Actionscript Cookbook

1.1. Using Mathematical Operators

Problem

You want to modify something over time, such as the rotation or position of a movie clip.

Solution

Use the compound assignment operators to change a variable or property in increments. Or, if incrementing or decrementing by one, use the prefix or postfix increment or decrement operators.

Discussion

Often, you’ll want the new value of a variable or property to depend on the previous value. For example, you might want to move a movie clip to a new position that is 10 pixels to the right of its current position.

In an assignment statement—any statement using the assignment operator (an equals sign)—the expression to the right of the equals sign is evaluated and the result is stored in the variable or property on the left side. Therefore, you can modify the value of a variable in an expression on the right side of the equation and assign that new value to the very same variable on the left side of the equation.

Although the following may look strange to those who remember basic algebra, it is very common for a variable to be set equal to itself plus a number:

// Add 6 to the current value of myNum and assign that new value back to myNum. For
// example, if myNum was 3, this statement sets it to 9.
myNum = myNum + 6;

However, when performing mathematical operations, it is often more convenient to use one of the compound assignment operators, which combine a mathematical operator with the assignment operator. The +=, -=, *=, and /= operators are the ...