To do this like you're attempting requires the use of the eval function, but be very careful in its use. Google 'eval matlab' for more information on why. And actually, the way you've got it set up wouldn't work. Your for loop needs to run from 1:29 because there's no L31.
for i = 1:29
eval(['P(' num2str(i) ')=L' num2str(i) '-L' num2str(i+1) ';'])
end
Is there any reason why your L variables aren't in a single L vector?
