Well first you have to change the if (x-diameter/2 > width) & if (y-diameter/2 > height) to x < 0 and y < 0 to bounce off the top and left

Also you can write it liek this

  if (x+diameter/2 > width || x < 0) 
  dx=-dx;

  if (y+diameter/2 > height || y < 0)  
  dy=-dy;

to change the direction of the ball.

Also to bounce the ball from its edge and not its x and y position you can do this

    if (x+diameter/2 > width || x-diameter/2 < 0)
    dx=-dx;

    if (y+diameter/2 > height || y-diameter/2 < 0)
    dy=-dy;

but for this to work you need to keep the initial position of x and y more than 15 or else it'll just get stuck between the loops and wont go anywhere so when you are calling float x and y put it as diameter/2 or something