Solution

Replace multiplication and division with addition and subtraction.

1 ylen = (yend - ystart); 2 b = (xend - xstart); 3 y = ystart; 4 for (x = xstart; x < xend; x++) { 5 t += ylen; 6 if (t > b) { 7 t -= b; 8 y++; 9 } 10 plot (x, y); 11 }

This is called Bresenham's Algorithm
Think of it as pretending to do division.
No, you couldn't just precompute ylen / xlen and add it each step, because this would require floating point, which is slow.