How can I find the max of two vectors?

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

Answer 1

[C.J. Harris]

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

[Adrienne]

Yes I think that's what I need to do!!! Thanks!!!

[Guillaume]

I understood that you wanted to sort on b, and for identical b find the highest value in a (a multiple key sort). If that is the case, then this answer is completely wrong.

consider

a = [4.84 3.55 2.09 4.20 1.14 0.15 2.45 3.41 1.66 2.75 3.87]; %only changed one value the 5.15 to 0.15
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 =
     3.87    4.84     3.55
     8       7        8

If that is indeed what you wanted, then you certainly had a convoluted way of expressing it.

See my answer for a multi-key sort.

[C.J. Harris]

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]

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.

Answer 2

[the cyclist]

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.

Answer 3

[Guillaume]

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)