MATLAB Answers

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

218 views (last 30 days)
navan
navan on 20 May 2015
Answered: 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]

  0 Comments

Sign in to comment.

Accepted Answer

Guillaume
Guillaume on 20 May 2015
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.

  3 Comments

Bruno Luong
Bruno Luong on 28 Oct 2018
zahra najja: Please read the other ANSWERS bellow, or even better the DOC of MIN

Sign in to comment.

More Answers (3)


Thomas Koelen
Thomas Koelen on 20 May 2015
Edited: 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

Thomas Koelen
Thomas Koelen on 20 May 2015
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
Guillaume
Guillaume on 20 May 2015
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 on 20 May 2015
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

Sign in to comment.


Thorsten
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);

  0 Comments

Sign in to comment.