I've written a simple piece of code to display the Ulam Spiral ( http://en.wikipedia.org/wiki/Ulam_spiral)
Nothing fancy: the algorithm draws the spiral pixel by pixel, and checks if each new number is a prime - if so, it'll draw a point on that pixel.

In the first line, int dim = 200 sets the matrix to 200x200. If I set this value to anything over 500 in my MacBookPro, the sketch takes forever to run  - I've never seen it compile when set to 1000.

Is this just the way it is, or is there a better, less processor intensive way to check if a number is prime? Commenting out the call to checkNumber() makes the code run in a jiffy, so the problem must be within this function alright.

Thanks in advance!

1. int dim = 200;
   int matrixDim = dim-1;
   int centerPoint = ((matrixDim-1)/2);
   int cellCount = 1;
   
   void setup() {
     size(dim,dim);
     background(255);
     int currentPosX = centerPoint;
     int currentPosY = centerPoint;
     int highestVal=matrixDim*matrixDim;
     float diagVal;
   
     //start at center point, defined as (matrixDim-1/2, matrixDim-1/2);
     //in an odd dimension matrix. such as 7x7, center point is (3,3);
     point (centerPoint,centerPoint);
     //starting at the center point, the algorithm for drawing the Ulam spiral is as follows:
     //for a step N
     //  Right 2N-1  Steps
     //  then Up 2N-1  Steps
     //  then Left  2N  Steps
     //  then Down 2N
     for (int counter=1;counter<matrixDim;counter++) {
       int RUCounter = (2*counter)-1;
       int LDCounter = 2*counter;
   
       for (int i=0;i<RUCounter;i++) {
         currentPosX++;
         cellCount++;
         checkCell(currentPosX,currentPosY,cellCount);
       }
   
       for (int i=0;i<RUCounter;i++) {
         currentPosY++;
         cellCount++;
         checkCell(currentPosX,currentPosY,cellCount);
       }
   
       for (int i=0;i<LDCounter;i++) {
         currentPosX--;
         cellCount++;
         checkCell(currentPosX,currentPosY,cellCount);
       }
   
       for (int i=0;i<LDCounter;i++) {
         currentPosY--;
         cellCount++ ;
         checkCell(currentPosX,currentPosY,cellCount);
       }
     }
   }
   
   void checkCell(int posX, int posY,int number) {
     if (checkNumber(number)) {
       point(posX,posY);
     }
   }
   
   boolean checkNumber(int n) {
     boolean isPrime = false;
     for (int i=2;i<n;i++) {
       if (n%i== 0) {
         isPrime=false;
         break;
       }
       else {
         isPrime=true;
       }
     }
     return isPrime;
   }