Class: MEL Programming Basics
Instructor: Mike Harris email
Class: 4
Date: March 9, 2006

MEL Programming Basics

Conditional Branching:

An action will be performed only if a specified condition is evaluated to be true

                      // if condition //
                        if ( condition is true )
                                do something;

                      // if-else condition //
                        if ( first condition is true )
                                do something;
                        else
                                do something else;

                      // if-else if-else condition //
                        if ( first condition is true )
                                do something;
                      else if ( second condition is true )      // Multiple else if tests are optional and valid
                                do something else;
                      else if ( third condition is true )
                                do something else;
                      else if ( fourth condition is true )
                                do something else;
                        else
                                do something else;

Switch Case is a way of shortening the amount of typing for a large if-else if-else conditional branching test, replacing the need for multiple else if tests
Syntax:

                        switch (controlValue )
                        {
                            case value1:
                                statement;
                                break;
                            case value2:
                                statement;
                                break;
                            case value3:
                                statement;
                                break;
                            default:
                              statement;
                        }

You can remove the "break;" line to "fall through" to the next statement if the value is true. This means that if value2 was met and there was no break in that group, the statements for both value2 and value3 would execute until the "break;" of value3 was reached.

Scope:

Global vs. Local:

Global variables are seen everywhere in Maya and can become problematic with naming conflicts and the possibility of changing someone else's data.
Local variables are seen only within the "nested" level they are created in and all of that levels "nested" children.

Nesting is a heirarchy way of making levels of code depend on the parent level of code.

Example:

                                                                                  // Global scope!
                                grandparent level
                                {                                                   // start gransparent scope
                                        parent level
                                        {                                           // start parent scope
                                                child1 level
                                                {                                 // start child1 scope

                                                }                                 // end child1 scope
                                                child2 level
                                                {                                 // start child2 scope

                                                }                                 // end child2 scope
                                        }                                           // end parent scope
                                }                                                   // end gransparent scope

If a variable is declared inside the curly braces of a multi-line statement (loop, conditional branch, or proceedure) the value of that variable is only visible while you are inside the curly braces. So when you leave the multi-line statement, it is removed from memory.
If you declare the variable outside the curly braces it will be seen at it's level as well as all nested "child" levels of the script.
Example:

                         for ( $i = 0; $i < 10; $i++ )
                         {
                                string $badscope = "Hello World!";
                                print ( "This is pass number: " + $i + " through the loop\n" );
                        }
                        print ( $badScope );   // This will give you an error because Maya no longer sees the variable $badScope

This should be written:

                        string $goodScope = "Hello World!";
                        for ( $i = 0; $i < 10; $i++ )
                        {
                                print ( $goodScope + "\n" );
                                print ( "This is pass number " + $i + " through the loop.\n" );
                        }
                        print ( $goodScope );   // Because the variable was declared outside the curly braces of the loop, Maya sees it both inside and outside of the nested loop

Functions vs. Procedures:

A function is a built in piece of code that will perform a set task based on the user input
Examples of functions:

size( )
rand( )
sphrand( )
rad_to_deg( )
deg_to_rad( )

A proceedure is a small piece of code that performs a specific task allowing you to moduralize your code into more managable pieces.
Sometimes code is used more than one time in a script. This is not a loop, but simply code that can serve more than one purpose at a different time in the script.
Basic syntax:

                        proc procName( arguments )   // arguments are optional
                        {
                                // Tasks to perform...
                        }

Passing Arguments to Procedures:

Since variables are only seen within the block of code they were created, arguments are ways that we can pass variables from one procedure to another without having to make them global.
Example:

                        proc add2Numbers( float $numA, float $numB )
                        {
                                float $total = ( $numA + $numB );
                                print ( "The sum of " + $numA + " and " + $numB + " is: " + $total + "\n" );
                        }

                        add2Numbers( 3.5, 25.63 );