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ActionScript 3.0 Cookbook by Joey Lott, Darron Schall, Keith Peters

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Checking Equality or Comparing Values

Problem

You want to check if two values are equal.

Solution

Use the equality (or inequality) or strict equality (or strict inequality) operator to compare two values. To check whether a value is a valid number, use isNaN().

Discussion

Equality expressions always return a Boolean value indicating whether the two values are equal. The equality (and inequality) operators come in both regular and strict flavors. The regular equality and inequality operators check whether the two expressions being compared can be resolved to the same value after converting them to the same datatype. For example, note that the string “6” and the number 6 are considered equal because the string “6” is converted to the number 6 before comparison:

trace(5 == 6);    // Displays: false
trace(6 == 6);    // Displays: true
trace(6 == "6");  // Displays: true
trace(5 == "6");  // Displays: false

Note that in a project with default settings, the previous code example won’t even compile. That’s because it is compiled with a strict flag, causing the compiler to be more exact in checking datatypes at compile time. It complains that it is being asked to compare an int with a String. To turn off the strict flag, go to the ActionScript Compiler section of the project’s properties, and uncheck the box next to “Enable compile-time type checking (-strict)”. It is suggested, however, that you leave this option on for most projects, as it gives you better protection against inadvertent errors.

The logical inequality operator (!=) returns false if two values are equal and true if they aren’t. If necessary, the operands are converted to the same datatype before the comparison:

trace(5 != 6);    // Displays: true
trace(6 != 6);    // Displays: false
trace(6 != "6");  // Displays: false
trace(5 != "6");  // Displays: true

Again, this example only compiles if strict type checking is disabled.

On the other hand, if you have turned off the strict flag, but you want to perform a strict comparison in one section of code, you can use the strict equality and inequality operators, === and !==. These first check whether the values being compared are of the same datatype before performing the comparison. Differences in datatypes causes the strict equality operator to return false and the strict inequality operator to return true:

trace(6 === 6);    // Displays: true
trace(6 === "6");  // Displays: false
trace(6 !== 6);    // Displays: false
trace(6 !== "6");  // Displays: true

Warning

There is a big difference between the assignment operator (=) and the equality operator (==). If you use the assignment operator instead of the equality operator, the variable’s value will change rather than testing its current value.

Using the wrong operator leads to unexpected results. In the following example, quantity equals 5 at first, so you might expect the subsequent if statement to always evaluate to false, preventing the trace() from being executed:

var quantity:int = 5;
// The following code is wrong. It should be if (quantity == 6) instead
if (quantity = 6) {
  trace("Rabbits are bunnies.");
}
trace("quantity is " + quantity);  // Displays: quantity is 6

However, the example mistakenly uses the assignment operator (=) instead of the equality operator (==). That is, the expression quantity = 6 sets quantity to 6 instead of testing whether quantity is 6. When used in an if clause, the expression quantity = 6 is treated as the number 6. Because, any nonzero number used in a test expression converts to the Boolean true, the trace() action is called. Replace the test expression with quantity == 6 instead. Fortunately, the ActionScript 3.0 compiler is smart enough to recognize this common error and although the code still compiles, you aren’t given a warning.

You can check an item’s datatype using the is operator, as follows:

var quantity:int = 5;
if (quantity is int) {
  trace("Yippee. It's an integer.");
}

Tip

Note that the new ActionScript 3.0 types int and uint will also test positive as Numbers.

However, some numeric values are invalid. The following example results in quantity being set equal to NaN (a constant representing invalid numbers, short for not a number) because the calculation cannot be performed in a meaningful way:

var quantity:Number = 15 - "rabbits";

Despite its name, NaN is a recognized value of the Number datatype:

trace(typeof quantity);   // Displays: "number"

Therefore, to test if something is not just any number, but a valid number, try this:

var quantity:Number = 15 - "rabbits";
if (quantity is Number) {

  // Nice try, but this won't work
  if (quantity != NaN) {
    trace("Yippee. It's a number.");
  }
} 

Warning

However, you can’t simply compare a value to the constant NaN to check whether it is a valid number. The ActionScript 3.0 compiler even gives you a warning to this effect. Instead, you must use the special isNaN() function to perform the test.

To determine if a number is invalid, use the special isNaN() function, as follows:

var quantity:Number = 15 - "rabbits";
if (isNaN(quantity)) {
  trace("Sorry, that is not a valid number.");
}

To test the opposite of a condition (i.e., whether a condition is not true) use the logical NOT operator (!). For example, to check whether a variable contains a valid number, use !isNAN(), as follows:

var quantity:Number = 15 - "rabbits";
if (!isNaN(quantity)) {

  // The number is not invalid, so it must be a valid number
  trace ("That is a valid number.");
}

Of course, you can perform comparisons using the well-known comparison operators. For example, you can use the < and > operators to check if one value is less than or greater than another value:

trace(5 < 6);    // Displays: true
trace(5 > 5);    // Displays: false

Similarly, you can use the <= and >= operators to check if one value is less than or equal to, or greater than or equal to, another value:

trace(5 <= 6);   // Displays: true
trace(5 >= 5);   // Displays: true

You should also be aware that ActionScript compares datatypes differently. ActionScript datatypes can be categorized either as primitive (string, number, and Boolean) or composite (object, sprite, and array). When you compare primitive datatypes, ActionScript compares them “by value.” In this example, quantity and total are considered equal because they both contain the value 6:

var quantity:Number = 6;
var total:Number = 6;
trace (quantity == total);         // Displays: true

However, when you compare composite datatypes, ActionScript compares them “by reference.” Comparing items by reference means that the two items are considered equal only if both point to exactly the same object, not merely objects with matching contents. For example, two arrays containing exactly the same values are not considered equal:

// Create two arrays with the same elements.
var arrayOne:Array = new Array("a", "b", "c");
var arrayTwo:Array = new Array("a", "b", "c");
trace(arrayOne == arrayTwo);          // Displays: false

Two composite items are equal only if they both refer to the identical object, array, or sprite. For example:

// Create a single array
var arrayOne:Array = new Array("a", "b", "c");
// Create another variable that references the same array.
var arrayTwo:Array = arrayOne;
trace(arrayOne == arrayTwo);          // Displays: true

See Also

Recipe 5.8

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