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# Tetris collision detection (java)

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Hi! I'm currently writing a Tetris game, just for some practice and also, because it's fun. I figured out how I want to do collision detection, I'm aware of that there are probably thousand other ways to do it, that are better than how I do it. But I'll stick with my way of doing it for now, and if I run in to problems later that will be a lesson learned. So please don't comment on ineffective coding/bad coding, because I already know that it is. So I just wrote the part that's supposed to check for a collision when a piece is falling. I'm using 2 2d arrays, one for the location of the falling piece, and one for the location of the "inactive" pieces. To check if there's a collision between a falling piece and an inactive piece I do this:

for(int i = 0; i < width - 1; i++)
{
for(int n = 0; i < height - 1; n++)
{
if(fallingpiece[n] > 0)
{
if(inactivepiece[n + 1] > 0)
{
}
else
{
}
}
}
}

Before you say I'm stupid, I know that I don't actually do anything. But as soon as I run that code the program just doesn't continue, to the point after the code. width = 10 height = 20 the 2d arrays are both [10][20]. I'm probably just missing something obvious here, but I can't see where or what. The only thing I can think of is that I'm doing something with 2d arrays that I can't do. I never worked with 2d arrays before. If you need more information just say so, and I'll tell you what you want to know :). Thanks for any help in advance! PS. Forgot to mention the reason that I check if fallingpiece[n] is greater than 0, is because it can be any integer between 1 and 7, and each number is one of the colors. [Edited by - Bluestream on March 8, 2008 7:17:25 AM]

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Quote:
 Original post by Bluestreamwidth = 10height = 20the 2d arrays are both [10][20].

Aren't arrays constructed as array[height][width]? If so, you might be having some odd results from switching these.

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Quote:
 Original post by ShakedownAren't arrays constructed as array[height][width]? If so, you might be having some odd results from switching these.

I don't see the difference. Please, correct me if I'm wrong but the array in itself is just 10 * 20 storage places, I just chose to use it the way I did. And as far as I understand it wouldn't make any difference the other way around(?). What I mean is that you, instead of creating an array like int[200] you can create a 2d array like, int[10][20]. Just because it's easier to work with.

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Hello,

It's a classic error and very easy to miss ;-)

for(int n = 0; i < height - 1; n++)

you're checking i insteed of n

kind regards
Uncle

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You have created an infinite loop, because the condition for the inner most loop is checking the value of "i", but this variable is never incremented.

for(int i = 0; i &lt; width - 1; i++)        {           // Problem is here: "i &lt; (height - 1)"           for(int n = 0; i &lt; height - 1; n++)            {                if(fallingpiece[n] &gt; 0)                {                    if(inactivepiece[n + 1] &gt; 0)                    {                    }                    else                    {                    }                }            }        }

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Quote:
 Original post by UncleRemusHello,It's a classic error and very easy to miss ;-)for(int n = 0; i < height - 1; n++)you're checking i insteed of nkind regardsUncle

Ah, of course! Thanks a lot! Thought it was odd :).

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Quote:
 Original post by Bluestreamfor(int i = 0; i < width - 1; i++)width = 10height = 20the 2d arrays are both [10][20].

Valid array indices range from 0 to size-1, but your loop only iterates from 0 to width-2 (because width-1 is not smaller than width-1). This is the correct idiom:

for(int i = 0; i < width; i++)

This will iterate all valid indices, from 0 to size-1.

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Another one:

The line

if(inactivepiece[n + 1] > 0)

will get an ArrayIndexOutOfBoundsException if n ever reaches height - 1. Array indices start by 0 and go up to array length - 1. +1 is one too much.

You might put a safety check there.

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