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

hello.. i hv a question. what is the command to call the non diagonal entries of a matrix? tq very much...

Answer 1

[Rick Rosson]

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

[srycandy]

eg: i have 3x3 matrix. A = [2 3 5;3 6 8;5 8 4]. then , i want to multiply all the off-diagonal entries by 2. Can i use ur recommended code in my situation? tq for ur cooperation.

[Rick Rosson]

Do you want to keep the diagonal entries as well, or do you want to zero them out? Also, do you want the result to be a 3x3 matrix, or a 6x1 column vector (eliminating the 3 diagonal elements)? Please let me know.

[srycandy]

i wanna keep the diagonal entries as well..and i also want the result also in 3x3 matrix..tq

[Rick Rosson]

I modified my answer above.

Answer 2

[Nathan Greco]

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.

[srycandy]

eg: i have 3x3 matrix. A = [2 3 5;3 6 8;5 8 4]. then , i want to multiply all the off-diagonal entries by 2. is it clear enough? tq for ur cooperation.

[srycandy]

i used the 1st one from ur answer.. tq very2 much... :)

[Nathan Greco]

You could either do the above methods and multiply (.*) the result by 2, or use indexing as such:

idx = find(~eye(size(A))); %get NON diagonal entries

A(idx) = A(idx).*2;

[Nathan Greco]

OR: A.*2-diag(diag(A)) would work.

[srycandy]

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..

Answer 3

[dor eivensitz]

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

[James Tursa]

Please open up a new question for this, and provide a short example in that new question.

Answer 4

[James Tursa]

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)

Answer 5

[Niba Kainat]

hi can someone tell me how to compute sum of no diagonal entries.

[Voss]

M = magic(4) % some square matrix

M = 4x4

16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1

<mw-icon class=""></mw-icon>

<mw-icon class=""></mw-icon>

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