Since the equation for a circle on radius r centered at (0,0) is
x^2 + y^2 = r^2,
an obvious choice is to plot
y = +- SQRT (r^2 - x^2)
as -r <= x <= r, This works, but is inefficient because of the multiplications and square root operations. It also creates large gaps in the circle for values of x close to R.
A better approach, which is still inefficient but avoids the gaps is to plot
x = r cos 0
y = r sin 0
as 0 takes on values between 0 and 360 degrees.