More sophisticated code than a for loop with i is large

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Bran
Bran 2013-7-10
Hi there I am trying to calculate a scatter matrix for some data. so basically my Eigen values are in a matrix of the form; [e11 e12 e13; e21 e22 e23; e31 e32 e33; ...... e101 e102e103]
and my scatter matrix is of the form;
[e11^2 + e21^2 + e31^2 + ... e101^2, e11e12 + e21e22 + ... +e101e102, e11e13 + ... e101e103; e11e12 + e21e22 + ... +e101e102, e12+e22+e32+..e102, e12e13+e22e23+...+e102e103; e11e13 + ... e101e103, e12e13+e22e23+...+e102e103; e13^2+e23^2+...+e103^2]
Now in some cases instead of n = 10 as is the case above n =in the thousadns. is there a neat way to code for this?
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Jan
Jan 2013-7-10
How is "n=10" related to the posted code. I think, names like e101e103 are such ugly, that it is near to impossible to read the code.

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Andrei Bobrov
Andrei Bobrov 2013-7-10
编辑：Andrei Bobrov 2013-7-10
Let e = [e11 e12 e13; e21 e22 e23; e31 e32 e33; ...... e101 e102e103];
out = e.'*e;
out(2,2) = sum(e(:,2));

更多回答（1 个）

Hugo
Hugo 2013-7-10
Please notice that in the "scatter matrix" that you showed as an example there are elements that are repeated, and the structure is not exactly a matrix.
So, it seems that the matrix of eigenvalues, let's say A, is of size n x 3. Is the scatter matrix of this data what you want? You can simply do
A'*A, and it will return a 3x3 matrix where each element Aij has the values:
Aij= sum_n ( eni * enj ), which partially coincides with your example.

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