Wednesday, 6 June 2012

Arrays in GML : 5 things you probably don’t know

What is GML?

You have most probably heard of Gamemaker, the popular game design software.

Along with Drag-and-Drop (D&D) feature, it also features an extensive coding language, which is formally called “The Game maker Language (GML)”.

According to Mark Overmars, the original creator of Game Maker (which has been recently re-named as Gamemaker), “Game Maker contains a built-in programming language. This programming language gives you much more flexibility and control than the standard actions. This language we will refer to as GML (the Game Maker Language).”

Why this article?

The only official documentation of Game Maker is the help file included with the software. This help file, though complete, documents only what happens when something is used and the correct syntax to use it. But, it leaves out necessary details in some cases.

Among the many features of GML, I find the “Array” portion of the help file least detailed, which is (probably) supposed to be known by the user. Though this is acceptable, it often causes a lot of confusion when using arrays extensively for the first time in a new project. A lot of things have to be guessed. Now the problem lies with this guessing: Everyone with prior programming knowledge tries to think in terms of their previous concepts and experiences. Unfortunately, this often differs considerably due to the different approach of different programming languages.

This problem is concentrated further by the fact that compiled languages like C/C++ (which are most popular among programmers) support only fixed length arrays (without using dynamic memory allocation, of course) compared to GML which supports dynamic arrays (so, practically their lengths are not fixed and can be changed whenever required). Thus, for many programmers, previous concepts have to be tested in GML. This article tries to solve this problem, testing and publishing the result of every exceptional situation that may arise with arrays in GML. I have tried to answer every weird question about arrays that popped up in my head and thus, exposing some of the lesser-known facts about it. But on other hand, you are expected to know what is written in the help file.

Just a word of caution: You probably won’t find anything worth experimenting with arrays after you complete reading this.

So, let’s start…

1.     What does arr refer to when arr is an array? 

A: In GML, nothing is practically defined as a variable of some specific data type. The data type is instead implied based on the value assigned to it at any time. A variable is implied to be an array only when it is assigned a value in this format:
  • a)      arr[ind]=value;   /* in case of 1-D arrays or */  variable_local_array_set(name,ind,value);
  • b)      arr[ind1,ind2]=value; /*in case of 2-D arrays*/variable_local_array2_set(name,ind1,ind2,value);
Now that it is implied as an array, arr actually refers to the arr[0,0] ( which is the same as arr[0] ) position. 

2.     How are the 1-D and 2-D arrays related when a 1-D array is converted into 2-D array and vice-versa? 

 A: After a lot of experimenting, this is the final decision I came to: arr[ind1] is the same as arr[0,ind1] for all practical purposes. The value of arr[ind1] changes to the new value whenever the value of arr[0,ind1] is changed to a new value and vice versa.

3.     Is there any relation between numbers, strings and arrays? 

A: Two-dimensional arrays can be considered as the general data type for both numbers and strings as well as 1-D arrays (Hence, these are the special cases of an 2-D array).

      They are related in the following way:

  • Any number or string variable (let’s say,the variable  named numb_or_string , usually used in code like this:  numb_or_string=something;) is actually the [0,0] position of the 2-D array of the same name(i.e. numb_or_string[0,0] ). Thus, numb_or_string can be alternatively called as numb_or_string[0,0] and vice versa . 
  • Any 1-D array (for eg. arr[ind], used in code like this:  arr[ind]=something;) essentially refers to the [0,ind] position of the 2-D array with the same name(i.e.  arr[0,ind] ).
This answer generalizes the answers given in the first and second question.

As a side note, this is the reason why the same variable name can’t be used for two variables even when they are of different data types.

4.     Which elements of an array are filled with the default value?

 A: This idea is quite simple: I’ll explain it in terms of a 2-D array, though it can be easily applied with 1-D arrays (following the previous rules). The first and second index of a 2-D array is referred to as row and column respectively, for visualization purpose.

   The rules for filling up the elements of an array with the default value (zero) are:

  •   If it’s a global 2-D array, then the [0,0] element is filled with zero.
  •   In both local and global arrays, for any row, all the undefined elements between any two defined element is filled with the default value, zero.
5.     Is there always an error when a negative index is passed to an array?

 A: In terms of 2-d array, an error is always thrown when you use this type of syntax: “arr[ind1,ind2]=value; “ (where atleast one of the values among ind1 and ind2 is negative), to set or get a value. But only in one exceptional case, when you use variable_local_array2_set(name,ind1,ind2,value)  to set a value, such that ind1 is positive but ind2 is negative, no error is thrown. 

     I found out this last exception just out of curiosity. Though practically it is not of much use as the memory is ultimately leaked and so, the value can’t be accessed. IMO, it is some sort of a unimportant bug, so you should probably check if ind2 is negative, before using it in that function.


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