Recursive call in RPG ILE

A previous tip stated that RPG does not allow recursive calls. Actually ILE RPG does allow recursion. A program cannot call itself, but it can call a subprocedure that calls itself.

The procedure call should not use the CALLP opcode, rather it should use the EVAL opcode and treat the procedure as a function. For example, a procedure name CALCS would use the following code:

RETURN CALCS(value02)

The sample code calculates the n'th number in a Fibonacci sequence. A Fibonacci sequence is a sequence whereby any number is equal to the previous two numbers in the sequence:

Nbr(n) = Nbr(n-1) + Nbr(n-2).

In the sequence, the first number is 1, preceeded by an implied zero so that the second number has an "n-2" to use.

The first few elements then calculate to:
1 1 2 3 5 8 13 21 34

It is an interesting exercise to run this procedure in DEBUG and observe its action.

The subprocedure CALCS would be called from an ILE RPG program using a statement such as:

EVAL RESULT = CALCS(Nbr)

Note that if the recursion proceeds through too many recursive calls, that performance drastically slows down. In this example, anything over 30 calls really ran slowly. If the job ends abnormally while into a deep call stack, the end job process takes a very long time. Because of this, I cannot think of many practical applications of recursion in ILE RPG.

   * Procedure Prototype
                  
DCALCS          PR             9P 0    
D                              9P 0    
 * * * * * * * * * * * * * * * * * *  * Procedure Definition
P CALCS         B
*                                       D CALCS         PI             9P 0
D  NBR                         9P 0 
*
* Procedure variables 
D NM1             S            9P 0
D NM2             S            9P 0
*                                       C                   SELECT                    
 * Endpoint if inbound parm = 0
C      WHEN      NBR = 0         
C      RETURN    1               
 * Endpoint if inbound parm = 1
C      WHEN      NBR = 1         
C      RETURN    1               
 * Endpoint if inbound parm = 2
C      WHEN      NBR = 2         
C      RETURN    1               
 * Recursive call
C      OTHER
C      EVAL      NM1 = NBR - 1   
C      EVAL      NM2 = NBR - 2   
C      RETURN    CALCS(NM1)+ CALCS(NM2)

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