# assemble matrices for any matrix size with error of matrix dimension must agree

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Mark Sc on 30 Jan 2021
Commented: Walter Roberson on 31 Jan 2021
Hi all,
I am trying to assmeble matrices, I already wrote a code for which I could do, but I would like to do edit/develop commands so it work for any matrix size.
Now my matrix is 2*2 if i change it to 4*4, or 5*5 and so on.. I get matrix dimension must agree ?
Looking for any help!
clearvars
clc;
NE=2;
NN=NE+1;
NDOF=NN*2
ke=[1 -1;-1 1]; % IF I change this matrix to be 4*4 or 5*5 and so on... will not work to assemble
ke=[1 -1 3;-1 1 5];
K=zeros(NDOF);
for i=1:NE
K(i:i+1,i:i+1)=K(i:i+1,i:i+1)+ke
end
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### Accepted Answer

Walter Roberson on 31 Jan 2021
Edited: Walter Roberson on 31 Jan 2021
Why would it assemble? i:i+1 is a vector of length 2. You use it for both indices of K, so K(i:i+1,i:i+1) is going to talk about a 2 x 2 array. You take that 2 x 2 array extracted from K and you add all of ke to it. If ke is not a compatible size to the 2 x 2 array you extracted, then the addition is going to fail.
The compatible sizes of array that can be added to a 2 x 2 array are:
• 1 x 2 x anything
• 2 x 1 x anything
• 2 x 2 x anything
If you want to add all of ke to something extracted from K, then you need the extracted size to be compatible with the size of ke. For example,
NE=2;
NN=NE+1;
NDOF=NN*2
NDOF = 6
ke=[1 -1 3;-1 1 5];
[r, c, ~] = size(ke);
K=zeros(NDOF);
for i=1:NE
K(i:i+r-1,i:i+c-1)=K(i:i+r-1,i:i+c-1)+ke;
end
K
K = 6×6
1 -1 3 0 0 0 -1 2 4 3 0 0 0 -1 1 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
##### 2 CommentsShowHide 1 older comment
Walter Roberson on 31 Jan 2021
True, 1 x 1 x anything would work.

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