Game of life rules not working properly

Question

I have the following logic code for my Game of Life application in Java. I have the problem that the rules do not act like the default Conway's Game of Life rules. I have read up on them on Wikipedia, and they are the following;

Any live cell with fewer than two live neighbours dies, as if caused by under-population.
Any live cell with two or three live neighbours lives on to the next generation.
Any live cell with more than three live neighbours dies, as if by overcrowding.
Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.

I have attempted to duplicate these rules in the following code, but it acts unlike the normal Conway' Game of Life;

int surroundingLife = 0;
if (lifeMap[cX+1][cY]) { //Right
    surroundingLife++;
}
if (lifeMap[cX-1][cY]) { // Left
    surroundingLife++;
}
if (lifeMap[cX][cY+1]) { // Above
    surroundingLife++;
}
if (lifeMap[cX][cY-1]) { // Below
    surroundingLife++;
}
if (lifeMap[cX-1][cY-1]) { // Bottom left
    surroundingLife++;
}
if (lifeMap[cX+1][cY+1]) { // Top Right
    surroundingLife++;
}
if (lifeMap[cX-1][cY+1]) { // Some other corner (I don't know which one)
    surroundingLife++;
}
if (lifeMap[cX+1][cY-1]) { // Yet another corner (I don't know which one)
    surroundingLife++;
}
if (running) {
    // Logic for life
    if (surroundingLife < 2 && lifeMap[cX][cY]) {// Rule 1. Any live cell with fewer than two live neighbours dies, as if caused by under-population.
        lifeMap[cX][cY] = false;
    } else if (surroundingLife == 2 && lifeMap[cX][cY]) { // Rule 2. Any live cell with two or three live neighbours lives on to the next generation.
        lifeMap[cX][cY] = true;
    } else if (surroundingLife == 3 && lifeMap[cX][cY]) { // Rule 3. Same as above
        lifeMap[cX][cY] = true;
    } else if (surroundingLife > 3 && lifeMap[cX][cY]) { // Rule 4. Any live cell with more than three live neighbours dies, as if by overcrowding.
        lifeMap[cX][cY] = false;
    } else if (surroundingLife == 3 && !lifeMap[cX][cY]) { // Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.
        lifeMap[cX][cY] = true;
    }
}

This is how it looks after running a couple of generations;

GameOfBugs, or BugsOfLife.

It reminds me of the 'maze' ruleset, which is odd.

I don't believe there is a fault with my surroundingLife calculator, as it returns 8 when entities have 8 others surrounding them. Is the problem due to me looping through Y then X?

possible duplicate of [The game of life(Conway's game) - how to check for cell neighbours](http://stackoverflow.com/questions/8364301/the-game-of-lifeconways-game-how-to-check-for-cell-neighbours) — Paul Gregoire, Nov 23 '14 at 01:02

score 10 · Accepted Answer · answered Jan 03 '13 at 15:58

10

The problem is that you are modifying the grid in the same pass as you are evaluating what needs to change. Each time you change a cell, you are affecting the outcome of all future tests in the same pass that border that cell.

You need to make a copy of the grid. Always test (read) from that copy, and apply changes (write) to the original.

answered Jan 03 '13 at 15:58

cdhowie

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2

So have 2 grids, and sync them after the generation has completed? – Jack Wilsdon Jan 03 '13 at 15:59
3

I wish I had a nickel for every time I've seen this mistake; heck I probably did it myself the first time. – Dave Newton Jan 03 '13 at 15:59
You can do that, or you can use the approach I've suggested. Just make a copy of the array at the beginning of each pass, and use that array for all of your tests. Apply the actual changes to the original array. – cdhowie Jan 03 '13 at 15:59
I have done as suggested, but still get the problem. Here is my new code; https://gist.github.com/098a5b811a3a4cc7e124 – Jack Wilsdon Jan 03 '13 at 16:02
@HighPerformanceMark You can do that too, if after each pass you go back and copy the data from `old` to `new` before you begin, otherwise you will be making a pass using stale data. – cdhowie Jan 03 '13 at 16:03
It may be the best option but you don't "***need** to make a copy of the grid.*" You could make it work with one grid - but it would probably need 2 passes for each generation. – ypercubeᵀᴹ Jan 03 '13 at 16:06
3

@jackwilsdon You did not make a copy of the array, you made a copy of the array reference. Both variables are still pointing to the same array. – cdhowie Jan 03 '13 at 16:06
@cdhowie What do you mean? – Jack Wilsdon Jan 03 '13 at 16:06
@jackwilsdon `checkCopy = lifeMap;` This does not create a new copy of the array. It copies the reference to the array from one variable to another. `checkCopy` and `lifeMap` are both references to the *same array*. Changes you make to one will reflect in the other, because they are actually targeting the same object. – cdhowie Jan 03 '13 at 16:07
When you type `checkCopy = lifeMap;` both objects still point to the same array, and you're still editing only one array. You have to make a new object and copy the values into that one. – Lynn Jan 03 '13 at 16:07
I was thinking more of defining `lifeMap[m][n][2]` and setting `old = 1`, `new = 2`, then run the game one step updating `lifeMap[][][new]` from `lifeMap[][][old]`, then swap `old = 2`, `new = 1` and continue. I don't see any need for copying arrays of data. – High Performance Mark Jan 03 '13 at 16:09
@HighPerformanceMark I see what you mean, yes, that would also work. I was momentarily forgetting that each pass will update *every* cell. – cdhowie Jan 03 '13 at 16:10
using `checkCopy = lifeMap.clone();` still modifies `lifeMap` when I do `checkCopy[20][20] = true`. How do I completely disassociate the arrays? – Jack Wilsdon Jan 03 '13 at 16:11
1

@jackwilsdon Cloning the array should be enough to disassociate the arrays. If that's not working then there is another problem somewhere else. – cdhowie Jan 03 '13 at 16:18
Using a for loop to go through the code works fine; https://gist.github.com/4444628 – Jack Wilsdon Jan 03 '13 at 16:18
1

@jackwilsdon If you're still having trouble, [let's chat over here](http://chat.stackoverflow.com/rooms/22135/game-of-life-rules-not-working-properly) instead of cluttering this answer with comments. – cdhowie Jan 03 '13 at 16:25

Game of life rules not working properly

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