Hi,
The average voltage over a full cycle is 0, but the average of the absolute value is 0.6366 approximately or 2/pi exactly and this often applies to the full non rectified wave too.
The average voltage over a full cycle of a half wave rectified signal is 1/pi exactly which is approximately 0.3183 .
The RMS voltage over a full wave is 1/sqrt(2) which is approximately 0.7071 .
The RMS voltage over a full cycle of a half wave rectified wave is 1/(2*sqrt(2)) which is approximately 0.35355 .