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navan
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findout the second largest element in each row and its location in a matrix

Asked by navan
on 20 May 2015
Latest activity Answered by Steven Lord
on 28 Oct 2018
I have a (3*5) matrix i.e A=[1 2 3 4 5, 6 7 9 8 10, 11 12 14 13 15], i would like to find out the second largest element in each row and its location in the matrix. i expect 2 outputs 1)second_maxvalue_A=[4,9,14] 2)location_of_second_maxval=[4,3,3]

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4 Answers

Answer by Guillaume
on 20 May 2015
 Accepted Answer

One possible way is to find the largest values, replace them with a very small value (-Inf) and search for the new largest values:
A=[1 2 3 4 5; 6 7 9 8 10; 11 12 14 13 15] %your example A was a single row
m = max(A, [], 2); %find the max in each row
A(bsxfun(@eq, A, m)) = -Inf; %replace the max(s) by -Inf
m = max(A, [], 2) %find the new max which is the second largest.

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Thanks Guillaume,
That works.thanks alot
zahra najja: Please read the other ANSWERS bellow, or even better the DOC of MIN

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Answer by Thomas Koelen on 20 May 2015
Edited by Thomas Koelen on 20 May 2015

Edit: seems like I misread second largest...
, should be ;.
A=[1 2 3 4 5; 6 7 9 8 10; 11 12 14 13 15];
[m,i] = max(A,[],2)
gives you:
m =
5
10
15
i =
5
5
5
example:
A=rand(3,5);
[m,i] = max(A,[],2)
gives:
0.8147 0.9134 0.2785 0.9649 0.9572
0.9058 0.6324 0.5469 0.1576 0.4854
0.1270 0.0975 0.9575 0.9706 0.8003
m =
0.9649
0.9058
0.9706
i =
4
1
4

  3 Comments

Finding second or third biggest can be done this way:
m=3;n=4;ggg = rand(m,n);
[svals,idx] = sort(ggg(:),'descend'); % sort to vector
svals(2) % second largest value
[II,JJ] = ind2sub([m,n],idx(2)) % position in the matrix
Doesn't this find the second largest element in the matrix, rather than each row?
Secondly, if there are more than one element equal to the max, it still returns the max.
A generic way of finding the nth largest value of each row, if it exists:
function nl = nthlargestrow(m, n)
%m: a 2d matrix to search
%n: an integer
%nl: cell array containing the nth largest value of each row. If a row does not have n different values, the cell is empty
nl = cell(size(m, 1), 1);
u = cellfun(@unique, num2cell(m, 2), 'UniformOutput', false);
hasnelements = cellfun(@numel, u) >= n;
nl(hasnelements) = cellfun(@(row) row(end-n+1), u(hasnelements), 'UniformOutput', false)
end

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Answer by Thorsten
on 20 May 2015

A=[1 2 3 4 5; 6 7 9 8 10; 11 12 14 13 15];
% remove highest value in each row
sz = size(A);
[~, ind] = max(A, [], 2);
ind = ind + [0; cumsum(repmat(sz(2), [sz(1)-1 1]))];
A = A';
A(ind) = [];
A = reshape(A, [sz(2)-1 sz(1)])';
% find highest value in each row (i.e., second largest of original matrix)
[max2, ind2] = max(A, [], 2);

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Answer by Steven Lord
on 28 Oct 2018

If you're using release R2017b or later, use maxk.

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