Example 2-1:  Example of recursive function

        PROGRAM  FACTORIAL_TEST
  !
  !  This  is  just  a  simple  example  program  showing  recursion  in
  !  FORTRAN.
  !
  !  The  program  computes  and  displays  the  factorials  of  the

  !  integers  1:12;  factorial  of  13  is  too  big  for  a  longword
  !  integer.
  !
        INTEGER  FACTORIAL

        DO  N=1,12
           WRITE  (6,*)  N,'  factorial  is  ',FACTORIAL(N)
        END  DO
        END
        OPTIONS  /RECURSIVE

        INTEGER  FUNCTION  FACTORIAL  (N)
  !
  !  The  Factorial  of  an  integer  N  is  N  *  N-1  *  N-2  ...  *  1.
  !  This  is  often  written  as  N!.
  !
  !  If  N  is  greater  than  1,  call  ourselves  recursively,
  !  passing  N-1  to  perform  the  calculation.   Note  that
  !  each  recursive  call  to  FACTORIAL  will  get  its  own
  !  copy  of  N.
  !
  !  If  N  is  1  or  less,  the  result  is  1.   (We  assume  that
  !  N  is  not  negative.)
  !
        IF  (N  .GT.  1)  THEN
           FACTORIAL  =  FACTORIAL(N-1)  *  N
        ELSE
           FACTORIAL  =  1
        END  IF

        RETURN
        END

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