Array not set before reading (Bug?)

[pawbap]

Hi GMC!

I have a question about arrays that I'm not getting and I think it's best explained with a demonstration.

Consider the following code in the Create Event of a newly created object called objArrayTester:

//Example #1
function addOne(array) {
    array[0] += 1;
}

a = array_create(1, 0);
addOne(a);
show_debug_message(a);

This...results in a fatal error:


However, interestingly, if I use the @ accessor in the function like so:

//Example #2
function addOne(array) {
    array[@ 0] += 1;
}

a = array_create(1, 0);
addOne(a);
show_debug_message(a);

The game runs like normal and the output log tells me that a contains [ 1 ]:


Even more interestingly though, if I change line array[@ 0] += 1; to array[0] = array[0]+1;, so that it's:

//Example #3
function addOne(array) {
    array[0] = array[0]+1;
}

a = array_create(1, 0);
addOne(a);
show_debug_message(a);

The game will STILL run and also give me the same output. a contains [ 1 ]. I'm not returning the array at all, so this shouldn't be the case, right?

The manual explains the use of the @ accessor used to directly reference/modify the contents of an array instead of making a copy. Handy. It also explains that without it, a copy of the array is created, modifying that instead, not the original thing. So, I don't understand why Example #1 crashes without the use of the @ accessor or why the contents of a change in Example #3.

IDE Version: 2.3.1.542
Current Runtime: 2.3.1.409

Any replies appreciated!

 

[gnysek]

Can you add show_debug_message(array) in addOne function to check, what is actually passed into function? You can also use debugger to check.

 

[pawbap]

Can you add show_debug_message(array) in addOne function to check, what is actually passed into function? You can also use debugger to check.

I tried a couple more places.

//Example #3
function addOne(array) {
    show_debug_message("A "+string(array));
    array[0] = array[0]+1;
    show_debug_message("B "+string(array));
}

a = array_create(1, 0);
show_debug_message("C "+string(a));
addOne(a);
show_debug_message("D "+string(a));

EDIT: The output is this if the attachment doesn't work for whatever reason.

C [ 0 ]
A [ 0 ]
B [ 1 ]
D [ 0 ]

 

[pawbap]

I tried a couple more places.

//Example #3
function addOne(array) {
    show_debug_message("A "+string(array));
    array[0] = array[0]+1;
    show_debug_message("B "+string(array));
}

a = array_create(1, 0);
show_debug_message("C "+string(a));
addOne(a);
show_debug_message("D "+string(a));

View attachment 38019

EDIT: The output is this if the attachment doesn't work for whatever reason.

C [ 0 ]
A [ 0 ]
B [ 1 ]
D [ 0 ]

Alright, so after that experiment, I went ahead and commented out show_debug_message("A "+string(array));
Output is:

C [ 0 ]
B [ 1 ]
D [ 1 ]

Really...interesting!!