Q37310: LPRINT Forced to Screen Using OPEN in FUNCTION Procedure

Article: Q37310
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 28-DEC-1989

You should avoid invoking a FUNCTION procedure that performs I/O in
I/O statements.

For example, if a FUNCTION procedure that opens a file is invoked in
an LPRINT statement, the printing occurs on the screen instead of the
printer in the example below. The simplest way to work around this
limitation is to assign the FUNCTION procedure value to a temporary
variable and then LPRINT the temporary variable. You can also
eliminate the problem by taking the OPEN statement out of the FUNCTION
procedure.

This behavior applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50 for MS-DOS, to Microsoft BASIC Compiler Versions 6.00 and
6.00b for MS-DOS and MS OS/2, and to Microsoft BASIC PDS Version 7.00
for MS-DOS and MS OS/2.

This limitation for the FUNCTION statement is documented on Page 201
of the "Microsoft QuickBASIC 4.0: BASIC Language Reference" manual for
Versions 4.00 and 4.00b. The following information appears in this
manual:

   Because BASIC may rearrange arithmetic expressions for greater
   efficiency, avoid using FUNCTION procedures that change program
   variables in arithmetic expressions. Also avoid using FUNCTION
   procedures that perform I/O in I/O statements. Using FUNCTION
   procedures that perform graphics operations in graphics statements
   may also cause side effects.

To work around this problem, assign the value of the FUNCTION
procedure to a temporary variable, then use the temporary variable in
the arithmetic expression, I/O statement, or graphics statement.

Versions of QuickBASIC earlier than 4.00 do not have the FUNCTION
statement.

The following example, which LPRINTs a temporary variable, works
correctly:

   DECLARE FUNCTION code$ (a!)
   CLS
   x$=code$(a)
   LPRINT x$; "stuff to print"
   END
   FUNCTION code$ (a)
      OPEN "temp" FOR RANDOM AS #1 LEN=1
      code$="abcd"
      CLOSE #1
   END FUNCTION

The following example, which LPRINTs a FUNCTION procedure that uses
the OPEN statement, incorrectly prints to the screen:

   DECLARE FUNCTION code$ (a!)
   CLS
   LPRINT code$(a); "stuff to print"
   END
   FUNCTION code$ (a)
      OPEN "temp" FOR RANDOM AS #1 LEN=1
      code$="abcd"
      CLOSE #1
   END FUNCTION

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