FoxPro 2.6 for Dos use of memory variables

[wpeters]

HELP! I have been thrown into being a developer of an OLD system. I have done well with the knowledge I have thus far. EXCEPT, cannot find reference books, not too many forums out there. I need a quick breakdown of use of memory variables. Why did the previous programmer sometimes use m-> and sometimes a->?

 

[JRB-Bldr]

Value 'containers' (a.k.a. "variables") can be of a variety of types. Typically these are memory variables (which reside soley in the workstation's memory) and data table fields (which reside in a FP data table).

The OLD symbology of -> is synonymous with the newer symbology . (or 'dot').

Example:
   m->ThisValue
   m.ThisValue   && same as above

The m-> or m. was used to designate a memory variable - again resident in the workstation memory ONLY.

When a data table is used, its ALIAS is typically used to indicate a value from one of its fields. These values are typically shown as ALIAS + '->' + Fieldname or ALIAS + '.' + Fieldname

Example:
   USE MyTable IN 0
   SELECT MyTable
   m.MemVariable = MyTable.FldValue

This symbology comes in very handy if you should have both a memory variable and a data table field with the same name. How would you differentiate between the two and know when you were using which? Now you have a way.

From your example you have some data table which is aliased as 'A' either by workspace inheritance or by filename. And values referenced as A-> are data table field values.

Good Luck,
JRB-Bldr

 

[DSummZZZ]

To add to JRB-Bldr's post, the A-> notation stems from the old Fox days when there was a limit of 10 available work areas, or more specifically, open tables and index files.

You would open tables by specifying a work area by using:
SELECT 1
USE somedbf1
SELECT 2
USE somedbf2

and so on up to 10.

But in order to reference a field in the non-current work area, you couldn't use:

REPLACE 1.field1 WITH 'asdf'

you had to use the alpha version of the work area. So you would use:

REPLACE A->field1 WITH 'asdf' .

Most people found it easier to just reference the work areas with the letter:

SELECT A
USE somedbf1
SELECT B
USE somedbf2

etc., up to 'J'.

Since 'M' was beyond the limit of work areas, it would never be used as a work area to open a table, so it was always safe to use for memory variables. Which means you can pretty much assume any alpha character 'J' or lower, will be a table reference.



-Dave Summers-

Even more Fox stuff at:

http://www.davesummers.net/foxprolinks.htm