assemble global stiffness matrix

I am trying to assemble a matrix in which I would like to put small matrices into a big one but I don't know how to do that,

I know that I have to define the big matrix first and then show put the small ones inside

for example I have k1=[1 -1; -1 1] , K2=[1 -1; -1 1] and the define bigK in which bigK=zeros(3,3) and in the end I want it to be like bigK = [ 1 -1 0; -1 2 -1; 0 -1 1]

in which the small matrices will assemble in diagonal way

how I can do it using a for loop ?

Answer 1

[John D'Errico]

K = zeros(10,10);
k = [1 -1; -1 1];
for n = 1:9;
  rind = n + [0 1];
  cind = rind;
  K(rind, cind) = K(rind, cind) + k;
end

There are better ways to do this in terms of pure efficiency. But the above will suffice. First learn how to write at least decent code that solves the task at hand. Only when you decide if the code is a problem do you want to worry about optimizing it for speed.

[John D'Errico]

Please don't add answers just to make a comment. Moved to a comment by the op:

"rind = n + [0 1]; cind = rind; I did not understand why you did this"

[John D'Errico]

Why I did it? rind is the name I chose for RowIndex. cind stood for ColumnIndex.

So think about what the code does as I wrote it, and where in the matrix it puts k.

[abdelrahman alhammadi]

but why n+[0 1] ?

[abdelrahman alhammadi]

is there another way of doing it in a for loop in which I only define the small k function and size of big k and then it will do it by itself ?

[Torsten]

k=[1 -1; -1 1];
n=5;
K=zeros(n+1);
for i=1:n
  K(i:i+1,i:i+1)=K(i:i+1,i:i+1)+k;
end
K

Answer 2

[Torsten]

k1 = [1 -1; -1 1];
k2 = k1;
K = zeros(3,3)
K(1:2,1:2) = k1;
K(2:3,2:3) = K(2:3,2:3) + k2;
K

Answer 3

[abdelrahman alhammadi]

what if I have vector a1= [1; -1] and a2= [ 1 ;-1 ] and so on ... the big matrix F= [ 1; 0; 1] if the size was(2) how can I do that in a for loop ?

[John D'Errico]

Please stop adding multiple answers for every single comment!

[Beck Rinks]

Please only reply if you are giving answers. Thanks.

Answer 4

[Samba Dumbuya]

Hello,

I have computed the following stiffness matrix by hand;

[2/3 -1/6 -1/6 -1/3; -1/6 2/3 -1/3 -1/6;-1/6 -1/3 2/3 -1/6; -1/3 -1/6 -1/6 2/3]

Can someone please with codes that assemble the stiffness matrix into global matrix. The domain is discritize to square elemen.

thanks

[Samba Dumbuya]

*help me