All computer games require the manipulation of some type of data. This data may be collected as a result of interaction with the user or it may be stored internally within the game. Game Maker provides a number of different ways of storing data. One of the most basic ways of working with data in your applications is to embed within your code statements, as demonstrated here:

show_message("The value of pi is 3.14.");

Here, the value of the mathematic constant pi (3.14) is embedded within a string passed to a GML function named show_message(). A string is a collection of one or more characters enclosed within single or double quotation marks.

Although embedding within code statements provides an effective means of handling small amounts of static information that never changes, this approach is not satisfactory in most games, where data is constantly changing. Game Maker provides you with a number of different ways to store data within your applications, including variables, constants, and arrays.

Game Maker supports three different types of data: integers, real numbers, and strings. Each of these types of data is handled differently. Integers and real numbers can be added, subtracted, multiplied, and divided. Strings can be concatenated together, creating a new, larger string. However, strings cannot be subtracted, multiplied, or divided. An integer is a whole number, whereas a real number has a decimal point. As previously stated, a string is created by enclosing characters within single or double quotation marks.

As you define the variables used in your GML scripts, make sure you assign them descriptive names. This helps to make your code statements self-documenting and easier to understand. Game Maker variable names are case-sensitive. From Game Maker’s view, a reference to a variable named player1 is not the same thing as a reference to a variable named Player1. So be careful to use a consistent case when naming your variables. Variables must be uniquely named. When naming variables, you must follow the rules outlined below.

Game Maker does not require you to explicitly declare variables. Instead, all you have to do to create a new variable is to use it. In order to assign a value to a variable, you use the = operator, as demonstrated below.

name = "Alexander";

Here, a variable named name is created and assigned a string value of Alexander. If this variable already existed, Game Maker would simply change its value as instructed in the assignment statement.

Hint

When developing programming logic using Game Maker’s action icons, you can also work with variables using the actions shown in Figure 9.1 to create, modify, analyze, and display variable values.

Figure 9.1. These action icons facilitate the use of variables.

Game Maker supports three types of variables, each of which has a different scope. Scope defines the locations within your application from which variables can be accessed. The three types of variables that Game Maker supports are outlined here:

name = "Alexander";

x = playerOne.name;

global.age = 21;

global.age = 18;

globalvar age;
age = 21;

globalvar age, name, address, phone;

var age;
age = 21;

var age, name, address, phone;

As you have already seen through various action icons, Game Maker provides access to a large number of special variables that it automatically creates and maintains during program execution. These include both global and local variables. Examples of global variables are lives, score, and health. There are also global variables that provide information about the location and the operations of the mouse, including mouse_x, mouse_y, and mouse_button. Examples of Game Maker’s special local variables include:

You can retrieve and analyze the values stored in any of these variables, and in most cases, you can change their values as well, directly impacting the operations of your games as a result.

GML lets you create both one- and two-dimensional arrays. Single-dimensional arrays are created in GML by assigning them a name followed immediately by a [ bracket and a number representing the array’s index followed by a closing ] bracket, as demonstrated below.

Like variables, you do not have to declare arrays prior to using them. The syntax that you must follow when defining an array is outlined here:

arrayname[indices]

arrayname represents the name of the array being created, indices is a comma-separated list of numeric values that specifies whether the array has one or two dimensions. The following example demonstrates how to define an array named animals[] that is capable of storing six elements.

animals[5]

All array indices in GML begin at 0, so as defined above, the animals[] is capable of storing six entries (0 through 5). In GML, two-dimensional arrays are created by assigning them a name followed immediately by a [ bracket and two comma-separated numbers representing both of the array’s indices, followed by a closing ] bracket, as demonstrated here:

animals[4,4]

GML limits array index size to 32,000 and limits an array’s overall storage capability to 1,000,000 entries.

Game Maker arrays are zero-based. The first element (value) in the array starts at index location 0, and the second element in the array begins at location 1. The following example demonstrates how to create an array named names and assign it three elements.

{
   names[0] = "Molly";
   names[1] = "William";
   names[2] = "Alexander";
}

Once created, you can reference any element in an array by specifying its index number, as demonstrated here:

{
   names[0] = "Molly";
   names[1] = "William";
   names[2] = "Alexander";

   show_message("Match!");
}

Here, an array made up of three elements is created, and the second element (William) stored in an array is retrieved and displayed. Be repeating and modifying the last statements shown above, you could modify this example to display all of the array’s contents.

Rather than retrieving data an element at a time, it is usually a better idea to set up a loop to iterate through the array’s contents. For example, the following script creates an array and then loops through it using a for loop to iterate through and display its contents. More information on loops is provided in Chapter 10.

{
   names[0] = "Molly";
   names[1] = "William";
   names[2] = "Alexander";

   x_coord = 50;
   y_coord = 50;

   for (i=0; i<3; i=i+1) {
        draw_text(x_coord, y_coord, names[i]);
        y_coord = y_coord + 20
   }
}

An understanding of how to work with the if statement, along with its corresponding else statement, is required to develop even the most simple scripts. Using the if statement, you can implement any type of conditional test. Using the optional else statement, you can specify what you want to happen in the event the tested conditional evaluates as false.

if (expression){
     statements
}

else {
   statements
}

if (expression) statement;

secretNumber = round(random(2) + 1);
displayMsg = "I am thinking of a number from 1 to 3, try to guess it."

userGuess = get_string(displayMsg, "");

if (secretNumber == real(userGuess)) show_message('You guessed it!");

if (expression){
   statements
}

secretNumber = round(random(2) + 1);
displayMsg = "I am thinking of a number from 1 to 3, try to guess it."

userGuess = get_string(displayMsg, "");

if (secretNumber == real(userGuess)) {
    show_message("You guessed it!");
    sleep(3000);
    game_end();
}

secretNumber = round(random(2) + 1);
displayMsg = "I am thinking of a number from 1 to 3, try to guess it."

userGuess = get_string(displayMsg, "");

if (secretNumber == real(userGuess)) {
    show_message("You guessed it!");
} else {
    show_message("Sorry. Your guess was wrong.");
}

a = 5;
b = 10;
c = 20;

if (a == 5) {
    if (b == 10) {
        if (c == 20) {
            draw_text(200, 200, "Bingo!");
        }
    }
}

In addition to various adaptations of the if statement, Game Maker allows you to implement conditional logic using the switch statement. This statement is designed as a substitute for using multiple if statement code blocks. The switch statement is designed for situations where you need to compare one value against a range of different values. The switch statement must be used in conjunction with a number of other statements, including the case, break, and default statements, to create a code block as outlined here:

switch (expression){
    case expression:
         statement;
         .
         .
         .
         break;
    case expression:
         statement;
         .
         .
         .
         break;
   .
   .
   .
    default:
        statement;
        .
        .
        .
}

The switch statement code block begins with the switch statement followed by the expression to be evaluated and then a series of case blocks, each of which is designed to analyze a possible match with the expression. Each case block specifies its own expression that is compared to the switch statement’s expression. The statements embedded within the first case statement that results in a match are executed and the rest of the statements in the switch code block are skipped. In the event that none of the case statements results in a match, the statements embedded within the default code block are executed.

Note that the default statement and its code block are optional. If omitted and none of the expressions belonging to the case statements match the expression belonging to the switch statement, then nothing happens. Inclusion of the break statements is also optional and if omitted, any case statement that evaluates as true will be executed.

As an example of how to set up a switch statement code block, consider the following example.

{
   userGuess = get_string("Okay, what is the game rated?", "");

   switch (userGuess) {
       case "e": draw_text(200, 200, "Yes. Of course you may."); break
       case "t": draw_text(200, 200, "Yes. I suppose you can."); break
       case "m": draw_text(200, 200, "No. Wait until you are older."); break
       case "a": draw_text(200, 200, "No. Not in my house!"); break
       default: draw_text(200, 200, "No. I need to know more about it.");
   }

   sleep(3000)
   game_end()
}

Here, the user is prompted to enter a video game rating (e, t, m, or a). A switch statement code block is then used to analyze the user’s input and to compare it against a series of four case statements. A different text string is displayed based on the user’s input. If the user enters something other than e, t, m, or a, the string belonging to the default statement is displayed.