looping through symmetric matrices

Question

MiauMiau 2014-6-18

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https://ww2.mathworks.cn/matlabcentral/answers/135120-looping-through-symmetric-matrices

编辑： Andrei Bobrov 2014-6-19

Hi

I have two matrices T and Q, and I want to plot the corresponding elements of them such as in:

T = randi(10,10)
Q = randi(10,10)
hold on
for i = 1:10
    for j = 1:10
plot(T(i,j),Q(i,j))
    end
end

But the thing is that the matrices are symmetric, and for reasons of complexity I do not want to go through redundant elements twice..what can I do? Thanks

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Answer 1

J. van Delft 2014-6-18

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Hi,

First of all the matrices are not symmetric. If they would be multiplying T with its transpose T' would give an identity matrix, which is not the case.

If I understand your question correctly you do not want to plot elements in T and Q that are equal? What you could do then is add a comparison in the inner for-loop. If the elements are equal, break the loop:

T = randi(10,10)
Q = randi(10,10)
hold on
for i = 1:10
    for j = 1:10
        if T(i,j) == Q(i,j)
            break
        end
        plot(T(i,j),Q(i,j))
    end
end

Let me know if this answers your question.

J. van Delft

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MiauMiau 2014-6-19

well this was example code. the matrices in my code are symmetric.

MiauMiau 2014-6-19

No, so I want to plot all the Elements of T and Q which have the same index but if the pairs happen to have values as already encountered I do not want them to be considered.. so for [1 4; 4 5] and [2 3; 3 6] the pairs to be plottet would be (1/2), (4/3),(4/3),(5/6) but the repeated (4/3) should not be considered...

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Answer 2

Andrei Bobrov 2014-6-19

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https://ww2.mathworks.cn/matlabcentral/answers/135120-looping-through-symmetric-matrices#answer_141640

编辑：Andrei Bobrov 2014-6-19

在 MATLAB Online 中打开

T = randi(20,10);           %
Q = randi(20,10);           % 
T = tril(T) + tril(T,-1)';  %
Q = triu(Q) + triu(Q,1)';   % here we creating  symmetric matrices
tt = tril(ones(size(T))) > 0;
plot(T(tt),Q(tt));

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Answer 3

Joseph Cheng 2014-6-18

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https://ww2.mathworks.cn/matlabcentral/answers/135120-looping-through-symmetric-matrices#answer_141518

编辑：Joseph Cheng 2014-6-18

在 MATLAB Online 中打开

What you can do is use the unique() function

example:

T=randi(10,10);
Q=randi(10,10);
temp = [T(:) Q(:)];
utemp = unique(temp,'rows','legacy');
figure,plot(utemp(:,1),utemp(:,2),'.');

the unique will get rid of any repeating pairs leaving only unique elements.

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MiauMiau 2014-6-19

thats not doing what i want at all..I always want to plot the elements with the same index, and if these pairs are repeated they should not be plotted..

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