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How can I find the location of the minimum and maximum

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Tony Montgomery
Tony Montgomery on 31 Oct 2014
Commented: Matt Tearle on 31 Oct 2014
I am to test some ill condition matrices, so I want create a loop that creates 5 random nxn matrices. Then I want to be able to find the matrix with the minimum condition number, and the one with the maximum condition number. I have the following code:
for i=1:5
Ai = rand(n,n);
C(i) = cond(Ai);
end
max(C)
min(C)
So I was able to find the min and max values, but I want to know which matrix they are associated with. If i knew where they occurred I can figure it out, but how can I find WHERE the min and max occured??

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Answers (3)

SK
SK on 31 Oct 2014
Edited: SK on 31 Oct 2014
[maxvalue, index_of_max] = max(C);
[minvalue, index_of_min] = min(C);

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Orion
Orion on 31 Oct 2014
Edited: Orion on 31 Oct 2014
Hi,
just use the second output argument of the min and max functions.
n=10;
for i=1:5
Ai = rand(n,n);
C(i) = cond(Ai);
end
[MaxVal,MaxIndex] = max(C);
[MinVal,MinIndex] = min(C);
% check
disp([C(MaxIndex),MaxVal])
disp([C(MinIndex),MinVal])

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Matt Tearle
Matt Tearle on 31 Oct 2014
Others have explained how to get the index of the max or min, but I'm wondering if you actually want to know what the matrix is that it's associated with. Currently you overwrite your matrix each time through the loop, so there's no way to retrieve it. So
Ai = rand(n,n,5);
C = zeros(5,1);
for i = 1:5
C(i) = cond(Ai(:,:,i));
end
[idx,maxC] = max(C);
maxA = Ai(:,:,idx);

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Tony Montgomery
Tony Montgomery on 31 Oct 2014
I wold like to retrieve it. How can I prevent it from overwriting?
Matt Tearle
Matt Tearle on 31 Oct 2014
That's what my code does -- it keeps all the matrices in a 3-D array (n-by-n-by-5). Then once you have idx -- the index of the maximum -- you can index into Ai to get the idx element: maxA = Ai(:,:,idx);

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