indicated by a period followed by a letter or an underscore. The missing values are distinct and are all considered smaller than any actual number. .Z is the "largest". So PTYPE{I}<=.Z is basically testing if the value is missing. You could instead use MISSING(PTYPE{I}) to make the same test. The right hand side is writing out the name and the value of the variable in the array with a missing value and also the name and value of the variable named in the macro variable ID.