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supesfan

Dynamic Arrays in UnrealScript...

4 posts in this topic

If you know UnrealScript at all, I could sure use your help to understand this. 
 
What I don't understand is what I highlited in Bold. Specifically TestArray[CopyArray.length - 1]
 
What I am not sure about is, is this statement copying the value of TestArray to CopyArray? and if so why do we need to subtract to 1? Thanks for your help.
 
Let's say we had an array and PostBeginPlay set up like this:
 
var int TestArray[3];
var array<int> CopyArray;
function PostBeginPlay()
{
TestArray[0] = 9;
TestArray[1] = 5;
TestArray[2] = 6;
}
Without using numbers, how would we copy the TestArray values into the CopyArray?
Think about our experiment on adding to the end of arrays for hints, and when you are
ready; check the following code for the answer.
var int TestArray[3];
var array<int> CopyArray;
function PostBeginPlay()
{
TestArray[0] = 9;
TestArray[1] = 5;
TestArray[2] = 6;
 
CopyArray[CopyArray.length] = TestArray[CopyArray.length – 1];
CopyArray[CopyArray.length] = TestArray[CopyArray.length – 1];
CopyArray[CopyArray.length] = TestArray[CopyArray.length – 1];
}
 
If you tried TestArray[CopyArray.length] at first, you probably noticed an out of
bounds error in the log. Why is that? Well, an important thing to know about dynamic arrays
is that assigning a value to the end of the array increases the size of the array first, before
assigning the value. By the time the code reaches the right-hand side of the equals sign, the
array's length has already increased by 1.
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is this statement copying the value of TestArray to CopyArray?

 

That's exactly what it's doing... perhaps not the best way to do it, but it works none-the-less. :P

 

As for why you need to subract by 1, you need to understand three things. First of all, while the starting index of arrays is 0, the length variable starts at 1 (so an array with one element will only be indexed with 0, but will have a length of 1). Secondly, unlike static arrays, with unrealscript's dynamic arrays, you can extend an array simply by trying to write to an element out of the array's current bounds (so if CopyArray has 3 elements and you write to CopyArray[3], you will extend the array rather than getting an out of bounds error). And lastly, the length variable updates in real-time, so with the bolded lines, every time "CopyArray.length" is called a second time, CopyArray.length has already incremented.

 

Ehh, it's probably easier to see rather than have it explained, so I'll just go step-by-step through one of those lines.

 

CopyArray[CopyArray.length] = TestArray[CopyArray.length - 1];

At the start of the line, assuming this is the first time we wrote this, CopyArray has no elements, meaning it has a length of 0. CopyArray[CopyArray.length] is the same thing as writing CopyArray[0], meaning we just extended the array by one. So with the right hand side, CopyArray.length is 1 and to get the correct element from TestArray, we need the -1. Without that, when we reach the 3rd line, the code'll be accessing TestArray[3], which is out of the array's bounds.

 

Hope this explanation was somewhat understandable/helpful.

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Actually your explanation was helpful. I wasn't sure if UnrealScript was that popular of a language so I wasn't sure if anybody would be able to help. Glad you did. Thanks a lot. However there is still one thing that confuses me. Shouldn't we be able to copy the value of TestArray to CopyArray with only one statement, why would they need three of them? Is that because you can only copy over one index at a time?

 

Also, you said that writing CopyArray[CopyArray.length] is the same as writing CopyArray[0}, meaning we just extended the array by one.

 

Does this mean that just by writing and executing that statement the Array is automatically given a value of 1 (CopyArray[0]) before the program reaches the other side of the = sign? If so is that because Arrays are pre-initialized with a value of 1 in UnrealScript?

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Well... usually you'd use a for loop to copy two arrays. If you're copying a dynamic array to another dynamic array, you can simply set them equal to each other, like this:

var array<int> TestArray;
var array<int> CopyArray;

function PostBeginPlay()
{
   TestArray[0] = 9;
   TestArray[1] = 5;
   TestArray[2] = 6;

   CopyArray = TestArray;   //This actually works!!
}

However, as soon as static arrays are involved, that won't work. What you'd probably want to do then would be something like:

var int TestArray[3];
var array<int> CopyArray;

function PostBeginPlay()
{
   local int i;

   TestArray[0] = 9;
   TestArray[1] = 5;
   TestArray[2] = 6;

   //Copy the array
   for( i = 0; i < ArraySize(TestArray); i++ )
   {
       TestArray[i] = CopyArray[i];
   }
}

 

Also, when I said that writing CopyArray[CopyArray.length] was the same thing as writing CopyArray[0], I meant that only for the first time that line was written (when CopyArray.length was equal to 0). And dynamic arrays are pre-initialized with a length of 0, but when they are extended, the array's length variable will change BEFORE the right-hand side of the line is executed. Sort of confusing, and you'll rarely encounter code where you need to know that, but it's always nice to know. :D

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Well it's nice to know I will rarely encounter code like that because it hurts my brain to think about it...

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