Q65886: TAB() Function Is Affected by Nonprinting Control Characters

Article: Q65886
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900916-2 B_BasicCom B_GWBasicI B_BasicInt | mspl13_basic
Last Modified: 8-NOV-1990

Sending control characters to a device in conjunction with the TAB()
function may result in output not appearing as desired. This is
because the TAB() function does not distinguish what is and what is
not a control character. This behavior is due to the fact that control
characters and escape sequences are device and hardware specific but
the TAB() function treats them like any other character.

This information applies to the following products:

1. Microsoft QuickBASIC versions 1.00, 1.01, 1.02, 2.00, 2.01, 3.00,
   4.00, 4.00b, 4.50 for MS-DOS

2. Microsoft BASIC Compiler versions 6.00 and 6.00b for MS OS/2 and
   MS-DOS

3. Microsoft BASIC Professional Development System (PDS) versions 7.00
   and 7.10 for MS-DOS and MS OS/2

4. Microsoft GW-BASIC Interpreter versions 3.20, 3.22, 3.23 for MS-DOS

The TAB() function is a device I/O function that moves the print
position when used in the PRINT, PRINT #, and LPRINT statements. The
position it moves to is based on how many characters have been sent to
the file or device.

When you send ASCII text data to a file or device, TAB() functions as
desired. But when you send control characters or escape sequences
(that may be device or hardware specific), the TAB() function counts
these characters along with the other characters, which results in the
desired print position being off by the number of "nonprinting"
control characters sent. This device-independent behavior of TAB() is
by design.

To work around this behavior, the program must keep track of the
current print position and/or the number of control characters sent.
When keeping track of the current print position, the program can then
use the SPACE$() or SPC() function to move to the desired position.
When keeping track of the number of control characters sent, the
desired column of the TAB() function can be added to the number of
control characters sent to get the desired print position. The
following code examples demonstrate both methods.

Example 1
---------

This example uses the variable CTRLCHARS to keep track of the control
characters sent for a given line of text, and puts it in the TAB()
function when evaluating how far to move over:

   10 LPRINT CHR$(27) + CHR$(52)    'SET EPSON PRINTER IN ITALIC MODE
   20 CTRLCHARS = 2
   30 LPRINT TAB(10 + CRTLCHARS) "HELLO"    'PRINT HELLO ON LINE 10

Example 2
---------

This example uses the SPACE$() function to move the print position to
the desired column:

   10 LPRINT CHR$(27) + CHR$(52)    'SET EPSON PRINTER IN ITALIC MODE
   20 LPRINT SPACE$(10); "HELLO"    'SPACE OVER 10 PLACES AND PRINT HELLO

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