symbolic summation
in [Matlab]
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From: rams on 17 Jun 2010 12:08 I am trying to find root mean square deviation using following code... RMSD=(symsum((Rmea-Rmod).^2, 1, 664)/664).^0.5; when i run this..i am getting following error... ??? Undefined function or method 'symsum' for input arguments of type 'double'. may its because i don't have symbolic math toolbox...if its the case then how to perform symbolic summation without using symsum function....thanks in advance!
From: Walter Roberson on 17 Jun 2010 16:44 rams wrote: > I am trying to find root mean square deviation using following code... > > RMSD=(symsum((Rmea-Rmod).^2, 1, 664)/664).^0.5; > > when i run this..i am getting following error... > > ??? Undefined function or method 'symsum' for input arguments of type 'double'. > > may its because i don't have symbolic math toolbox...if its the case then how to perform symbolic summation without using symsum function....thanks in advance! The 1 to 664 -- do you know which variable symsum would substitute that for? Hint: symsum does not work on the basis of "whichever variable gets mentioned first". If you do not have the symbolic toolbox then you are going to need to find another way of calculating this, such as using some basic analysis: (Rmea-Rmod).^2 = Rmea.^2 - 2 .* Rmea .* Rmod + Rmod.^2 If you were to take the sum of that over a range of Rmea (arbitrarily chosen) then that is going to be logically be (sum Rmea.^2 for Rmea over the range) - (sum 2 .* Rmea .* Rmod for Rmea over the range) + (Rmod .^2 for Rmea over the range) The last term is going to be Rmod.^2 * (last Rmea - first Rmea + 1) which would be 664 * Rmod.^2 . You can likewise analyze the first two terms and find simpler ways of writing them and then add the three parts together to arrive at the final symbolic sum.
From: rams on 17 Jun 2010 14:00 Hi Walter... THanks for great hint. There are 664 elements in Rmea and Rmod so i want to calculate for all the elements.
From: Walter Roberson on 17 Jun 2010 18:12 rams wrote: > There are 664 elements in Rmea and Rmod so i want to calculate for all the elements. Hmmm, are _any_ of the values in Rmea or Rmod symbolic? If not then you do not want the symbolic sum, you just want to total the values. If Rmea and Rmod are vectors, then your symsum call was not correct, because symsum substitutes the given limits in for the default variable rather than _indexing_ the variable as would be necessary for vectors. Is what you want the sum of (Rmea(K) - Rmod(K)).^2 for K=1 to 664 and both vectors numeric? If so, then that is just sum((Rmea-Rmod).^2) with no symsum() call. Is what you want the sum of (Rmea(K) - Rmod(P)).^2 for K = 1 to 664 and P from 1 to 664 (that is, all pairwise combinations)? If so sum the lower (or upper) triangle, multiply by 2 and add the sum over the diagonal. You might, though, perhaps only be looking for the triangular sum. Please be more specific about what you are trying to add and what the input data types are.
From: rams on 18 Jun 2010 10:11
Hi Walter, Yes, i want to find sum of (Rmea(K) - Rmod(K)).^2 for K=1 to 664 and both vectors numeric. This solved my problem. Thanks again! |