In Matlab, when I plan to extract a submatrix from a large matrix in the following situation:
Say I want to extract row and column 1 and 4 from K, I can create an index matrix and do this:
idx = [4 1; 1 2; 2 3; 3 5];
k = K(idx(1,:),idx(1,:));
But in Mathcad, the built in function submatrix can only extract continuous row and column ranges.
Is there a way to let Mathcad do the same job as in Matlab?
Best
Shawn
ACCEPTED SOLUTION
Accepted Solutions
So far I guess the following should help. You can extract the appropriate row of your idx matrix using Primes row selector.
Say I want to extract row and column 1 and 4 from K,
You are not just looking for the extraction of a single matrix element, or are you?
Or do you really mean what you write - extracting the full first and fourth column and also the first and fourth row. This would mean that in your example you want to extract 16 values and four of them would be extracted twice. If that is what you are after, you would have to tell us in which way you would like the 16 (or 12) value being arranged - which data structure.
In case you are looking for a way to extract a couple of matrix elements whose indices are given by a matrix "idx" and collect those elements in a vector, the following two methods should help. Otherwise you would have to come back and specify more detailed what kind of result you are expecting.
BTW, the value 5 in your idx matrix seems to be wrong anyway, even if you set ORIGIN to 1.
Hi, Werner,
Sorry I truncate the original problem that results in some confusion.
The original one is this:
Thus there are 10 DOFs.
With Matlab, I can do this to assemble the matrix:
k = @(L,EI) [12 -12 6*L 6*L;
-12 12 -6*L -6*L;
6*L -6*L 4*L^2 2*L^2;
6*L -6*L 2*L^2 4*L^2]*EI/L^3; % Beam Local Stiffness Matrix
k1 = k(1/4,1); % beam local stiffness matrice
% Assume EI=1, L=1/4;
K = zeros(10,10); % In total there are 10 DOFs
idxc = [4 1 9 6;
1 2 6 7;
2 3 7 8;
3 5 8 10];
% Index for beam elemental DOFs
for i = 1:4 % Assemble global beam stiffness matrix
K(idxc(i,:),idxc(i,:)) = K(idxc(i,:),idxc(i,:)) + k1;
end
Ktt = K(1:5,1:5) - (K(6:10,1:5))'*(K(6:10,6:10))^-1*K(6:10,1:5);
%Stiffness Condensation
Ktt = Ktt(1:3,1:3); % Subtract Boundary Conditions & Rotational DOFs
Here idxc stores all the 4 DOFs for each element, with the DOFs not continuous labeled.
In Mathcad, however, the built in function submatrix cannot work in this way.
Hope this clarify the problem.
Best
Shawn
So far I guess the following should help. You can extract the appropriate row of your idx matrix using Primes row selector.
One additional remark:
If you really want to ADD the small matrix to the values of the larger one instead of just inserting it, you have to change the program:
