/*-------------------<-- Start of Description -->--------------------\
| Generate random variate from a gamma distribution                  |
|--------------------<--- End of Description -->---------------------|
|--------------------------------------------------------------------|
|--------------<--- Start of Files or Arguments Needed -->-----------|
| Arguments Need:                                                    |
|      seed - seed; Required, default is the current system time;    |
|      var  - the variable to save the generated random variates;    |
|      alpha- alpha;                                                 |
|      beta - beta;                                                  |
|      Note: mean=alpha*beta, variance=alpha*beta^2;                 |
|      temp - the temporary variable to update the seed;             |
|             default is _rangam0_;                                  |
|---------------<--- End of Files or Arguments Needed -->------------|
|--------------------------------------------------------------------|
|----------------<--- Start of Example and Usage -->-----------------|
|Example                                                             |
|   data one;                                                        |
|      do i=1 to 20;                                                 |
|      x=%_rangam(alpha=5, beta=2);                                  |
|      z=%_rangam(seed=1, alpha=10);                                 |
|      %_rangam(seed=1324, var=y, alpha=2, beta=5);                  |
|      output;                                                       |
|      end;                                                          |
|   run; %print(one);                                                |
|Usage: %_rangam(seed=%sysfunc(datetime(), 15.), var=, alpha=, beta=,|
|                temp=_rangam0_);                                    |
\-------------------<--- End of Example and Usage -->---------------*/
%macro _rangam(seed=%sysfunc(datetime(), 15.), var=, alpha=REQUIRED, 
               beta=, temp=_rangam0_);
/*--------------------------------------------\
| Author:  Duo Zhou;                          |
| Created: 3-22-2002 6:30pm;                  |
| Purpose: Random Gamma Generator;            |
\--------------------------------------------*/
%let _achk_=%sysfunc(rxmatch(%sysfunc(rxparse($a|$A|$w)),&alpha));
%if (%quote(&seed) eq) or &_achk_ %then %do;
   %if (%quote(&seed) eq) %then %do;
      %put ==> Error: This is not a valid seed!; 
      %if (%length(&var)) %then %do; &var=.; %end;
      %else %do; .;%end;
   %end;
   %if &_achk_ %then %do;
      %put ==> Error: &alpha is not a valid alpha!; 
      %if (%length(&var)) %then %do; &var=.; %end;
      %else %do; .;%end;
   %end;
   %goto finish;
%end;
%else %do;
   %if (%length(&var)) %then %do;
      %if (not %sysfunc(rxmatch(%sysfunc(rxparse(_|.|$a|$A|$w)),&seed))) %then %do;
         drop &temp;
         retain &temp &seed;
         %let seed=&temp;
      %end;
      call rangam(&seed, &alpha, &var);
      %if (%length(&beta)) %then &var=&var;
   %end;
   %else rangam(&seed, &alpha);
   %if (%length(&beta))  %then *β
%end;
%finish:
%mend _rangam;