Description of the differences among the macro substitution tool, the name expressions tool, and the EVALUATE function tool in FoxPro

MORE INFORMATION

The Visual FoxPro 3.0 Language Reference guide and the Visual FoxPro 3.0 and later versions EVALUATE function Help topic specify that the EVALUATE function and the name expressions must be used instead of macro substitution. The EVALUATE function and the name expressions run faster. However, it does not explain why this is so.

Macro Substitution

Macro substitution is just what it is called. A run-time "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.

Macro substitution is still supported for backwards compatibility purposes, and because it is still popular with many xBase developers. The only situation where macro substitution is still required is when the expression to evaluate contains the command to be executed, in whole or in part, as in the following code examples:

cClauses=" * FROM CUSTOMER WHERE cno='A'"
   SELECT &cClauses

cResetMenu="SET SYSMENU TO DEFAULT"
   &cResetMenu

Name Expression

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

Name expression compilation is optimized and does not require the creation of any "lookup" code, which makes it much more efficient than macro substitution. However, the limited scope of name expressions (they can only be used to evaluate variables used to name FoxPro components) required that another tool be used for evaluation of variables under other circumstances.

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
				

In the above example, if &cCondition is used as the condition, the loop will process one record only, because the condition is evaluated only when the mini-compiler executes on-the-fly. Instead, the condition should be "EVALUATE(cCondition)" (without the quotation marks), because EVALUATE() can be called repeatedly because it is a native FoxPro function.

CODE OPTIMIZATION:

To speed up run-time 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')
				

While it is true that reducing the size of the code in memory generally helps speed up the code at runtime, it is not always true that reducing the number of lines reduces the size of the application in memory, especially when using macro substitution, because of the extra code that is built in by the compiler. Also, even if the application size in memory is reduced, the overhead incurred during macro evaluation slows the execution down anyway. The following example illustrates this:

   * 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.
				

Note that these optimization concepts are fairly irrelevant with simplistic examples like these. In a real-world application, the speed difference will be more noticeable. OVERLAPPING OF USAGE: 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")
				

Although macro substitution could also be used here, its use is not demonstrated because of the reasons explained previously: speed, efficiency, and consistency.