How do I return rows of a matrix based on the value of the maximum element.

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Fil Okua 2021-4-24

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评论： Fil Okua 2021-4-25

Can someone help me? I am using MATLAB R2011b and I am having problems returning rows based on the maximum value in (:, 2:end). I have two .txt files with data in the format:

A =

1901 1.0 0.9 1.4 3.0 3.9 8.3 9.5 9.4 8.8 6.6 4.1 1.7

1902 0.3 2.4 1.5 3.7 5.1 8.3 10.4 10.3 7.6 5.9 2.7 0.7

1903 0.6 0.1 0.7 2.6 4.4 7.6 9.3 10.1 7.9 4.7 -0.7 1.2

B =

1901 5.9 7.0 8.2 10.5 14.3 17.1 17.6 18.6 16.5 13.2 10.2 6.6

1902 5.4 8.6 8.2 13.5 13.8 17.9 18.8 19.3 16.8 12.9 8.9 6.1

1903 5.3 6.1 7.7 11.0 14.3 18.0 17.5 18.1 16.6 11.9 5.8 6.5

If I use:

D = [A(:,1), (B(:,2:13)-A(:,2:13))] and get:

1901 4.9 6.1 6.8 7.5 10.4 8.8 8.1 9.2 7.7 6.6 6.1 4.9

1902 5.1 6.2 6.7 9.8 8.7 9.6 8.4 9.0 9.2 7.0 6.2 5.4

1903 4.7 6.0 7.0 8.4 9.9 10.4 8.2 8.0 8.7 7.2 6.5 5.3

I expect to get:

1903 4.9 6.1 6.8 7.5 10.4 8.8 8.1 9.2 7.7 6.6 6.1 4.9

1905 4.7 6.0 7.0 8.4 9.9 10.4 8.2 8.0 8.7 7.2 6.5 5.3

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DGM 2021-4-24

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I still don't understand what you're aiming for. Why is the difference of the 1901 and 1903 datasets now associated with 1903 and 1905 instead? When you say "return arrays of rows with the maximum value found in the right columns" are you talking about the right column of A or B, or the right column of the difference? I'm assuming you mean A or B, since the selected rows don't correspond to the maximal values in the last column of the difference.

If you're looking for the row with the maximal value in the rightmost position, why are there 2 rows instead of one? Are you looking for the N most maximal values? Is N strictly 2 as in this example?

In any case, I doubt that using a loop would be necessary.

Let's say I assume you meant to find the difference of the rows of the arrays corresponding to the two largest values in A(:,end)

A = [1901 1.0 0.9 1.4 3.0 3.9 8.3 9.5 9.4 8.8 6.6 4.1 1.7;
	1902 0.3 2.4 1.5 3.7 5.1 8.3 10.4 10.3 7.6 5.9 2.7 0.7;
	1903 0.6 0.1 0.7 2.6 4.4 7.6 9.3 10.1 7.9 4.7 -0.7 1.2];
B = [1901 5.9 7.0 8.2 10.5 14.3 17.1 17.6 18.6 16.5 13.2 10.2 6.6;
	1902 5.4 8.6 8.2 13.5 13.8 17.9 18.8 19.3 16.8 12.9 8.9 6.1;
	1903 5.3 6.1 7.7 11.0 14.3 18.0 17.5 18.1 16.6 11.9 5.8 6.5];
N = 2; % how many maximal values
[~,idx] = sort(A(:,end),'descend'); % finding maximal values in the end column of A
ddiff = B(idx(1:N),2:end)-A(idx(1:N),2:end); % only taking the difference of those rows
C = [A(idx(1:N),1) ddiff]

Excepting the first column, that would give the result you described.

DGM 2021-4-25

It returns both rows associated with 10.4, which is the global maximum of the data difference.

"I can see 2 values in D which are the same and they are the maximums."

Which is what you were saying.

Fil Okua 2021-4-25

@DGM thanks. I checked again and again. For some reason MATLAB has rounded up some field which was resulting in my confusion. All now good. Thanks 👍

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