Skip to main content

Restoring Work Areas: ALIAS() vs. SELECT()

Most programs like to save and restore the calling environment when they execute. The environment may include settings, work areas and more. This is something that is fairly unique to Visual FoxPro rather than VS environments that don't really have the concept of a work area. In Visual Basic for example, you retrieve your dataset object, execute your commands separately and basically treat it like an object (this isn't about VB so don't flame me on the basic description).

In Visual FoxPro however, you have the concept of work areas. So when you decide to work on the Customer file, you can say SELECT Customer and that becomes your main "area" that you are working with. This can be extremely handy for a variety of purposes. But coming back to the matter at hand, saving and restoring environments.

FoxPro provides you with a few functions that can tell you what you're working with. If you want to know what table you are currently working on, you can use DBF() to return the full file name or ALIAS() to return the alias of the currently selected table. If no table is open, ALIAS() returns blank. ALIAS() is often used when using temporary alias names as shown here:

lc = SYS(2015)
SELECT * FROM CUSTOMER INTO CURSOR &lc
SELECT CUSTOMER
SELECT &lc
...

Why would someone do that? Hard to say - a cursor is after all a temporary name (existing in memory only) but if you are having a rough time coming up with unique names, SYS(2015) always works well.

But Visual FoxPro also has a SELECT() function that returns the current work area. When you first start VFP, you are in work area 1. If you want to select the next unused area, you can say SELECT 0 or type in USE INVOICES IN 0.

So as with all things Fox, there are a number of ways you can accomplish a task, and often the most obvious way may not be quite what you want.

I was recently reviewing some code that was attempting to store and restore the environment. The code used the ALIAS() function.

lcSelect = ALIAS()
SELECT 0
USE xxxxx
** do some other stuff
SELECT &lcSelect

Works great, right? Wrong. This code assumes that lcSelect does have a value in it. If there was no table currently open, lcSelect would be empty. As a result, the final SELECT statement would say "SELECT " - which generates a syntax error.

The smarter approach is to use the SELECT() function. SELECT() returns a work area number and will always (?) give a value greater than 0 but even if it returned 0, SELECT 0 IS a valid function.

Update: As Colin Nicholls noted in the comments, if you use SET COMPATIBLE ON, SELECT() doesn't work the same. (it returns the number of the next unused area - kind of like saying SELECT 0) Instead, use SELECT(0) to be sure you're getting the correct result. Thanks Colin - shows how much I use SET COMPATIBLE.

lnArea = SELECT(0)
** do my code
SELECT (lnArea)

This code won't error out the same way the SELECT (lcArea) does .

It also returns code using the EVALUATION parentheses which is somewhat faster than the & approach.

For many experienced developers, this may seem obvious - but if you're just starting out working in FoxPro, the concepts of work areas may seem a little foreign and you feel better relying on the alias. Don't - work areas are your friends and SELECT() is a very handy function to keep in mind.

Comments

Anonymous said…
Andrew, I agree with your recommendation about SELECT(). However, something I didn't realise until recently: SET COMPATIBLE will change how SELECT() works in a devastatingly serious way. From the help file:

SELECT() returns the number of the current work area if SET COMPATIBLE is set to OFF. If SET COMPATIBLE is set to ON, SELECT() returns the number of the unused work area with the highest number.

So for goodness' sake, if you're writing code for other developers to use, get into the habit of specifying SELECT(0) rather than SELECT(). It's a lot safer!

Popular posts from this blog

Blogs and RSS come to Microsoft.com

MS has just introduced their portal and it's pretty comprehensive. Nothing quite like learning that some people use AIM instead of MSN messenger, or that there really may be a need for supporting 4 monitors ( Cyrus Complains ) However, it's really a great sign that MS is serious about supporting the blogging community which seems to have um, exploded in size in the past year. Blogs and RSS come to Microsoft.com

FoxInCloud Stats

FoxInCloud sent this link a while back about their statistics regarding visits to their site: http://foxincloud.com/blog/2017/12/27/VFP-community-lessons-from-foxincloud-site.html What's interesting here is the breakdown of people. Yes, I think it's understandable that the Fox community is getting older. Another factor is the growth of the mobile and web environments taking over development. These environments really do push people towards the newer non-SQL or free SQL/hosted environments but more towards hosted storage options like Amazon and Google. A tool like FoxInCloud that helps MOVE existing applications to the cloud inherently competes with those environments. But FoxInCloud also allows developers to extend their application further by giving them a starting point using Javascript and the basic CSS (such as Bootstrap). If you're not rebuilding your application from scratch, it's certainly a great step forward. FoxPro VFP

5 Great Reasons to attend Virtual FoxFest

What's coming up? Virtual FoxFest is coming up soon (sessions start October 14th). Like last year, the conference is entirely virtual yet includes great breakdown rooms and sessions to add that nice one-on-one feel that you get in person. It's also staggered so you can choose which days you want to attend - October 14th, 20th and 26th. This is great if you can't break away for a consecutive three days. But really, I've gone through the sessions and I see five great sessions that I'm eager to check out. 1. A Decade of Thor (Rick Schummer) Thor has been an extension for Visual FoxPro that many developers swear by, yet many don't know even exists. Visual FoxPro's built-in extensions are great but Jim Nelson's Thor supercharges your IDE. I can't believe it's been ten years - so Rick's session should be able to not just whet your appetite but give you all the reasons you should be using it. 2. VFP C++ compiler.  Last year, we saw DotNetX as well