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I am trying to create the Sieve of Eratosthenes problem where I print out a grid from 2 to 100 and then cover all non-prime numbers with a rectangle. I can only get it to check one divisor between 2 and 10. I cant get it to loop through all divisors. my current version doesn't print any rectangles what so ever but it feels like it should be because reading it it looks like if variable a is less than 10 check if the number in that location is divisible by a. if it is print a rectangle there. once it checks all of those it add 1 to a. Where am I going wrong here?

int a=2;

void setup()
{
  size(600, 600);

  rectMode(CORNER);
  textSize(17);
  background(0);
  for (int x = 0; x < 10; x++)
  {
    for (int y =0; y<11; y++)
    {
      if ((x)+((y-1)*10)+1>1)
      {
        fill(255);

        text((x)+((y-1)*10)+1, x*50+30, y*50);
      }
    }
  }
}

void draw()
{
  for (int x = 0; x < 10; x++)
  {
    for (int y =0; y<10; y++)
    {
      while (a<10)
      {
       
        if ((x)+((y-1)*10)+1%a==0)
        {
          fill(50, 50, 200);
          rect((x)*50+30, (y)*50+30, 30, 30);
        }
         a++;
      }
    }
  }
}
Samuel Ross
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  • Maybe start your question by mentioning you're using the Processing library from processing.org. I would even drop the [java] tag, because people knowing Java but not the Processing library (like me) will not be able to help you. – GeertPt Oct 11 '22 at 15:48
  • It appears that you have two separate issues here. A) your calculation code isn't producing enough calls to text/fill/rect, and B) those calls aren't producing any visible result on your screen. You should separate these two problems into two questions, and provide the simplest code necessary in each case to reproduce that problem. For the drawing problem, all you should need is the half dozen or so lines that set up your drawing environment and then draw something... – CryptoFool Oct 11 '22 at 16:16
  • For the problem with the algorithm itself, take out all the calls to the drawing library and replace the draw primitives with simple print statements, or do something else inside the loops (like just counting the iterations) to demonstrate the problem you're having with the computational logic. I would suggest that you separate these two issues for your own debugging as well. – CryptoFool Oct 11 '22 at 16:17
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    Introducing functions such as a function to place numbers in a grid and a separate function to draw a rectangle over a specific number might help. Get these smaller functions to work and then worry about how to combine them. – John Coleman Oct 11 '22 at 17:56
  • Print out the values for x + (y-1)*10 + 1 % a and you will see that it never equates to zero, therefore no rectangles are drawn. – apodidae Oct 11 '22 at 18:25

1 Answers1

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You have several problems in Your solution.

  • ((x)+((y-1)*10)+1>1) formula is overcomplex for this problem,
  • void draw() is a loop, so Your 'a' variable will never go back to value 2.
  • Use System.out.println(); wherever You are not sure what is going on.

So, here is the cleaner concept of Your problem:

  • in Your case there is no need to call draw(),
  • set 'a' value every time before while(), not just once before setup(),
  • draw Your boxes just inside the same place as text()
    void setup() {
        size(600, 600);
        background(0);
        fill(255);
        for (int y=0; y<10; y++) {
            for (int x=1; x<=10; x++) {
                System.out.println(x + ", " + y + ": " + (x+y*10));
                int a=2;
                textSize(26);
                while (a<=Math.sqrt(x+y*10)) {
                    if ((x+y*10)%a == 0) {
                        System.out.println(a+": "+x+", " +y+": "+(x+y*10)); 
                        textSize(12);  
                    }
                a++;
                }
                text(x+y*10, x*50, y*50+50);
            }
        }
    }   
    void draw() {
    }
Mruk
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