Recipe 1.7. Using Mathematical Operators

Problem

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

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 sprite to a new position that is 10 pixels to the right of its current position.

In an assignment statementany 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 some number:

// Add 6 to the current value of quantity, and assign that new  // value back to quantity. For example, if quantity was 4, this  // statement sets it to 10. quantity = quantity + 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 most prevalent compound assignment operators. When you use one of these compound assignment operators, the value on the right side of the assignment operator is added to, subtracted from, multiplied by, or divided into the value of the variable on the left, and the new value is assigned to the same variable. The following are a few examples of equivalent statements.

These statements both add 6 to the existing value of quantity:

quantity = quantity + 6; quantity += 6;

These statements both subtract 6 from the existing value of quantity:

quantity = quantity - 6; quantity -= 6;

These statements both multiple quantity by factor:

quantity = quantity * factor; quantity *= factor;

These statements both divide quantity by factor:

quantity = quantity / factor; quantity /= factor;

There should be no space between the two symbols that make up a compound assignment operator. Additionally, if you are incrementing or decrementing a variable by 1, you can use the increment or decrement operators.

This statement adds 1 to quantity:

quantity++;

and has the same effect as either of these statements:

quantity = quantity + 1; quantity += 1;

This statement subtracts 1 from quantity:

quantity --;

and has the same effect as either of these statements:

quantity = quantity  1; quantity -= 1;

You can use the increment and decrement operators (-- and ++) either before or after the variable or property they operate upon. If used before the operand, they are called prefix operators. If used after the operand, they are called postfix operators. The prefix and postfix operators modify the operand in the same way but at different times. In some circumstances, there is no net difference in their operation, but the distinction is still important in many cases. When using prefix operators, the value is modified before the remainder of the statement or expression is evaluated. And if you're using postfix operators, the value is modified after the remainder of the statement has executed. Note how the first example increments quantity after displaying its value, whereas the second example increments quantity before displaying its value:

var quantity:Number = 5; trace(quantity++);  // Displays: 5 trace(quantity);    // Displays: 6 var quantity:Number = 5; trace(++quantity);  // Displays: 6 trace(quantity);    // Displays: 6

Getting back to the original problem, you can use these operators to modify a property over time. This example causes the specified sprite to rotate by five degrees each time the method is called:

private function onEnterFrame(event:Event) {   _sprite.rotation += 5; }

Note that in ActionScript 3.0, you would have to add an event listener to the enterFrame event and set this method as the event handler for this to work properly. See Recipe 1.5 for information on how to handle the enterFrame event.

See Also

Recipe 1.5