Macro Subst, Name Expr, and EVALUATE() Runtime Evaluation

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for Windows, version 2.6a

SUMMARY

This article describes and explains the differences in behavior between the three Runtime Evaluation tools provided by the FoxPro language: "&" (macro substitution), "()" (name expression), and the "EVALUATE()" function.

MORE INFORMATION

The "Language Reference" guide specifies that EVALUATE() and name expressions should be used instead of macro substitution because they run faster. However, it does not explain why this is so.

Macro Substitution

Macro substitution is just what it is called. A runtime "macro" is built into the tokenized code to evaluate the expression at runtime. The compiler inserts some compiling-linking code into the application, and when the code is run, on-the-fly compilation and linking takes place to tokenize the expression before executing it. This is highly inefficient for two reasons:

• It increases the size of applications because the mini-compiler code has to be built for each expression to evaluate.
• It decreases execution speed because of the overhead incurred by the on-the-fly compilation.

• cClauses=" * FROM CUSTOMER WHERE cno='A'" SELECT &cClauses
• cResetMenu="SET SYSMENU TO DEFAULT" &cResetMenu

This is the preferred method for referencing FoxPro components that can be named, like windows, menus, popups, and files (including screens, menus, and reports) in the commands that run them, as in the following examples:

• cWindowName="test" DEFINE WINDOW (cWindowName) FROM 1,1 TO 20,50 SYSTEM
• cScreen="Myscreen.spr" DO (cScreen)
• cReport="Myreport.frx" REPORT FORM (cReport) PREVIEW ENVIRONMENT

EVALUATE()

This function is, like every FoxPro function and command, built into the language library and therefore more efficient than macro substitutions because there is no need for on-the-fly compilation. EVALUATE() can be used to return the values of expressions anywhere in the code, except when name expressions are required (see above). Typically, this function is used to retrieve sorting, filtering, or querying information from the user at runtime, as the following example illustrates:

   cCondition="CNO='A'"
   @ 2,2 SAY "Type filter condition for appending records or hit <ENTER>:";
      GET cCondition VALID CHECK()
   READ
   USE customer
   APPEND FROM customer FOR EVALUATE(cCondition)

   FUNCTION CHECK
   * Validation code for expressions typed
   RETURN .t. && or .f.

Special Considerations

EVALUATE() VS. MACRO SUBSTITUTION IN LOOPING CONDITION:

Consider the following syntax:

   USE customer
   cCondition = "CNO='A'"
   SCAN FOR <condition>
      * Processing code here
   ENDSCAN

CODE OPTIMIZATION:

To speed up runtime execution, developers usually reduce the number of lines in their code, thinking that if the size in memory is reduced, the application will execute faster. For example, the following would be a valid optimization:

   * Original code
   ...
   IF CHECK() == .t.
      WAIT WINDOW "This is the customer we're looking for"
   ENDIF

   FUNCTION CHECK
   PRIVATE lRetVal

   IF cno == 'A1592'
      lRetval = .t.
   ELSE
      lRetVal = .f.
   ENDIF

   RETURN lRetVal

   * Optimized code
   IF CHECK()
      WAIT WINDOW "This is the customer we're looking for"
   ENDIF

   FUNCTION CHECK
   RETURN (cno=='A1592')

   * Original code
   * Parameter is numeric, determines which order to use
   FUNCTION CustSort
   PARAMETERS nSortType

   DO CASE
   CASE nSortType == 1
      SELECT * FROM customer ORDER BY cno
   CASE nSortType == 2
      SELECT * FROM customer ORDER BY contact
   CASE nSortType == 3
      SELECT * FROM customer ORDER BY company
   OTHERWISE
      SELECT * FROM customer
   END CASE

   RETURN .t.

   * Optimized code (but it will most likely not run faster)
   * Parameter is character string, contains 'order by' clause
   FUNCTION CustSort
   PARAMETERS cOrderBy

   IF !EMPTY(cOrderBy)
      SELECT * FROM customer &cOrderBy
   ELSE
      SELECT * FROM customer
   ENDIF

   RETURN .t.

For more information about the EVALUATE() function, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q96294
   TITLE     : Difference Between EVALUATE() and Macro Substitution

   ARTICLE-ID: Q113172
   TITLE     : How the EVALUATE() Function Works

In some instances, EVALUATE() and name expressions can be interchanged. In this case, there is no preferred method, although it would help the developer to use name expressions whenever possible for consistency's sake. Here is a situation where either method is acceptable:

   cProc="Myproc.prg"
   DO (cProc)   && or DO EVALUATE("cProc")