If you have the Signal Processing Toolbox, you can use downsample() to simply downsample without lowpass filtering. Or use decimate() or resample() to first lowpass filter and then downsample.
n = 0:159;
x = cos(pi/4*n);
% downsample by 2 starting with the first value
y = downsample(x,2,0);
% downsample by 2 starting with the second value
y1 = downsample(x,2,1);
You can of course do the above with simple MATLAB commands:
y = x(1:2:end);
y1 = x(2:2:end);
In general, do consider either decimate()or resample() as these afford anti-aliasing through filtering.
