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

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navan
navan 2015-5-20
回答： Steven Lord 2018-10-28
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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Guillaume
Guillaume 2015-5-20
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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Steven Lord
Steven Lord 2018-10-28
If you're using release R2017b or later, use maxk.
Thomas Koelen
Thomas Koelen 2015-5-20
编辑：Thomas Koelen 2015-5-20
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
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Guillaume
Guillaume 2015-5-20
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.
Guillaume
Guillaume 2015-5-20
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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Thorsten
Thorsten 2015-5-20
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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