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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