Problems with modulo operator

Programming Questions

alphajuno

Problems with modulo operator

in Programming Questions • 1 year ago

I am writing a 2D cellular automata that uses a 16x16 grid of objects. The objects are stored in a multi-dimensional array representing the grid. To calculate the values of the next generation, I need to check the state of the surrounding cells (either on or off). If the cell to be checked is at the edge of the grid, then I'd like to have the method wrap around the grid,

I am doing this using the method below method, but am running into problems using the modulo operator:

// Scan neighbours, find out how many are active
int findNeighbours(int x, int y) {
int count = 0;
for(int i = -1; i<1; i++) {
for(int j = -1; j<1; j++) {
// Grid size is 16
int xPos = (x+i)%gridSize;
int yPos = (y+j)%gridSize;
// Check state
if(grid[xPos][yPos].on == true) {
count++;
}
}
}
return count++;
}

The problem is, that I would expect:

(-1) mod 16 = 15

Instead, I get:

(-1) mod 16 = -1

Resulting in an out of bounds error.

What's happening here?

Replies(6)

jbum

Re: Problems with modulo operator

1 year ago

I avoid using negative numbers with modulo. Some languages it works (when it's implemented as true modulo), some it doesn't (when it's implemented as remainder, as in Java/Processing). With either type, you can always get a decrementing counter using

(x + gridsize - 1) % gridsize

Edit: Updated my comments per DavBol's insightful post, below.

alphajuno

Re: Problems with modulo operator

1 year ago

Thanks for the help. As far as I can tell everything is now how it should be, but it doesn't perform as expected.

Here's my entire code:

// Number of rows

int gridSize = 16;

// Holds cells

Cell[][] grid, futureGrid;

void setup() {

size(600, 600);

background(0);

stroke(200);

frameRate(4);

rectMode(CORNER);

// Create grids

grid = new Cell[gridSize][gridSize];

futureGrid = grid;

// Size of cell

float unitSize = (float) width/gridSize;

for (int x = 0; x <= gridSize-1; x++) {

for (int y = 0; y <= gridSize-1; y++) {

// Location of each cell

float xPos = unitSize * x;

float yPos = unitSize * y;

grid[x][y] = new Cell(xPos, yPos, unitSize, false);

}

// Starting pattern

grid[3][8].alive = true;

grid[4][8].alive = true;

grid[5][8].alive = true;

}

void draw() {

drawGrid();

getNextGen();

// Swap new for the old

Cell[][] temp = grid;

grid = futureGrid;

futureGrid = temp;

}

// Display grid

void drawGrid() {

for (int x = 0; x < gridSize; x++) {

for (int y = 0; y < gridSize; y++) {

grid[x][y].drawCell();

}

// Update the future grid

void getNextGen() {

for (int x = 0; x < gridSize; x++) {

for (int y = 0; y < gridSize; y++) {

int nb = countNumberOfNeighbours(x,y);

// Check rules of the game

if (grid[x][y].alive == true && nb < 2) {

futureGrid[x][y].alive = false; // Death by isolatialive

} else if (grid[x][y].alive == true && nb > 3) {

futureGrid[x][y].alive = false; // Overcrowding

} else if (grid[x][y].alive == false && nb == 3) {

futureGrid[x][y].alive = true; // Birth

} else {

futureGrid[x][y].alive = grid[x][y].alive; // Survive

}

// Scan neighbours, find out how many are active

int countNumberOfNeighbours(int x, int y) {

int count = 0;

for (int i = -1; i<1; i++) {

for (int j = -1; j<1; j++) {

int xPos = (x+gridSize+i)%gridSize;

int yPos = (y+gridSize+j)%gridSize;

if (grid[xPos][yPos].alive == true) {

count++;

}

return count++;

}

class Cell {

float xPos, yPos;

float cellSize;

boolean alive;

Cell(float xPos,float yPos,float cellSize, boolean on) {

this.xPos = xPos;

this.yPos = yPos;

this.cellSize = cellSize;

this.alive = on;

}

void drawCell() {

if(alive)

{

fill(0);

} else {

fill(255);

}

rect(xPos, yPos, cellSize, cellSize);

}

jbum

Re: Problems with modulo operator

1 year ago

Two sets of problems.

1) Your count neighbors has a few bugs. In your loops you want it to go from -1 to 1, so you should use

<= 1 instead of < 1.

1b) You should not count the cell itself

if (i == 0 && j == 0)

continue;

1c) You should not increment count when you return it.

2) Your cell arrays are pointing to the same set of cell objects, which is screwing with your logic. You can code this much more simply if you omit the Cell object entirely, and store the cells as arrays of booleans or bytes.

Then just write a function to draw the cell at a particular address.

void drawCell(x,y) {

boolean alive = grid[x][y];

// same drawing code as before...

}

alphajuno

Re: Re: Problems with modulo operator

1 year ago

Thanks!! I've fixed it now.

davbol

Re: Problems with modulo operator

1 year ago

to your original question, (not to its use in your CA),

(and really just as future reference for any others searching this question):

java has no modulo operator, "%" is the remainder operator.

it's frequently mislabelled, but the distinction is important in several cases, negative numbers being one.

in true modular math (like a clock), -1%12 gives 11

(where -1 means "1 hour before midnight/noon" which is 11)

in remainder math (like java's), -1%12 gives -1

(such that truncated result of division, times divisor, plus remainder, equals original: (a/b)*b+(a%b)=a)

(run the example on -1%16 and you'll see the result of division is 0, so remainder must be -1 to satisfy equation)

esoteric stuff, granted, but hth

jbum

Re: Problems with modulo operator

1 year ago

Awesome clarification - thanks.

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