RECURSIVE Procedure

Fortran procedures can be RECURSIVE, meaning that they can either directly or indirectly call themselves. The RECURSIVE keyword must be used in the procedure declaration.

Description

A RECURSIVE procedure declaration implies that the procedure may call itself.

Syntax
 

     RECURSIVE SUBROUTINE sub-name([dummy-args])
 

     or
 

     RECURSIVE [type-spec] FUNCTION func-name([dummy-args])
         [RESULT(result-name)]
 Where:
 	

         

             sub-name is the name of the subroutine.
         

             dummy-args is a comma-separated list of dummy argument names.
         

             type-spec is
             

                 INTEGER [kind-selector] REAL [kind-selector] DOUBLE PRECISION
                 COMPLEX [kind-selector] CHARACTER [char-selector] LOGICAL [kind-selector]
                 TYPE (type-name)
         

         

             kind-selector is ( [KIND=] kind )
         

             char-selector is ( [LEN=] length [, ) KIND=kind])
         

             or (KIND=kind [, LEN=length]) or * char-length [,]
         

             kind is a scalar INTEGER expression that can be evaluated at compile time.
         

             length is a scalar INTEGER expression or *
         

             char-length is a scalar INTEGER literal constant or (*)
         

             func-name is the name of the function.
         

             result-name is the name of the result variable.
     
 

Remarks

The RECURSIVE keyword must be present if the procedure directly or indirectly calls itself or a procedure defined by an ENTRY statement in the same subprogram. RECURSIVE must also be present if a procedure defined by an ENTRY statement directly or indirectly calls itself, another procedure defined by an ENTRY statement, or the procedure defined by procedure declaration.

Result

If the recursive procedure is a funcrion, the result is the type and kind of the function declaration or of the result variable.

Example 1
       recursive subroutine sub3(i) ! recursive required if the
           i=i-1                    ! subroutine calls itself
           if(i > 0) call sub3(i)   ! directly or indirectly
       end subroutine
Example 2
       !   recursive function with result variable
       recursive function func4(a,b) result(res)
         real :: res     ! result type defined here
         real :: a,b
         if (a >= b) then
             res=a-b   ! function returns
         else if (a < spacing(b)) then
           res=-b        ! function returns
         else if (a < 0.) then
           b=func5(-a,b) ! indirect recursion
         else
           a=func4(a,-b) ! direct recursion
         end if
       end function
       !   recursive function without result variable
       recursive function func5(a,b)
         real :: func5      ! result type defined here
         real :: a,b
         if (a < b) then
           func5=func4(b,a) ! result variable not required
         else                 ! if recursive function does
           func5=func4(a,b) ! not invoke itself directly
         end if
       end function
     

See Also

SUBROUTINE statement

FUNCTION statement