//adapted from code by Mark Russo

int positionX = 250;
int positionY = 250;
int deltaX = 0;
int deltaY = 0;

void setup() {
  size(500, 500);
  smooth();
}

void draw() {
  background(255);
  
  // Increment position and clip value
  positionX = positionX + deltaX;
  positionY = positionY + deltaY;
  
  // Clip values
  if (positionX < 0)      positionX = 0;
  if (positionX > width)  positionX = width;
  if (positionY < 0)      positionY = 0;
  if (positionY > height) positionY = height;
  
  // Draw ellipse
  ellipse(positionX, positionY, 50, 50);
}

void keyPressed() {
    // Change direction based on key code
    if (keyCode == 37) {                  //left arrow
      deltaX = -2;
      deltaY = 0;
    }
    else if (keyCode == 39) {             //right arrow
      deltaX = 2;
      deltaY = 0;
    }
    else if(keyCode == 38) {              //up arrow
      deltaY = -2;
      deltaX = 0;
    }
    else if (keyCode == 40) {            //down arrow      
      deltaY = 2;
      deltaX = 0;
    }
    else if (keyCode == 32) {            //space
      deltaX = 0;
      deltaY = 0;
    }
}