/***************************************************************************/
/* Determine characterisitcs of Control ID variable (numeric or character)
/***************************************************************************/
%let __dsid = %sysfunc(open(&cntl_dsn,i));
%let __varnum = %sysfunc(varnum(&__dsid, &cntl_idvar));
%let __cntl_type = %sysfunc(vartype(&__dsid, &__varnum));
%let __rc = %sysfunc(close(&__dsid));
%put &__cntl_type;

请问下SAS 大大们， SAS 中的__ 与_ 有什么区别呀？