MATLAB: Matrix manipulation: Changing Diagonal values

diagonal matrixmatlab matrixmatrix

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.

Best Answer

>> 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
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