Square grid

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int rectWidth = 25;

int rectHeight = 25;

void setup() {

  size(396,396);

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}

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void draw() {

  background(0);

  fill(255);

  for (int y= 5; y<height; y += 30){

    for (int x= 5; x<width; x += 30){

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      rect(x, y,rectWidth, rectHeight);

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      if (mouseX >= x && mouseX <= x+rectWidth && 

        mouseY >= y && mouseY <= y+rectHeight) {

        fill(0);

        rect(x, y,rectWidth, rectHeight);

      }

      fill(255);

    }

  }

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}

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