What is wrong with my for loop?

%check for stitching global nxn matrix 
elements = 4;
nodes = 5; %nodes
s=3;        %size of matrix nxn
k=[1 1 1; 1 1 1; 1 1 1];

size = (s* elements) - (elements-1);
K = zeros(size,size);
K(1:s,1:s) = k(1:s,1:s);
%this fills the matrix with values
for i =2:elements-1
    % K((s:((i*s)-(i-1))), (s:((i*s)-(i-1)))) = k(1:s,1:s);
    for j = 1:elements-1
    K((j*s-j-1):(i*s-i-1), (j*s-j-1):(i*s-i-1)) = k(1:s,1:s);
    end 
%      K((s:((2*s)-(2-1))), (s:((2*s)-(2-1)))) = k(1:s,1:s);
%      K((s:((3*s)-(3-1))), (s:((3*s)-(3-1)))) = k(1:s,1:s);
    %K(s:((j*s)-(j-1)),s:((j*s)-(j-1))) = k(1:s,1:s);
%     K(s:5,s:5) = k(1:s,1:s);
%     K(5:7,5:7) = k(1:s,1:s);
%     K(7:9,7:9) = k(1:s,1:s);
end
%this adds matrix elements at stitching location
for i = 1:elements-1
   K(((i*s)-(i-1)), ((i*s)-(i-1))) = k(1,1)+k(s,s);
end

K

the lines including K(s:5,s:5) to K(7:9,7:9) was my verification to see if the matrix was right and it is. the code above in the for loops should give me the same answers as these but I keep getting an error saying unable to perform assignment because the size of the left matrix doesnt match the right

this is the error I recieve:

Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is

3-by-3.

[Kyle McLaughlin]

using those verification lines of code and deleting the first for loop gives me what I want and looks like this:

K =

1 1 1 0 0 0 0 0 0

1 1 1 0 0 0 0 0 0

1 1 2 1 1 0 0 0 0

0 0 1 1 1 0 0 0 0

0 0 1 1 2 1 1 0 0

0 0 0 0 1 1 1 0 0

0 0 0 0 1 1 2 1 1

0 0 0 0 0 0 1 1 1

0 0 0 0 0 0 1 1 1

[madhan ravi]

Why did you delete your question?

[Kyle McLaughlin]

I deleted it to post it correctly so there would be no further confusion

[Kyle McLaughlin]

I was able to come up with the following code which is closer to what I want but doesnt quite fit:

elements = 4;

nodes = 5; %nodes

s=3; %size of matrix nxn

k=[1 1 1; 1 1 1; 1 1 1];

size = (s* elements) - (elements-1);

K = zeros(size,size);

%K(1:s,1:s) = k(1:s,1:s);

for j = 2:elements

for ii = 0: elements -2

%K(j*s-s+ii*(s-1):j*s-1+ii*(s-1), j*s-s+ii*(s-1):j*s-1+ii*(s-1)) = k(1:s,1:s);

%K(j*s-s-1:j*s-1, j*s-s-1:j*s-1) = k(1:s,1:s);

K(j*s-s-ii:j*s-1-ii,j*s-s-ii:j*s-1-ii)= k(1:s,1:s);

end

end

%K(size-s+1:size,size-s+1:size) = k(1:s,1:s);

for i = 1:elements-1

K(((i*s)-(i-1)), ((i*s)-(i-1))) = k(1,1)+k(s,s);

end

K

K =

1 1 1 0 0 0 0 0 0 0 0

1 1 1 1 0 0 0 0 0 0 0

1 1 2 1 1 0 0 0 0 0 0

0 1 1 1 1 1 0 0 0 0 0

0 0 1 1 2 1 1 0 0 0 0

0 0 0 1 1 1 1 1 0 0 0

0 0 0 0 1 1 2 1 1 0 0

0 0 0 0 0 1 1 1 1 1 0

0 0 0 0 0 0 1 1 1 1 1

0 0 0 0 0 0 0 1 1 1 1

0 0 0 0 0 0 0 0 1 1 1

Answer 1

[Kyle McLaughlin]

elements = 4;

s=3; %size of matrix nxn

k=[1 1 1; 1 1 1; 1 1 1];

length = (s* elements) - (elements-1);

K = zeros(length,length);

for j = 0:elements-1

index =(1:s);

n = index + j*(s-1);

K(n,n) = k(1:s,1:s);

end

for i = 1:elements-1

K(((i*s)-(i-1)), ((i*s)-(i-1))) = k(1,1)+k(s,s);

end

K

This does exactly what I want I answered my own question nevermind. Here it is for reference.

[madhan ravi]

Please use the formatting tools. Last time when I edited your code you deleted your question, so edit it yourself appropriately.

[Kyle McLaughlin]

Excuse me, clearly i didnt see any answer you posted which is why it was deleted. Sorry you didnt answer my question and get an accepted answer. I am aware of how it works now, dont beat a dead horse.

[madhan ravi]

Lol, good luck

Answer 2

[Matt J]

It seems clear that when i=j, the left hand side of

K((j*s-j-1):(i*s-i-1), (j*s-j-1):(i*s-i-1)) = k(1:s,1:s);

will be 1x1 whereas the right hand side will always be 3x3. Generally speaking, the size of the left hand side is changing in a highly i,j-dependent way whereas the right hand side is not.

[Kyle McLaughlin]

I am trying to avoid them being equal with the nested for loop in the larger one, additionally I begin the larger for loop with i =2 and begin the nested loop with j =1, so shouldnt they never be the same? how do I make it such that they are not the same value at the same time if this is not the case

[Matt J]

I'm not sure what you are trying to achieve. If the idea is just to make tiled copies of k, then youcould just use repmat

K=repmat(k,elements,elements)

[Kyle McLaughlin]

This is the begining of an FEA script, the code that I am writing here assembles the global stiffness matrix from the element matricies. I need to apply this to the mass matrix and damping matrix for nxn size so that it is modular in the event of different sized element matrixies for given element types. The goal is to add the first and lass matrix values which creates the nodal connection between the two elements and apply that to the lot of them. The first comment I made in the post, directly under my question, shows the desired results of the matrix.