If you have a list of desired x points and they are not integral then
ysubset = interp1(x, y, xsubset, 'nearest');
If you have R2015a or newer you could use
[tf, idx] = ismembertol(xsubset, x);
xfound = xsubset(tf);
ysubset = ysubset(idx(tf));
The interp1 and ismembertol techniques can also be used if your target x are integers. However, if your criteria is that you want to extract all of the values that correspond to integer x and leave out the others then,
tf = x == floor(x);
xsubset = x(tf);
ysubset = y(tf);
