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  [SOLVED] Math problem with array .length  (Read 673 times)
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Offline Yemto

Junior Devvie

Exp: 3 years

« Posted 2013-08-20 09:20:44 »

I believe this is quite simple. But I have never been good at math so I ask here before doing some thing dumb. In my game the world is build up in chunks, which is 32x32 tiles. The problem I have is moving a object to another chunk. It works as long as the coordinates are not negative. But if they are, it won't work. I have tried to do it on my own. But what that would require is an awful if statement to handles negative coordinates, and I think it could be done without an if statement

here is the code I have got working so far. I remove all attempts to do negative coordinates
int chunkX = getChunkX() + (x / Chunk.SIZE);
int chunkY = getChunkY() + (y / Chunk.SIZE);
x = x%Chunk.SIZE;
y = y%Chunk.SIZE;

so what needs to happened. If a object is at 0,0 in chunk 5,5 and gets a order to move to 0,-1 the coordinates most change to 0,31 chunk 5,4

EDIT: I going to try to make it easier to understand.

I have big and small arrays set up like this.

The P is the player/Object. Now I want to move him to 3,-1 which would result in java.lang.ArrayIndexOutOfBoundsException So instead I want to move the Player/Object to 3x8 but in the big 0,-1 array. The problem is I don't know the math I need to do so I get the right big array, and the small array coordinates.

chunkX and chunkY is the big array x and y
x and y  is the small array x and y
Offline Stranger

Senior Devvie

Medals: 9

I'm going slightly mad. It finally happened.

« Reply #1 - Posted 2013-08-20 11:15:40 »

This might help you.

Offline Yemto

Junior Devvie

Exp: 3 years

« Reply #2 - Posted 2013-08-20 13:57:59 »

This might help you.

I don't get it.
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Offline Riven
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Hand over your head.

« Reply #3 - Posted 2013-08-20 14:01:43 »

To wrap on an axis (both positive and negative) do this:
x = (((x+n) % DIM) + DIM) % DIM

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Offline Yemto

Junior Devvie

Exp: 3 years

« Reply #4 - Posted 2013-08-20 20:07:43 »

To wrap on an axis (both positive and negative) do this:
x = (((x+n) % DIM) + DIM) % DIM

Thank you so much. Me and some guys at work went nuts over this problem. It's finally fixed.

Here is the code I ended up using. Just to see if someone sees any issues.
public void setTileFromWorld(Tile t, int x, int y){
    int DIM = Chunk.SIZE;

    int cx = (int)Math.floor(x / (double)DIM);
    x = ((x % DIM) + DIM) % DIM;

    int cy = (int)Math.floor(y / (double)DIM);
    y = ((y % DIM) + DIM) % DIM;

    for(Chunk c : chunks){
        if(cx == c.getX() && cy == c.getY()){
            c.setTile(t, x, y); return;
    System.err.println("An error occurred while trying to place a tile");
    System.err.println("Chunk X: "+cx+" X: "+x);
    System.err.println("Chunk Y: "+cy+" Y: "+y);

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