Making Decisions with Conditionals and switch

Making Decisions with Conditionals and `switch`

During the course of the day, you make countless decisions. Everything you do depends on decisions large and small. Just eating a meal involves decisions, from what to eat to whether to use a fork or spoon.

With code, complex behaviors can be broken down into a series of simple decisions. These are called conditionals , or tests. A test is run, and depending on the result, an action is taken. For example, you may want to check whether the user entered the correct password before running more of the program.

Before jumping into conditionals, I want to briefly mention operators. Operators are the "glue" of ActionScripting. They are symbols that tell the code to do something specific. For instance, when you work with conditionals you'll often see the ==, &&, and operators. The == operator compares two elements of a statement, and requires that the elements be equivalent before another section of code can run. The && operator requires multiple conditional statements to be true. The operator requires one or another conditional statement to be true. This is why it is called the or operator.

There are two main formats for independent conditionals in Flash: 1) if, if-else , and if-else-if , and 2) switch .

Using the `if` Statement

When the Flash compiler encounters an if statement, it looks at the condition within the parentheses. If the condition is true, the code within the curly braces is executed. A general if statement looks like this:

if(condition){
    //do something
}

A simple condition compares two expressions, and returns true if the condition is met. Several types of comparisons can be made between expressions. For example, you may want to know whether two expressions are equal or different. Or you may want to know whether one expression is greater than another.

Let's say that you have a project where you have a movie clip instance for the sky in the background of an animation. What if you want to change the color of that instance to blue, when timeOfDay is "dawn" ? Before changing the color of the movie clip instance, you first need to determine what color to change it to. In ActionScript, you might write the following, which compares the value of timeOfDay to "dawn" . If they are equal, the code between the curly braces is run and the variable skyColor is assigned the value of "blue" .

if(timeOfDay == "dawn"){
    skyColor:String = "blue";
}

Although this will work, you might want to change the color at various times of the day. An else after the first closing curly bracket , followed by a second set of curly brackets enclosing another line of code, presents an alternative code to execute if the condition is not true. In this case, if timeOfDay is "dawn", skyColor is "blue" . If not, skyColor is "black" . This is called an if-else conditional.

if(timeOfDay == "dawn"){
    skyColor:String = "blue";
} else {
    skyColor:String = "black";
}

You can set another conditional after an else . In fact, you can write a whole sequence of if-else-if statements. In the following example, the variable daytime has a range of numeric values. Notice that the conditional statements are more complex. By using the && logical operator, you can test for two or more conditions at once. Here, it tests whether the value is between specific number ranges:

if(daytime > 0500 && daytime 1130)
    skyColor:String = "blue";
} else if (daytime >1130 && daytime <= 1230)
    sky Color:String = "bright_blue;
} else if (daytime > 1230 && daytime <= 1900){
    skyColor:String = "blue";
} else {
    sky Color:String =" black";
}

Using the `switch` Statement

The switch style of conditionals can be used when you have a list of specific values that you are testing for equality. It works just like an if-else statement, but the code is formatted differently. First, you state that this is a switch conditional, with the variable to be tested in parentheses. Then you list each case that you want to test. The syntax for each case is as follows . Note the colon after the case value, which serves as shorthand for the curly braces.

case "value":
    //code to execute goes here;

The following example shows a series of cases to compare values for the variable daytime . Remember that the keyword case tells the compiler to test whether the next expression is equal to the variable in parentheses.

switch(daytime){

    case "dawn":
        skyColor:String = "dark_blue";
        break;
    case "morning":
        skyColor:String = "blue";
        break;
    case "noon":
        skyColor:String = "light_blue";
        break;
    case "afternoon":
        skyColor:String = "blue";
        break;
    case "evening":
        skyColor:String = "dark_blue";
        break;
    case "night":
        skyColor:String = "black";
}

Switch statements are sometimes easier to read, and are better suited to specific cases than to a range of possible values.