Notes: Fortran-90 and Recursion

Math 61 -- D. C. Smolarski, S.J.
Santa Clara University, Department of Mathematics and Computer Science

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Recursion

The concept of recursion in Fortran-90 is identical to the concept of recursion in any other language that implements it. In brief, a function or subroutine is recursive if it calls itself.

For a general introduction to recursion (in C++) see this link.

Note that recursion is not implemented in older versions of FORTRAN!

Recursive Fortran 90 Subroutines

To designate a Fortran-90 SUBROUTINE as recursive, one prepends the word RECURSIVE before the keyword SUBROUTINE in the code for that subprogram. No other adaptations are required. SUBROUTINES designated as recursive are then used as they would be in other languages.

The following is an example of the recursive Towers of Hanoi code implemented in Fortran-90. (It is also available for downloading at this link.) 

     program hanoimain
     ! recursive towers of hanoi program in 
     ! fortran 90
         integer n
	 n = 5
	 call hanoi(n,'a','c','b')
     end
     !
     recursive subroutine hanoi(n,frompeg,auxpeg,topeg)
         integer n
         character*1 frompeg,auxpeg,topeg
         if (n == 1) then
            write(6,*)"Move disk 1 from peg ", frompeg, " to peg ", topeg
         else
            call hanoi(n-1,frompeg,auxpeg,topeg)
            write(6,101) n, frompeg,topeg
            101 FORMAT(1x,"Move disk ",i1," from peg ", A1, " to peg ", A1)
            call hanoi(n-1,auxpeg,topeg,frompeg)
         endif
    end

Recursive Fortran 90 Functions

To designate a Fortran-90 FUNCTION as recursive, one prepends the word RECURSIVE before the keyword FUNCTION in the code for that subprogram. In addition, in the header line after the parameter list, one includes the keyword RESULT followed, in parentheses, by the internal variable that will be used to store the value the function returns. Some authors recommend that this variable use the suffice _RESULT but this is not required.

As in other languages, the recursive FUNCTION can be called on the right side of an assignment statement within the FUNCTION. The RESULT variable must be given a return value somewhere within the code.

(Recall that in a non-recursive Fortran FUNCTION, the return value is assigned to the name of the FUNCTION which is considered a local variable within the subprogram. In a recursive FUNCTION, the FUNCTION name retains its purpose as a means to invoke itself, and so another identifier must be designated as the means to return the value computed by the subprogram.)

FUNCTIONS designated as recursive as then used as they would be in other languages.

The following is an example of a recursive fibonacci function implemented in Fortran-90. (It is also available for downloading at this link.) 

     program fibonmain
     ! recursive fibonacci program in
     ! fortran 90
         integer n, final, fibon
	 n = 5
	 final = fibon(n)
         write(6,*) "Final fibonacci value for input of",n," is",final
     end
     !
     recursive function fibon(n) result (fib_result)
         integer  :: n, fib_result
	 if (n <= 2) then
	     fib_result = 1
         else
	     fib_result = fibon(n-1) + fibon(n-2)
         endif
     end

This page is maintained by Dennis C. Smolarski, S.J. dsmolarski@scuacc.scu.edu
© Copyright 1998 Dennis C. Smolarski, SJ, All rights reserved.
Last changed: 9 April 1998.