Why does this macro work? (it does) The loop is able to start, despite the fact that the nrows variable is defined in the set statement inside the loop. Does SAS read the set statement before starting the loop? Where can I find documentation on this issue (which statements inside loops, if any, are executed before the loop starts)?
%macro get_last_n_rows(n, existing, new);
data &new.;
do _i_ = 1 + nrows - &n. to nrows;
set &existing. point = _i_ nobs = nrows;
output;
end;
stop;
run;
%mend get_last_n_rows;
The short answer to your question is: yes, SAS reads the number of rows available prior to the loop executing. In fact, SAS reads the number of rows available before the data step executes; it's determined at data step compile time. See for example, this paper and this paper, among many others.
See specifically the SAS documentation for SET:
At compilation time, SAS reads the descriptor portion of each data set and assigns the value of the NOBS= variable automatically. Thus, you can refer to the NOBS= variable before the SET statement. The variable is available in the DATA step but is not added to any output data set.
Note this has nothing to do with the do loop; this is true for the entire data step (which is itself one large loop, of course).