Arrays can be extremely useful and are an essential part of making games. It is essentially a type of variable that can hold multiple values as a "list" -- consider the following code:

numbers = [ 0, 1, 2, 3, 4, 5 ];

fruits = [ "Apples", "Oranges", "Mangoes" ];

Using the [item, item, item] syntax we are creating an array that is stored in a variable. The items stored in an array can later be accessed through that variable using an integer number, starting at 0, which is placed inside [] brackets:

first_fruit = fruits[ 0 ];
second_fruit = fruits[ 1 ];
// ...and so on.

1-Dimensional Arrays1-Dimensional Arrays

Before going any further let's clarify what an array actually is and how it's structured. An array is simply a data type that is assigned to a variable, and it can contain not just one value, but multiple values. The image below shows a schematic for a basic array:

This is called a 1D (one-dimensional) array, and as you can see the array is stored in the variable "a" and contains multiple values. To access the array you would do something like the following:

var _val = a[0];

The above code gets the value from position 0 of the array "a" then outputs it to the console, which - based on the contents of the array shown in the image above - would output 125. If you did the following:

var _val = a[3];

The output would show "Hi!".

As you can see, you give the array a variable name and then a value in square brackets [], where the value is the position in the array to get the data from. So essentially, an array is a container with a number of slots to store values, and each position in the container has a specific number to identify it, which is what we put in the []. It's worth noting that the contents of an array always start at 0 and can never be negative!

       Creating Arrays

We've shown how to check an array for data, but how do we create the array to start with? First it has to be initialized before we can use it or GameMaker Studio 2 will give us an error. Initializing an array just means that we give each slot of the array an initial value in preparation for it to be used elsewhere in the project code. This is important to remember as it means that you have to do a certain amount of planning before using arrays, but it is easy enough to initialize one using a repeat loop like this:

var i = 9;

    array[i] = 0;
    i -= 1;

This simple code will initialize a ten-slot array (from 0 to 9) to hold 0, ie: each slot in the array contains the value 0. You will notice that the array has been initialised backwards, with the last value being defined first. This is not strictly necessary but is the optimal way to do it as it will reserve a space in memory that is the exact size of the array, whereas if you initialize an array from 0 upwards, the memory has to be re-allocated for every additional value added (so for a ten-slot array, initialising it in a loop would change the memory allocation ten times). The speed difference is negligible for smaller arrays, but larger ones should be optimised as much as possible in this way.

NOTE: The HTML5 export is the exception to the above rule, and when targeting that you should initialise arrays in consecutive order from 0 upwards.

You can also use the GML function array_create() to initialize an array with a fixed size, and you can even create "empty" arrays with no values, for example:

my_array = [];

This tells GameMaker that the variable "my_array" is an array, and you can then add values to it at any time in the future. However, if you try to access a value in an empty array then you will get an error.

If you already know which items you want to put into the array, you can add comma-separated values between the brackets when declaring the array:

my_array = ["Steve", 36, "ST-3V3 - Steve Street"];

       Array Bounds

You should always take care to only access valid array positions, as trying to access a value outside of an array will also give an error. For example, this will cause the project to crash when run:

my_array = array_create(5, 0);
var _val = my_array[6];

The array was only initialised with 5 positions, but we've tried to get position 7 - since arrays are numbered from 0, array[6] is position 7 - therefore the game generates an error and crashes.

       Using Arrays

Now how do we use an array practically? Exactly the same as we would use a normal variable, as shown in the following examples:

// Add two array values together
total = array[0] + array[5];

// Check an array value
if array[9] == 10
    // Do something

// Draw an array value
draw_text(32, 32, array[3]);

Since arrays are numbered sequentially, this means you can loop through them to perform extra actions too, just like we did to initialize it:

var total = 0;

for (var i = 0; i < 10; ++i)
    total += array[i];
    draw_text(32, 32 + (i * 32), array[i]);

draw_text(32, 32 + (i * 32), total);

The above code will add up all the values in our array, draw each one of them and then draw the total value at the end.

       Deleting Arrays

The last thing to mention about arrays is that you can delete an array simply by "re-assigning" the variable that defines it to a single value. This will free up the memory associated with all the positions and values for that array. For example:

// Create an array
for (var i = 9; i > -1; --i)
    a[i] = i;

// Delete the array
a = -1;

If the array has multiple dimensions (see below), they will all be cleaned up too, and note that when you create arrays in instances, these do not need to be cleaned up when the instance is removed from the game, as they will be removed automatically by the garbage collector on Destroy or Room End. However, if any of the array positions hold references to dynamic assets, such as particle systems, buffers, or data structures, then these will need to be destroyed before the array is deleted, the instance is destroyed or the room ends.


Multi-Dimensional ArraysMulti-Dimensional Arrays

We now know what a 1-dimensional array is, but in GameMaker Studio 2 you can have arrays with multiple dimensions, which are essentially structured as an array inside an array inside an array... For example, the following is a 2D (two-dimensional) array:

array[0][0] = 5;

This is essentially telling GameMaker that the array is actually comprised of various 1D arrays. Here's an extended example:

array[0][0] = 0;
array[0][1] = 1;
array[0][2] = 2;

array[1][0] = 3;
array[1][1] = 4;
array[1][2] = 5;

In the above code, array[0] holds another array, and so does array[1].

A multi-dimension array needs to be initialised before use, the same as a single 1D array, and can hold real numbers, strings, and any other data type, just like any variable, making them ideal candidates for any game that needs to store large amounts of data in an easily accessible way (remember, you can loop through an array easily).

You can also initialize a multi-dimensional array within one statement by nesting 1-dimensional arrays:

two_dimensional_array = 
    ["Apple", 10, 2],
    ["Orange", 5, 2],
    ["Mango", 15, 4],
    // ...and so on.

Multi-dimension arrays are also not limited to just two dimensions, and you can have 3, 4 or more dimensions to an array as required in your code, just by adding [n] further arguments, eg:

array[0][0][0] = 1;     // A three dimensional array
array[0][0][0][0] = 1;  // A four dimensional array
// etc...

It should be noted too that the length of each dimension in an array can be different, so you can have the initial array dimension with a length of 3, but the second dimension entry can be a different length for each slot in the first dimension; for example:

array[2][2] = "3";
array[2][1] = "2";
array[2][0] = "1";

array[1][3] = "four";
array[1][2] = "three";
array[1][1] = "two";
array[1][0] = "one";

array[0][1] = 2;
array[0][0] = 1;

In the above code, array[0] has 2 slots, array[1] has 4 slots and array[2] has 3 slots.

       Extended Example

Here is one final example of how this may be used in an actual game: Say you want to spawn four different enemies at four different points in your game depending on a random value. Well, we can use an array with 2 dimensions to do this and save writing out a load of code.

First we should initialize the array we are going to use in the Create event of our "controller" object (note the use of comments to remind you what each array entry does):

enemy[3][2] = 448;       //y position
enemy[3][1] = 32;        //x position
enemy[3][0] = obj_Slime; //Object
enemy[2][2] = 448;
enemy[2][1] = 608;
enemy[2][0] = obj_Skeleton;
enemy[1][2] = 32;
enemy[1][1] = 608;
enemy[1][0] = obj_Knight;
enemy[0][2] = 32;
enemy[0][1] = 32;
enemy[0][0] = obj_Ogre;

We now have the objects to spawn instances of and their corresponding x and y spawn coordinates within the room all stored in our array. This can now be used as follows in another event of the controller object (an alarm for example, or a key press event):

//get a random number from 0 to 3, inclusive
var i = irandom(3);

//Use the array to create the object
instance_create_layer(enemy[i][1], enemy[i][2], "Enemy_Layer", enemy[i][0]);

That short code will now spawn a random enemy in the game room, and it uses far less code than an "if / then / else" structure or even a "switch", and as the array is initialized all together in the create event it is MUCH easier to edit and change any of those values as they are not hard-coded into the rest of the project code.


Arrays as Arguments

Just like normal variables, you can pass arrays through to script functions and method variables as arguments, and then return them back to the instance that called the function. To do this, you simply have to specify the array variable (no need for each of the individual positions, nor the [] brackets) and the entire array will be passed by reference into the function.

However, should you change any of the array's values, the array will be copied into a temporary array just for that function. Note the use of the word temporary here! You are not actually passing the array itself into the function (as you would a variable), but instead you are requesting that the function create a copy of this array, which you will change. This means that you must always return the array from the function if you wish to change any array values (this behaviour is called "copy on write").

NOTE: Due to the way that this works internally, passing arrays to functions may affect performance, especially if the array is very large. So use this functionality with care!

As an example, consider the following code. First we create the array we want to use, and then we pass that array into a function:

for (var i = 9; i > -1; --i)
    a[i] = i;


The function itself is something simple like this:

my_array_func = function(array)
    for (var i = 9; i > -1; --i)
        array[i] = i * 100;

Now you would expect the final array to hold the values 0, 100, 200, etc... BUT this will not be the case, since we did not return the array from the function, so all we changed was the temporary copy that was created when we passed the array as an argument into the function, and when the function has finished the copied array basically disappeared too. To rectify this we should have formatted the code as follows:

for (var i = 9; i > -1; --i)
    a[i] = i;

a = my_array_func(a);

And the function should now look like this:

my_array_func = function(array)
    for (var i = 9; i > -1; --i)
        array[i] = i * 100;

    return array;

NOTE: The above code is not necessary if you are not changing any of the array's values, but rather referencing them. Referencing an array will not copy it and will be faster to parse.

It is also worth noting that you can use the accessor @ to reference an array from an argument and change its values directly, which saves the CPU overhead of having to make a temporary copy. This means that you do not need to return the array from the function and can edit it directly:

my_array_func = function(array)
    for (var i = 9; i > -1; --i)
        array[@ i] = i * 100;

You can find out more information on accessors and how they work, along with an example for arrays, from the following page: