Q28165: "String Formula Too Complex" with Recursive String Function

Article: Q28165
Product(s): See article
Version(s): 6.00 6.00b 7.00 | 6.00 6.00b 7.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | B_QuickBas | mspl13_basic
Last Modified: 2-FEB-1990

The program below, which recursively invokes a string function, causes
a "String Formula Too Complex" run-time error. This program shows a
design limitation in Microsoft BASIC Compiler Version 6.00 and 6.00b,
Microsoft BASIC Professional Development System (PDS) Version 7.00,
and in QuickBASIC that is not usually encountered.

In the following program, QuickBASIC is computing RIGHT$(TEXT$,1),
assigning the value to a temporary string descriptor, and then
recursing on REVERSE$(LEFT$(TEXT$,LEN(TEXT$)-1)):

   DECLARE FUNCTION Reverse$ (Text$)
   CLS
   PRINT Reverse$ ("123456789 1234567890")

   FUNCTION Reverse$ (Text$)
   IF LEN(Text$) = 0 THEN
     EXIT FUNCTION
   ELSE
     Reverse$ = RIGHT$(Text$, 1) + Reverse$ (LEFT$(Text$, LEN(Text$) -1))
   END IF
   END FUNCTION

The temporary string descriptor is not released until the recursion is
completed. Each recursive REVERSE$ function call locks one string
descriptor. Because QuickBASIC allows only 20 temporary descriptors
(more than enough for most applications), after around 20 function
calls the program gives the error "String Formula Too Complex."

The workaround is to use a real string descriptor as follows (which
will be allocated from stack space):

   Temp$ = Reverse$(LEFT$(Text$,LEN(Text$)-1))
   Reverse$ = RIGHT$(Text$,1) + Temp$

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