#B4X Expression Compiler and Eval Library

MteEval is a library for compiling and evaluating expressions at runtime. Expressions are converted to bytecode and then executed on demand with a simple virtual machine.

There are three editions of the library: Android (B4A), iOS (B4i), and Java (B4J).

See Anywhere Software to learn more about B4A, B4i, and B4J cross-platform development tools.

##Application

The ability to create and evaluate expressions at runtime is a powerful tool, allowing calculations and program flow to be customized after installation which would otherwise require a physical update or a custom build of an application. For example, an application designed to manage a sales compensation plan could benefit from runtime expressions, where the end-user may want to modify the plan's formulas by team members, product mixes and sales goals.

##Codeblocks

MteEval adopts the "code block" format for expressions from the venerable 1990's xBase compiler Clipper 5. Codeblocks begin with an open brace, followed by an optional parameter section between pipes, then the expression, and end with a closing brace.

{|<parameters>|<expression>}

##Examples

You only need to compile a Codeblock once. Once compiled you can evaluate it as many times as needed, all while supplying different parameter values.

Example 1: Codeblock without parameters

Dim cb as Codeblock
cb.Initialize
cb.Compile( "{||5 * 3}" )
Result = cb.Eval           'Result=8

Example 2: Codeblock with parameters

Dim cb as Codeblock
cb.Initialize
cb.Compile( "{|length,width|length*width}" )
Area = cb.Eval2( Array( 3, 17 ) )    'Area=51

When evaluating a Codeblock with parameters, you use the Eval2 method.

Example 3: Codeblock compile, eval and repeat

Dim cb as Codeblock
cb.Initialize
cb.Compile( "{|sales,r1,r2| r1*sales + iif( sales > 100000, (sales-100000)*r2, 0 ) }" )
Commission1 = cb.Eval2( Array( 152000, .08, .05 ) )    'Commission1=14760
Commission2 = cb.Eval2( Array( 186100, .08, .07 ) )    'Commission2=20915
Commission3 = cb.Eval2( Array( 320000, .08, .05 ) )    'Commission3=36600

##Operator support

The library supports C/Java style operators along side a growing list of B4X native functions.

• Math operators: +-*/%
• Relational: > < >= <= != ==
• Logical: || && !
• Bitwise: << >> & ^ |
• Functions: abs(), iif(), min(), max(), sqrt(), power()

##Linking to your project

• To use the Android or Java editions, add the .JAR and .XML files to your Additional Libraries folder and check the MteEval library in the Libraries Manager of the IDE.
• For iOS, copy the modules Codeblock.bas, Codegen.bas, PCODE.bas, and Run.bas to your project folder or place them in the Shared Modules folder. Then add the modules to the project through the IDE.