Question: How do you use is_undefined in GameMaker?

Answer

In GameMaker, the is_undefined function is used to check if a variable has been defined or not. This is particularly useful when working with instances or data structures where you might not be sure if a certain value has been set.

Here's an example of how you can use is_undefined:

// Check if the variable 'playerScore' is defined
if (is_undefined(playerScore)) {
    // If it's undefined, initialize it with a default value
    playerScore = 0;
}

In the above code, if playerScore wasn't previously declared and assigned a value, it would be considered undefined, and thus the condition would be true, leading to playerScore being initialized to zero.

It's important to note that undefined in GameMaker is a special constant representing a variable that has not been defined. It differs from simply having a variable with no value or a null value as seen in some other programming languages.

You can also use is_undefined when dealing with arrays or data structures:

// Assuming 'inventory' is an array that may have uninitialized indices
var item = inventory[5];
if (is_undefined(item)) {
    // The item at index 5 hasn't been added to the inventory yet
    show_message("There is nothing in this slot.");
}

With the introduction of GameMaker Studio 2.3.0, using is_undefined became less common because of the improved variable scope and initialization. However, it remains a helpful function, especially in scenarios involving dynamic data or data received from external sources where uncertainty about variable definition might exist.