数一数。 来自元数据的 SAS 数据集的观察

Question

code to count the no. of observations of a data set:

data _NULL_;
 if 0 then set sashelp.cars nobs=n;
 put "no. of observations =" n;
 stop;
run;

What does if=0 means? how this is condition working?

Answer 1

zero is the intrinsic false value in SAS. Therefore the Set Statement in the if 0 then set.. statement is never executed. The nobs=n is set at compile time. Therefore, if your only goal is to find the number of observations in a SAS data set, there is no need to read any of the actual data.

Answer 2

When evaluating boolean logic SAS will treat zero or missing values as FALSE and any other value as TRUE. So the IF 0 THEN allows you to have code that the data step compiler sees, but that never actually gets run. The variable created by the NOBS= option gets assigned a value before the data step starts running.

For this simple step you could get the same result by placing the SET statement after the STOP statement.

data _null_;
  put "no. of observations =" n;
  stop;
  set sashelp.cars nobs=n;
run;