On Tue, 25 Mar 2008 09:54:54 -0400, Richard Wright
>Under SAS, like many other languages, an expression that evaluates to non-
>zero is boolean "true"; "false" is an expression that evaluates to 0.
technically, sas evaluates zero *and* missiing to false; everything else if
true. This may be important because it is quite arbitrary and can differs
from language to language. for example, in stata, missing is true.
In R, the situation is much more complicated. NA is of logical type by
default, which is neither TRUE or FALSE. NA is not equal to anything -- it
is not even equal to itself! I am not kidding. :-)