Let's start with some basics. Here's a DATA step that is able to establish whether a specific data set has an index...
dsid = open('SASHELP.CLASS'); /* Open the data set */
HasIndex = attrn(dsid,'ISINDEX'); /* Get num attr */
rc = close(dsid); /* Close the data set */
The following is returned in the log:
dsid=1 HasIndex=0 rc=0 _ERROR_=0 _N_=1
In essence, the OPEN function provides a numeric identifier for the data set in question; the identifier can then be passed to a myriad of functions (such at ATTRN) to "do stuff" with the data set; the identifier should then be returned back to SAS with the CLOSE function when you've finished with the data set (in order to close it and free-up memory). The OPEN function (and its associated functions) can be used in any DATA step, regardless of whether one or more other data sets are already open with a SET statement, for example.
The functionality provided by ATTRN is simple, but with its cousin ATTRC it provides a large number of different attributes. There are many more associated functions that use the data set identifier returned by the OPEN function. Some examples are: VARNUM, GETVARC/N, FETCH, FETCHOBS, and CALL SET. I'll talk about these in subsequent posts.
We're not restricted to using these DATA step functions within a DATA step. We can use them within macro code, or a PROC FCMP function. Their use within macro code can be particularly valuable. For instance, to get the value of a column from a specific row in a table and place it into a macro variable, we could use a DATA step thus:
%let dsid = %sysfunc(open(SASHELP.CLASS)); /* Open the ds */
%let HasIndex = %sysfunc(attrn(&dsid,ISINDEX)); /* Get attr */
%let rc = %sysfunc(close(&dsid)); /* Close the data set */
%put HasIndex is set to: &HasIndex;
And the resulting log shows:
HasIndex is set to: 0
It possibly looks more complicated than the DATA step alternative, but it has advantages too. We didn't need to run PROC CONTENTS, get the results into a data set, and then read the data set with a DATA step. And we didn't need to read SASHELP.VMEMBER, which can be resource-intensive.
Note that we've used macro statements, but they didn't need to be within a macro definition.
I'll show some more advanced uses in subsequent posts.
DATA SET FUNCTIONS:
1. NOTE: The OPEN Function (getting data set information in macros)
2. NOTE: The OPEN Function (reading data sets in macros)
3. NOTE: The OPEN Function (error trapping)