Programming fundamentals

First of all, it’s useful to have a conceptual idea of what a computer program is and what it does. There are two main aspects to a computer program:

• A program is a series of instructions or steps for the computer to carry out.

• Each step ultimately involves manipulating some piece of information or data.

In a general sense, a computer program is just a list of step-by-step instructions that you give to the computer, which it performs one by one. Each individual instruction is known as a statement. In ActionScript, each statement is written with a semicolon at the end.

In essence, all that a given instruction in a program does is manipulate some bit of data that’s stored in the computer’s memory. A simple example is instructing the computer to add two numbers and store the result in its memory. A more complex example is if there is a rectangle drawn on the screen and you want to write a program to move it somewhere else on the screen. The computer remembers certain information about the rectangle: the x, y coordinates where it’s located, how wide and tall it is, what color it is, and so on. Each of those bits of information is stored somewhere in the computer’s memory. A program to move the rectangle to a different location would have steps like “change the x coordinate to 200; change the y coordinate to 150.” In other words, it would specify new values for the x and y coordinates. Behind the scenes, the computer does something with this data to actually turn those numbers into the image that appears on the computer screen. However, at the basic level of detail it’s enough to know that the process of “moving a rectangle on the screen” just involves changing bits of data in the computer’s memory.

Programming mainly involves changing pieces of information in the computer’s memory. Consequently, it’s important to have a way to represent a single piece of information in a program. A variable is a name that represents a value in the computer’s memory. As you write statements to manipulate values, you write the variable’s name in place of the value. Any time the computer sees the variable name in your program, it looks in its memory and uses the value it finds there. For example, if you have two variables named value1 and value2, each containing a number, to add those two numbers you could write the statement:

value1 + value2

When it’s actually carrying out the steps, the computer looks to see the values in each variable and adds them together.

In ActionScript 3.0, a variable actually consists of three different parts:

• The variable’s name

• The type of data that can be stored in the variable

• The actual value stored in the computer’s memory

You’ve seen how the computer uses the name as a placeholder for the value. The data type is also important. When you create a variable in ActionScript, you specify the specific type of data that it is meant to hold. From that point on, your program’s instructions can store only that type of data in the variable. You can manipulate the value using the particular characteristics associated with its data type. In ActionScript, to create a variable (known as declaring the variable), you use the var statement:

var value1:Number;

This example tells the computer to create a variable named value1, which can hold only Number data. (“Number” is a specific data type defined in ActionScript.) You can also store a value in the variable right away:

var value2:Number = 17;

A constant is similar to a variable. It is a name that represents a value in the computer’s memory with a specified data type. The difference is that a constant can only be assigned a value one time in the course of an ActionScript application. Once a constant’s value is assigned, it is the same throughout the application. The syntax for declaring a constant is almost the same as that for declaring a variable. The only difference is that you use the const keyword instead of the var keyword:

const SALES_TAX_RATE:Number = 0.07;

A constant is useful for defining a value that is used in multiple places throughout a project, which don’t change under normal circumstances. Using a constant rather than a literal value makes your code more readable. For example, consider two versions of the same code. One multiplies a price by SALES_TAX_RATE. The other multiplies the price by 0.07. The version that uses the SALES_TAX_RATE constant is easier to understand. In addition, suppose the value defined by the constant does change. If you use a constant to represent that value throughout your project, you can change the value in one place (the constant declaration). In contrast, you would have to change it in various places if you used hard-coded literal values.

In ActionScript, there are many data types that you can use as the data type of the variables you create. Some of these data types can be thought of as “simple” or “fundamental” data types:

• String: a textual value, like a name or the text of a book chapter

• Numeric: ActionScript 3.0 includes three specific data types for numeric data:

  • Number: any numeric value, including values with or without a fraction

  • int: an integer (a whole number without a fraction)

  • uint: an “unsigned” integer, meaning a whole number that can’t be negative

• Boolean: a true-or-false value, such as whether a switch is on or whether two values are equal

The simple data types represent a single piece of information: for example, a single number or a single sequence of text. However, most of the data types defined in ActionScript could are complex data types. They represent a set of values in a single container. For example, a variable with the data type Date represents a single value (a moment in time). Nevertheless, that date value is represented as several values: the day, month, year, hours, minutes, seconds, and so on, all of which are individual numbers. People generally think of a date as a single value, and you can treat a date as a single value by creating a Date variable. However, internally the computer thinks of it as a group of several values that, put together, define a single date.

Most of the built-in data types, as well as data types defined by programmers, are complex data types. Some of the complex data types you probably recognize are:

• MovieClip: a movie clip symbol

• TextField: a dynamic or input text field

• SimpleButton: a button symbol

• Date: information about a single moment in time (a date and time)

Two words that are often used as synonyms for data type are class and object. A class is simply the definition of a data type. It’s like a template for all objects of the data type, like saying “all variables of the Example data type have these characteristics: A, B, and C.” An object, on the other hand, is just an actual instance of a class. For example, a variable whose data type is MovieClip could be described as a MovieClip object. The following are different ways of saying the same thing:

• The data type of the variable myVariable is Number.

• The variable myVariable is a Number instance.

• The variable myVariable is a Number object.

• The variable myVariable is an instance of the Number class.