ActionScript for Flash MX: The Definitive Guide, Second Edition

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Chapter 5: Operators

The left side of an assignment operation can never involve an operation. For exam-

ple, this code is common in mathematics, but it is illegal in programming:

x + y = 10; // ERROR!

As we’ll see later, equality comparisons between two expressions are performed with

the

==and != operators. For example, the following checks whether x+yis equal to

10:

var x = 1;

var y = 8;

trace(x + y = = 10); // Displays: false

Again, this is not quite the same as mathematics, where x+y=10asserts that x+y

is equal to 10, rather than checking whether that is the case.

Combining Operations with Assignment

Assignment operations are often used to set a variable’s new value, based in part on

its old value. For example:

counter = counter + 10; // Add 10 to the current value of counter

xPosition = xPosition + xVelocity; // Add xVelocity to xPosition

score = score / 2; // Divide score by two

Don’t confuse an equals sign, which is used to assign a value to a vari-

able, with the algebraic equals sign. For an explanation of the differ-

ence, see “Assigning Values to Variables” in Chapter 2.

ActionScript supports a shorthand version of assignment, called compound assign-

ment, that combines operators such as +, –, and / with the assignment operator to

form a single “calculate-while-assigning” operation. ...