Array editing in Android

Ive been working on an app for android much alike the program "Powder Toy". In order to simulate particle "stacking" I h开发者_运维技巧ave an array that monitors every column and if a particle goes past the value of the corresponding element, it is stopped and the element is increased for the next particle to land a little higher. When the element is changed it doesn't just change by one, instead I got numbers changing from, for example: 320 to -1211, 320 to -602, 320 to 14, and so on. I might be missing something basic about array editing, any ideas how to get around this?

for(int i = 0; i < intspan; i++){
    tempx = particlex[i];
    tempy = particley[i];

    if(tempx == 0 && tempy == 0) {
        return;
    } else {
        canvas.drawPoint(tempx, tempy, paint);

        if(tempy < columns[tempx]) {

            particley[i] += 1;
        }
        else if (tempy >= columns[tempx]){
            columns[tempx]--;
        }
    }
}

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In the part

else if (tempy >= columns[tempx]){
    columns[tempx]--;
}

Your are decreasing columns[tempx], when it is smaller than tempy. Therefore it is getting smaller and smaller. And as it is inside a for loop, it could be decreased by up to the number of your particles in a single run.