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How do I access and modify only the non diagonal entries in a matrix?

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srycandy
srycandy el 28 de Jul. de 2011
Comentada: Voss el 18 de Jul. de 2024 a las 14:57
hello.. i hv a question. what is the command to call the non diagonal entries of a matrix? tq very much...

Respuesta aceptada

Rick Rosson
Rick Rosson el 28 de Jul. de 2011
Please try the following:
M = 8;
N = 5;
X = randn(M,N);
idx = eye(M,N);
Y = (1-idx).*X;
Z = X(~idx);
HTH.
Best, Rick
Responding to your comment:
To multiply the non-diagonal elements by 2, please try:
A = [2 3 5;3 6 8;5 8 4];
idx = eye(size(A));
idx = idx + 2*(1-idx);
Y = idx.*A;
HTH.
Best, Rick
  4 comentarios
srycandy
srycandy el 28 de Jul. de 2011
i wanna keep the diagonal entries as well..and i also want the result also in 3x3 matrix..tq

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Más respuestas (4)

Nathan Greco
Nathan Greco el 28 de Jul. de 2011
If tmp is your matrix, try:
tmp-diag(diag(tmp)) %works only with square matrices
OR
triu(tmp,1)+tril(tmp,-1)
Both of these set the diagonal entries to zero, essentially ignoring them. If this isn't what you want, please clarify.
  5 comentarios
Nathan Greco
Nathan Greco el 28 de Jul. de 2011
OR: A.*2-diag(diag(A)) would work.
srycandy
srycandy el 28 de Jul. de 2011
yup.. A.*2-diag(diag(A)) is the one i'm searching for.. i forgot to mentioned that i wanna keep the diagonal entries as well.. anyway, tq..

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dor eivensitz
dor eivensitz el 2 de Nov. de 2017
i want to keep both diagonal in matrix size i don't know, defined by n, and all other elements to multiply by them self plus 1. tnx
  1 comentario
James Tursa
James Tursa el 2 de Nov. de 2017
Please open up a new question for this, and provide a short example in that new question.

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James Tursa
James Tursa el 2 de Nov. de 2017
Another way for a square matrix:
M = your matrix
x = ~logical(eye(size(M)));
M(x) = 2*M(x); % <-- or whatever function you want on the rhs using M(x)

Niba Kainat
Niba Kainat el 18 de Jul. de 2024 a las 14:23
hi can someone tell me how to compute sum of no diagonal entries.
  1 comentario
Voss
Voss el 18 de Jul. de 2024 a las 14:57
M = magic(4) % some square matrix
M = 4x4
16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1
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idx = ~eye(size(M)) % logical index indicating non-diagonal entries
idx = 4x4 logical array
0 1 1 1 1 0 1 1 1 1 0 1 1 1 1 0
result = sum(M(idx),'all') % sum the non-diagonal entries
result = 102

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