Q139156: PRB: DO WHILE Loop Does Not See Changes Made to Macro Variable

Article: Q139156
Product(s): Microsoft FoxPro
Version(s): WINDOWS:3.0,3.0b
Operating System(s): 
Keyword(s): kbcode
Last Modified: 11-FEB-2000

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The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 3.0b 
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SYMPTOMS
========

When you assign a field name to a variable and use a DO WHILE command on the
variable with macro substitution, the DO WHILE command doesn't recognize any
changes made to the variable while in the DO WHILE loop. For example, if the
variable x has been assigned a field name, then using DO WHILE &x will
evaluate the contents of the field. But if the contents of the variable changes
to another field name inside the DO WHILE loop, the new field name in the
variable is not recognized when the DO WHILE command is looped through again.
This could cause the DO WHILE loop to cycle indefinitely.

CAUSE
=====

The behavior of DO WHILE loops with macro substitution is controlled by the SET
COMPATIBLE setting. With the syntax SET COMPATIBLE OFF, Visual FoxPro only
evaluates the macro substitution the first time it goes through the loop. This
is significantly faster than evaluating it each time through the loop. With SET
COMPATIBLE ON, Visual FoxPro evaluates the macro substitution each time through
the loop.

RESOLUTION
==========

Use the following code instead. This code will stop when the variable x is
assigned a different field because the DO WHILE command is evaluating the actual
variable y instead of a macro substituted variable.

     CREATE TABLE test ;
            (fld1 C(10), fld2 C(5))
     INSERT INTO test ;
            (fld1, fld2) VALUES ("abcdef", "abcde")
     x = "fld1"
     y = &x
     DO WHILE y = "abcdef"
        WAIT WINDOW y
        x = "FLD2"
        y = &x
     ENDDO

STATUS
======

This behavior is by design.

MORE INFORMATION
================

Steps to Reproduce Behavior
---------------------------

Run the following program:

     CREATE TABLE test ;
         (fld1 C(10), fld2 C(5))

     INSERT INTO test ;
         (fld1, fld2) VALUES ("abcdef", "abcde")

     x = "fld1"
     DO WHILE &x = "abcdef"
        WAIT WINDOW &x
        * Note that the Wait Window shows the correct contents each time.
        x = "fld2"
     ENDDO

Additional query words: VFoxWin

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Keywords          : kbcode 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP300b
Version           : WINDOWS:3.0,3.0b

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