How can I find the max of two vectors?

Question

Adrienne 2014-11-27

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评论： Guillaume 2014-11-28

a=[4.84 3.55 2.09 4.20 1.14 5.15 2.45 3.41 1.66 2.75 3.87]
b=[ 7    8    8    7     7   9    8    6    5    7    8]

I want to find the max such that the max(a) is in the same column as max(b). I want to find the top 3. First should be column 6 Second should be column 11 Third should be column 1

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Answer 1

C.J. Harris 2014-11-27

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Do you actually want to sort by sum of columns?

For example:

a = [4.84 3.55 2.09 4.20 1.14 5.15 2.45 3.41 1.66 2.75 3.87];
b = [ 7    8    8    7     7   9    8    6    5    7    8];
[~,idx] = sort(a+b,'descend');
nTopThree = [a(idx(1:3));b(idx(1:3))];
nTopThree =
    5.1500    3.8700    4.8400
    9.0000    8.0000    7.0000

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C.J. Harris 2014-11-28

This answer is not completely wrong as you suggest. It does exactly what I said it does; it sorts by the total sum of the columns, as your example proves.

I think maybe you just misunderstood the question Guillaume.

Guillaume 2014-11-28

I'm fairly certain that Adrienne was looking for something completely different (like a multikey sort or something else). As I said, in that case, the answer is completely wrong.

Yes, I may have misunderstood the question, and if Adrienne wanted a sort based on column sums your solution is indeed right, but as I also said, the question was certainly asked in a convoluted way.

In the end, we will never know until Adrienne comes back and tell us.

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Answer 2

the cyclist 2014-11-27

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I don't fully understand what you want to do. But I think you can do what you need with the sort and max commands. Note in particular that second output of the sort command that gives the sorting index.

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Answer 3

Guillaume 2014-11-27

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编辑：Guillaume 2014-11-27

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Sounds like you want to perform a sort on multiple keys. sortrows is the function for that. As it operates on row rather than columns, you have to transpose your data:

a = [4.84 3.55 2.09 4.20 1.14 5.15 2.45 3.41 1.66 2.75 3.87];
b = [ 7 8 8 7 7 9 8 6 5 7 8];
ab = [a; b]'; 
[c, col] = sortrows(ab, [2 1]); %sort on b first, then on a
%to get order by max, flip the results up/down
c = flipud(c);
col = flipud(col);
%to get the the first column for each unique value of b:
colmax = col(diff([NaN; c(:, 2)]) ~= 0)