Try this:
numPoints = 1000 % Total number of points to save.
numLow = 1;
numHigh = 1;
% Start collecting elements using acceptance if in range,
% and rejection if out of range.
for k = 1 : 10000000 % enough to make sure we will collect at least numPoints elements.
r = rand();
% Save the number if it's in the low range.
if r <= 0.2 && numLow <= 0.20 * numPoints
% In range. Store/save the number.
lowKeepers(numLow) = r;
numLow = numLow + 1;
else
% Not in range so reject/ignore it.
end
% Save the number if it's in the high range.
if r >= 0.8 && numHigh <= 0.80 * numPoints
% In range. Store/save the number.
highKeepers(numHigh) = r;
numHigh = numHigh + 1;
else
% Not in range so reject/ignore it.
end
end
