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3.4. Datatype Conversion

Take a closer look at the example from the previous section. In that example, each datum -- "1" and 2 -- belonged to its own datatype; the first was a string and the second a number. We saw that the interpreter joined the two values together to form the string "12". Note that the interpreter first had to convert the number 2 into the string "2". Only after that automatic conversion was performed could the value "2" be joined (concatenated) to the string "1".

Datatype conversion simply means changing the type of a datum. Not all datatype conversions are automatic; we may also change a datum's type explicitly in order to override the default datatyping that ActionScript would otherwise perform.

3.4.1. Automatic Type Conversion

Whenever we use a value in a context that does not match its datatype, the interpreter attempts a conversion. That is, if the interpreter expects data of type A, and we provide data of type B, the interpreter will attempt to convert our type B data into type A data. For example, in the following code we use the string "Flash" as the righthand operand of the subtraction operator. Since only numbers may be used with the subtraction operator, the interpreter attempts to convert the string "Flash" into a number:

999 - "Flash";

Of course, the string "Flash" can't be successfully converted into any legitimate number, so it is converted into the special numeric data value NaN (i.e., Not-a-Number). NaN is a legal value of the number datatype, intended specifically to handle such a situation. With "Flash" converted to NaN, our expression ends up looking like this to the interpreter (though we never see this internal step):

999 - NaN;

Both operands of the subtraction operator are now numbers, so the operation can proceed: 999 - NaN yields the value NaN, which is the final value of our expression.

An expression that yields the numeric value NaN isn't particularly useful; most conversions have more functional results. For example, if a string contains only numeric characters, it can be converted into a useful number. The expression:

999 - "9";  // The number 999 minus the string "9"

converts internally to:

999 - 9;    // The number 999 minus the number 9

which yields the value 990 when the expression is resolved. Automatic conversion is most common with the plus operator, the equality operator, the comparison operators, and conditional or loop statements. In order to be sure of the result of any expression that involves automatic conversion, we have to answer three questions: (a) what is the expected datatype of the current context? (b) what happens when an unexpected datatype is supplied in that context? and (c) when conversion occurs, what is the resulting value?

To answer the first and second questions, we need to consult the appropriate topics elsewhere in this book (e.g., to determine what datatype is expected in a conditional statement, see Chapter 7, "Conditionals").

The next three tables, which list the rules of automatic conversion, answer the third question, "When conversion occurs, what is the resulting value?" Table 3-1 shows the results of converting each datatype to a number.

Table 3-1. Converting to a Number

Original Data

Result After Conversion

undefined

0

null

0

Boolean

1 if the original value is true; 0 if the original value is false

Numeric string

Equivalent numeric value if string is composed only of base-10 numbers, whitespace, exponent, decimal point, plus sign, or minus sign (e.g., "-1.485e2")

Other strings

Empty strings, non-numeric strings, including strings starting with "x", "0x", or "FF", convert to NaN

"Infinity"

Infinity

"-Infinity"

-Infinity

"NaN"

NaN

Array

NaN

Object

The return value of the object's valueOf( ) method

Movieclip

NaN

Table 3-2 shows the results of converting each datatype to a string.

Table 3-2. Converting to a String

Original Data

Result After Conversion

undefined

"" (the empty string).

null

"null".

Boolean

"true" if the original value was true; "false" if the original value was false.

NaN

"NaN".

0

"0".

Infinity

"Infinity".

-Infinity

"-Infinity".

Other numeric value

String equivalent of the number. For example, 944.345 becomes "944.345".

Array

A comma-separated list of element values.

Object

The value that results from calling toString( ) on the object. By default, the toString( ) method of an object returns "[object Object]". The toString( ) method can be customized to return a more useful result (e.g., toString( ) of a Date object returns: "Sun May 14 11:38:10 EDT 2000").

Movieclip

The path to the movie clip instance, given in absolute terms starting with the document level in the Player. For example, "_level0.ball".

Table 3-3 shows the results of converting each datatype to a Boolean.

Table 3-3. Converting to a Boolean

Original Data

Result After Conversion

undefined

false

null

false

NaN

false

0

false

Infinity

true

-Infinity

true

Other numeric value

true

Nonempty string

true if the string can be converted to a valid nonzero number, false if not; in ECMA-262, a non-empty string always converts to true (Flash 5 breaks the rules in order to maintain compatibility with Flash 4)

Empty string ("")

false

Array

true

Object

true

Movieclip

true

3.4.2. Explicit Type Conversion

If the automatic (implicit) type-conversion rules do not suit our purpose, we can manually (explicitly) change a datum's type. When we take matters into our own hands, we must remember that the rules listed in the preceding tables still apply.

3.4.2.6. Converting to a number using the parseInt( ) and parseFloat( ) functions

The parseInt( ) and parseFloat( ) functions convert a string containing numbers and letters into a number. The parseInt( ) function extracts the first integer that appears in a string, provided that the string's first non-blank character is a legal numeric character. Otherwise, parseInt( ) yields NaN. The number extracted via parseInt( ) starts with the first non-blank character in the string and ends with the character before either the first non-numeric character or the first occurrence of a decimal point.

Some parseInt( ) examples:

parseInt("1a")                 // Extracts 1
parseInt("1.3a"                // Extracts 1
parseInt("     1a")            // Extracts 1
parseInt("I am 14 years old")  // Yields NaN (the first non-blank
                               // character is not a number)
parseInt("14 years old")       // Extracts 14

The parseFloat( ) function extracts the first floating-point number that appears in a string, provided that the string's first non-blank character is a valid numeric character. (A floating-point number is a positive or negative number that contains a decimal value, such as -10.5 or 345.678.) Like parseInt( ), parseFloat( ) yields the special numeric value NaN if the string's first non-blank character is not a valid numeric character. The number extracted by parseFloat( ) is a series of characters that starts with the first non-blank character in the string and ends with the character before the first non-numeric character (any character other than +, -, 0-9, or a decimal point).

Some parseFloat( ) examples:

parseFloat("1.3a");             // Extracts 1.3
parseFloat("2.75 years old")    // Extracts 2.75
parseFloat("1nce upon a time")  // Extracts 1
parseFloat("I'm 3.5 feet tall") // Yields NaN

For more information on parseInt( ) and parseFloat( ), see Part III, "Language Reference".

3.4.4. Flash 4-to-Flash 5 Datatype Conversion

In Flash 4, the string operators and the numeric operators were completely distinct -- one set of operators worked only with numbers, and a second set worked only with strings. For example, the string concatenation operator in Flash 4 was &, but the mathematical addition operator was +. Similarly, string comparisons were done with the eq and ne operators, but numeric comparisons were accomplished via = and <>. Table 3-4 lists the Flash 5 syntax for analogous Flash 4 operators.

Table 3-4. Flash 4 Versus Flash 5 Operators

Operation

Flash 4 Syntax

Flash 5 Syntax

String concatenation

&

+ or add

String equality

eq

==

String inequality

ne

!=

String comparison

ge, gt, le, lt

>=, >, <=, <

Numeric addition

+

+

Numeric equality

=

==

Numeric inequality

<>

!=

Numeric comparison

>=, >, <=, <

>=, >, <=, <

Some Flash 5 operators can operate on both strings and numbers. For example, when used with strings, the + operator concatenates its operands together to form a new string. But when used with numbers, the + operator adds its two operands together mathematically. Similarly, the equality operator (==) and inequality operator (!=) in Flash 5 are used to compare strings, numbers, and other datatypes.

Because many Flash 5 operators work with multiple datatypes but Flash 4 operators do not, an ambiguity arises when a Flash 4 file is imported into Flash 5. Therefore, when importing Flash 4 files, Flash 5 automatically inserts the Number( ) function around any numeric data that is used as an operand of the following potentially ambiguous operators (unless the operand is a numeric literal):

+, ==, !=, <>, <, >, >=, <=

Flash 4 files converted to Flash 5 will also have the string concatenation operator (&) changed to the new add operator. Table 3-5 contains examples of Flash 4-to-Flash 5 operator translation.

Table 3-5. Sample Flash 4-to-Flash 5 Operator Translations

Flash 4 Syntax

Flash 5 Syntax

Loop While (count <= numRecords)
while (Number(count)<=
Number(numRecords))
If (x = 15)
if(Number(x) == 15)
If (y <> 20)
if(Number(y) != 20)
Set Variable: "lastName" = "kavanagh"
lastName = "kavanagh"
Set Variable: "name" = "molly" & lastName
name = "molly" add lastName

3.4.5. Determining the Type of an Existing Datum

To determine what kind of data is held in a given expression before, say, proceeding with a section of code, we use the typeof operator, as follows:

typeof expression;

The typeof operator returns a string telling us the datatype of expression, according to Table 3-6.

Table 3-6. Return Values of typeof

Original Datatype

typeof Return Value

Number

"number"

String

"string"

Boolean

"boolean"

Object

"object"

Array

"object"

null

"null"

Movieclip

"movieclip"

Function

"function"

undefined

"undefined"

Here are a few examples:

trace(typeof "game over");    // Displays: "string" in the Output window

var x = 5;
trace(typeof x);              // Displays: "number"

var now = new Date( );
trace(typeof now);            // Displays: "object"

As shown in Example 3-1, when combined with a for-in statement, typeof provides a handy way to find all the movie clip instances on a timeline. Once identified, we can assign the clips to an array for programmatic handling. (If you can't follow all of Example 3-1, revisit it after completing Part I, "ActionScript Fundamentals".)

Example 3-1. Populating an Array with Dynamically Identified Movie Clips

var childClip = new Array( );
var childClipCount = 0;

for (i in _root) {
   thisItem = _root[i];
   if (typeof thisItem == "movieclip") {
     // Notice the use of the postfix increment operator
     childClip[childClipCount++] = thisItem;  
  }
}

// Now that our array is populated, we can use it
// to manipulate the clips it contains
childClip[0]._x = 0;  // Place the first clip on the left of the Stage
childClip[1]._y = 0;  // Place the second clip at the top of the Stage


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