calculate sum over n

Hello! I try to calculate a sum over n of matrices K,L such as K(m,n)*L(n,t)*L(n,j). So far I have the code below, but I am doubing I wrote it correctly. Could you please give advise on that?

K=rand(20,20);
Ln=rand(20,20);
sum11=zeros(1,20);
for n=1:20
    summ=K(:,n).*Ln(n,:).*Ln(n,:);
    sum11(n)=sum11(n)+summ;
end

[KSSV]

Is the given code working first?

[Walter Roberson]

You should not have the period between K and the (

nn is not defined.

[Ben Andersson]

sorry. The result must be an array, and I get a matrix.

K=rand(20,20);
Ln=rand(20,20);
sum11=zeros(1,20);
for n=1:20
    summ=K(:,n).*Ln(n,:).*Ln(n,:);
    sum11(n)=sum11(n)+summ;
end

[Walter Roberson]

K(:,n).*Ln(n,:).*Ln(n,:)

is

K(:,n).*Ln(n,:).^2

Are you sure that is what you want? K(:,n) would be 20 x 1, and Ln(n,:).^2 would be 1 x 20, and 20 x 1 .* 1 x 20 is going to give you a 20 x 20 result.

Answer 1

[Image Analyst]

Why not just try:

K =  rand(20, 20);
Ln = rand(20, 20);
n = 5; % Whatever.
% Compute the matrix product of 
% a 5 matrix by a 5 matrix
% to get a 20x20 matrix.
LSquared = Ln .^ 2;
theProduct = K(:, 1:n) * LSquared(1:n, :)
% Now sum up.
theSum = sum(theProduct(:))

Unless you have specific numbers for t and j, then it would be different.

[Ben Andersson]

thanks, but it gives me one number and not a vector. I confused array with the vector above.

[Ben Andersson]

then something like this should be, right?

sum(theProduct(1:n,:))

[Image Analyst]

Why would it be a vector? For a given, specified t, j, and m, you're multiplying single elements together to get a single number, and summing them up. This is what I get using your new formula you edited and placed in your original post:

% Declare constants for t, j, and m
m = 10;
t = 2;
j = 3;
% Define two 20-by-20 matrices.
K =  rand(20, 20);
L = rand(20, 20);
% Make sure n is not greater than the number of columns in K
% or the number of rows in L
maxIndex = min([size(K, 2), size(L, 1)])
theSum = 0;
for n = 1 : maxIndex
	LntLnj = L(n, t) .* L(n, j);
	thisKelement = K(m, n);
	theProduct = thisKelement * LntLnj;
	theSum = theSum + theProduct;
end
theSum % Show result in command window.

[Ben Andersson]

Thanks!