The for loop allows you to iterate through a variable for a specific range of values. You must supply three expressions in a for statement: a variable that is set to an initial value, a conditional statement that determines when the looping ends, and an expression that changes the value of the variable with each loop. For example, the following code loops five times. The value of the variable i starts at 0 and ends at 4, and the output will be the numbers 0 through 4, each on its own line.

var i:int; 
for (i = 0; i < 5; i++) 
{ 
    trace(i); 
}

The for..in loop iterates through the properties of an object, or the elements of an array. For example, you can use a for..in loop to iterate through the properties of a generic object (object properties are not kept in any particular order, so properties may appear in a seemingly random order):

var myObj:Object = {x:20, y:30}; 
for (var i:String in myObj) 
{ 
    trace(i + ": " + myObj[i]); 
} 
// output: 
// x: 20 
// y: 30

You can also iterate through the elements of an array:

var myArray:Array = ["one", "two", "three"]; 
for (var i:String in myArray) 
{ 
    trace(myArray[i]); 
} 
// output: 
// one 
// two 
// three

What you cannot do is iterate through the properties of an object if it is an instance of a user-defined class, unless the class is a dynamic class. Even with instances of dynamic classes, you will be able to iterate only through properties that are added dynamically.

The for each..in loop iterates through the items of a collection, which can be tags in an XML or XMLList object, the values held by object properties, or the elements of an array. For example, as the following excerpt shows, you can use a for each..in loop to iterate through the properties of a generic object, but unlike the for..in loop, the iterator variable in a for each..in loop contains the value held by the property instead of the name of the property:

var myObj:Object = {x:20, y:30}; 
for each (var num in myObj) 
{ 
    trace(num); 
} 
// output: 
// 20 
// 30

You can iterate through an XML or XMLList object, as the following example shows:

var myXML:XML = <users> 
    <fname>Jane</fname> 
    <fname>Susan</fname> 
    <fname>John</fname> 
</users>; 
 
for each (var item in myXML.fname) 
{ 
    trace(item); 
} 
/* output 
Jane 
Susan 
John 
*/

You can also iterate through the elements of an array, as this example shows:

var myArray:Array = ["one", "two", "three"]; 
for each (var item in myArray) 
{ 
    trace(item); 
} 
// output: 
// one 
// two 
// three

You cannot iterate through the properties of an object if the object is an instance of a sealed class. Even for instances of dynamic classes, you cannot iterate through any fixed properties, which are properties defined as part of the class definition.

The while loop is like an if statement that repeats as long as the condition is true . For example, the following code produces the same output as the for loop example:

var i:int = 0; 
while (i < 5) 
{ 
    trace(i); 
    i++; 
}

One disadvantage of using a while loop instead of a for loop is that infinite loops are easier to write with while loops. The for loop example code does not compile if you omit the expression that increments the counter variable, but the while loop example does compile if you omit that step. Without the expression that increments i , the loop becomes an infinite loop.

The do..while loop is a while loop that guarantees that the code block is executed at least once, because the condition is checked after the code block is executed. The following code shows a simple example of a do..while loop that generates output even though the condition is not met:

var i:int = 5; 
do 
{ 
    trace(i); 
    i++; 
} while (i < 5); 
// output: 5