For loop trouble!

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skagawa23
skagawa23 on 22 Sep 2016
Edited: Christoph F. on 22 Sep 2016
What I am trying to do is, assume the first vector to be [1; 1; 1] and run the loop until difference in u is less than 0.0001. Before going into the loop, for some reason, I cannot set the first vector by saying u(1) = [1; 1; 1]
Here is what I have:
w = 100;
gravity = 32.2*12;
m = w/gravity;
k = 326.32;
K = [2 -1 0; -1 2 -1; 0 -1 1];
K = k*K
Kinv = inv(K)
M = [1 0 0; 0 1 0; 0 0 0.5];
M = m*M
u(1) = [1; 1; 1];
i = 1;
for i = 1:10;
Mstar = u(i) * M * u(i)'
Kstar = u(i) * K * u(i)'
L(i) = Kstar(i) / Mstar(i)
RHS = L(i) * M * u(i)'
u(i+1) = Kinv * RHS(i)
Diff = u(i+1) - u(i);
if Diff > 0.0001;
u(i+1) = u(i+1)
else
break
end
end

Answers (3)

Walter Roberson
Walter Roberson on 22 Sep 2016
u(1) is a specific numeric array locations. Numeric array locations can only hold a single numeric value; you are trying to store three numeric values in that one location.
You probably just want
u = [1; 1; 1];

Christoph F.
Christoph F. on 22 Sep 2016
Edited: Christoph F. on 22 Sep 2016
I assume you want to keep the intermediate values of u?
In that case, use u(:, 1) = [1; 1; 1] (or u = [1; 1; 1]). Then you can access the i-th version of u with u(:, i).
However, you may want to initialize u as a matrix, to avoid time-consuming resizing of u in the loop. This can be done by initializing u with u = zeros(3, 11); u(:, 1) = [1; 1; 1];

Andrei Bobrov
Andrei Bobrov on 22 Sep 2016
Edited: Andrei Bobrov on 22 Sep 2016
w = 100;
gravity = 32.2*12;
m = w/gravity;
k = 326.32;
K = [2 -1 0; -1 2 -1; 0 -1 1];
K = k*K;
M = [1 0 0; 0 1 0; 0 0 0.5];
M = m*M;
u = [1; 1; 1];
for ii = 1:10;
Mstar = u(:,ii)' * M * u(:,ii);
Kstar = u(:,ii)' * K * u(:,ii);
L = Kstar / Mstar;
RHS(:,ii) = L * M * u(:,ii);
u(:,ii+1) = K\RHS(:,ii);
Diff1 = u(:,ii+1) - u(:,ii);
if norm(Diff1) < 0.0001
break
end
end

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