SAS - See if macro variable exists; delete macro variable

This program shows how to see if a macro variable exists, as well as how to delete a macro variable.

%let chk = %symexist(myvar);
%put chk = &chk;        /* output: chk = 0   */
%let myvar = xyz;
%let chk = %symexist(myvar);
%put chk = &chk;        /* output: chk = 1   */
%symdel myvar;          /* delete macro var  */
%let chk = %symexist(myvar);
%put chk = &chk;        /* output: chk = 0   */
%symdel myvar;          /* gives a warning   */
%let chk = %symexist(myvar);
%put chk = &chk;        /* output: chk = 0   */
%symdel myvar /nowarn;  /* gives no warning  */
%let chk = %symexist(myvar);
%put chk = &chk;        /* output: chk = 0   */


Here is the log:

NOTE: Copyright (c) 2002-2010 by SAS Institute Inc., Cary, NC, USA.
NOTE: SAS (r) Proprietary Software 9.3 (TS1M1)
      Licensed to FIRST ANALYTICS, Site 70073576.
NOTE: This session is executing on the W32_7PRO  platform.



NOTE: Updated analytical products:

SAS/STAT 9.3_M1, SAS/ETS 9.3_M1, SAS/OR 9.3_M1

NOTE: SAS initialization used:
      real time           3.98 seconds
      cpu time            3.05 seconds

1    %let chk = %symexist(myvar);
2    %put chk = &chk;        /* output: chk = 0   */
chk = 0
3    %let myvar = xyz;
4    %let chk = %symexist(myvar);
5    %put chk = &chk;        /* output: chk = 1   */
chk = 1
6    %symdel myvar;          /* delete macro var  */
7    %let chk = %symexist(myvar);
8    %put chk = &chk;        /* output: chk = 0   */
chk = 0
9    %symdel myvar;          /* gives a warning   */
WARNING: Attempt to delete macro variable MYVAR failed. Variable not found.
10   %let chk = %symexist(myvar);
11   %put chk = &chk;        /* output: chk = 0   */
chk = 0
12   %symdel myvar /nowarn;  /* gives no warning  */
13   %let chk = %symexist(myvar);
14   %put chk = &chk;        /* output: chk = 0   */
chk = 0

Here are the SAS doc links:
For symexist:  http://support.sas.com/documentation/cdl/en/mcrolref/61885/HTML/default/viewer.htm#a002635883.htm
For symdel:  http://support.sas.com/documentation/cdl/en/mcrolref/61885/HTML/default/viewer.htm#a001651249.htm