Although find is very attractive for beginners, it is actually faster and neater to use logical indexing, so the equivalent to your code above would be:
ind_10 = data(:,2) >= 10;
dens_10 = data(ind_10,3);
This also allows us to easily define not only which elements we select from an array, but also those that we allocate into an array:
ind_10 = data(:,2) >= 10;
dens_10(ind_10) = data(ind_10,3);
You will probably need to consider what values to put in the non-allocated positions, something like this:
dens_10(~ind_10) = NaN; % or 0?
