# assemble global stiffness matrix

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abdelrahman alhammadi on 12 Oct 2018
Commented: Torsten on 12 Oct 2018
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 ?

John D'Errico on 12 Oct 2018
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.

abdelrahman alhammadi on 12 Oct 2018
but why n+[0 1] ?
abdelrahman alhammadi on 12 Oct 2018
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 on 12 Oct 2018
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

Torsten on 12 Oct 2018
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

abdelrahman alhammadi on 12 Oct 2018
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 ?

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John D'Errico on 12 Oct 2018