sas判断宏变量是否存在，宏变量值是否为空，数据集在当前环境是否存在

/*判断宏变量的值是否为空*/
/*%if &a=%str()*/

%let a=;

%macro check;
data _null_;
%if &a=%str() %then %put "value of a is null";
%else %put "value of a is not  null";
run;
%mend check;

%check;

/*判断宏变量是否存在*/
/*symexist("var")*/

%macro test;
%local y;
data _null_;
     if symexist("x") then  put "x exists";
     else put "x does not exist";
     if symexist("y") then put "x exists";
     else put "y does not exist";
run;
%mend test;
%test;

/*判断数据集是否存在*/
%let e=%sysfunc(exist(sashelp.class));
%put &e;