Which MATLAB function can remove the diagonal elements of a NxN matrix

Question

Jinlong Wei 2011-2-1

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编辑： oren Tadmor 2014-6-12

Do you know which MATLAB function can do the following work: to remove the diagonal elements of a (N+1)x(N+1) matrix to generate a new NxN matrix.

For say, the old matrix is

[ 0  1  2  3
0  2  3
2  0  3
2  3  0 ]

the generated new matrix is

[ 1  2  3
  1  2  3
  1  2  3 ]

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Kenneth Eaton 2011-2-1

It's not clear what you are asking. In your example, removing the main diagonal still leaves you with (N+1)*N elements, too many to make an N-by-N matrix. You would have to remove the main diagonal plus either the first superdiagonal or the first subdiagonal to end up with N*N elements.

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Answer 1

Matt Fig 2011-2-1

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Your example doesn't add up, as Kenneth mentioned. However, in general you could do something like this:

A = [ 0 1 2 3
1  0  2  3
1  2  0  3
1  2  3  0 ];
A(logical(eye(size(A)))) = []; % Or A = A(~eye(size(A)))
A = reshape(A,4,3);  % Or 3,4 or whatever.

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Paulo Silva 2011-2-1

A(A==0)=[]

A = reshape(A,4,3);

James Tursa 2013-1-30

编辑：James Tursa 2013-1-30

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Another variation using linear indexing:

m = size(A,1)+1;
A(1:m:end) = [];
A = reshape(A,4,3); % or whatever

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Answer 2

Paulo Silva 2011-2-1

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b=triu(a)
b=b(:,2:end)
c=tril(a)
c=c(:,1:end-1)
b+c

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ahahahaaron 2013-1-30

this one produces an overlap and doesn't really produce what the other guy wants.

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Answer 3

oren Tadmor 2014-6-12

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编辑：oren Tadmor 2014-6-12

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remove_diagonal = @(t)reshape(t(~diag(ones(1,size(t, 1)))), size(t)-[1 0]);

So :

>> m = magic(5)
m =
      17    24     1     8    15
      23     5     7    14    16
       4     6    13    20    22
      10    12    19    21     3
      11    18    25     2     9
ans =
      23    24     1     8    15
       4     6     7    14    16
      10    12    19    20    22
      11    18    25     2     3

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Answer 4

Youssef Khmou 2013-1-31

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编辑：Youssef Khmou 2013-1-31

在 MATLAB Online 中打开

hi, Jinlong Wei

According to your example : Here is a function :

function Z=diagrem(X)
N=size(X);
Z=X;
if N(1)~=N(2)
  error('Matrix is not square');
end
for x=1:N(1)
  Z(x,x)=0;
end
for y=1:N(1)-1
  Z(y,y+1)=0;
end
Z(Z==0)=[];
Z=reshape(Z,N(1)-1,N(2)-1);

It produces exactly your example ,it removes R(i,i) and in the second loop it removes R(j,j+1) with 1<=i<=n , 1<=j<n .

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Youssef Khmou 2013-1-31

In case your matrix already contains zeros, then change the code by giving Z(x,x) and Z(y,y+1) a NaN value per example or something else then remove them.

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