Matrix manipulation: Changing Diagonal values

Hello Everyone!

I have a matrix data which is from a nxn uitable inputted by a certain user, using this short code

 S = get(handles.Table,'data');

I have successfully saved the data in the variable S.

My next goal is to create another matrix out of this matrix. My general formula is

 NewMatrix(1,1) = S(1,1)-S(1,2)-S(1,3),...S(1,n)
 NewMatrix(2,2) = S(2,2)-S(2,1)-S(2,3),...S(2,n)

So on and so forth. Yes, it is the diagonal element of the matrix which I would like to manipulat. And I wish to multiply by (-1) all of the remaining values in the S matrix.

The NewMatrix will have different diagonal values while all of the other data is the negative of S Matrix.

I hope someone could help me. Cause I am starting to get cray on this. Thank you.

Answer 1

[Stephen23]

>> S = randi(9,3,5)
S =
     4     3     9     4     4
     6     4     2     2     9
     6     1     1     5     9
>> idx = 1==eye(size(S));
>> new = -S;
>> new(idx) = sum(new,2) + 2*S(idx)
new =
   -16    -3    -9    -4    -4
    -6   -15    -2    -2    -9
    -6    -1   -20    -5    -9

And checking:

 NewMatrix(2,2) = S(2,2)-S(2,1)-S(2,3),...S(2,n)

>> S(2,2)-S(2,1)-S(2,3)-S(2,4)-S(2,5)
ans =
   -15
>> new(2,2)
ans =
   -15

[Abbey]

Thank you so much!

Answer 2

[Walter Roberson]

-1 * (S - diag(diag(S))) + diag(NewValuesToInsert)

Or more compactly but less readable,

   diag(diag(S)) - S + diag(NewValuesToInsert)

diag(diag(S)) first extracts the diagonal of S to a vector for the first diag call, and then the second call makes that vector the diagonal of a full matrix, with the end result that you have a matrix that is just the diagonal.

[Abbey]

Thank you sir, however I am having difficulty using this in a nxn matrix.