MATRIX COFACTOR

I need to know a function to calculate the cofactor of a matrix, thank a lot!

[Quilee Simeon]

cofactor matrix for a matrix A is just det(A)*inv(A)

[Zoe Herrick]

det(A)*inv(A) gives the adjugate or classical adjoint of matrix A which is the Transpose of the cofactor matrix.

This wiki article gives a brief layout of this:

https://en.wikipedia.org/wiki/Adjugate_matrix

[Franco Salcedo Lópezz]

Here I leave this code, I hope it helps. Regards..

function v = adj(M,i,j)
  t=length(M);
  v=zeros(t-1,t-1);
ii=1;
ban=0;
for k=1:t
   jj=1;
  for m=1:t
    if ( (i~=k)&&(j~=m) )
      v(ii,jj)=M(k,m);
      jj++;
      ban=1;
    endif 
  endfor
   if(ban==1)ii++;ban=0;endif
endfor

[Walter Roberson]

ii++ is not valid MATLAB though. And endif and endfor are not MATLAB either.

[Fernando Salinas]

I wrote this in GNU/Octave but I guess it should work on MATLAB

function cofactor = matrizCofactores(A)
  [rows, cols] = size(A);
  if rows == cols
    for i = 1 : rows,
      for j = 1 : cols,
        Menor = A;
        Menor(i,:) = [];
        Menor(:,j) = [];
        if mod((i+j),2) == 0
          cofactor(i,j) = det(Menor);
        else
          cofactor(i,j) = -det(Menor);
        endif
      endfor
    endfor
  endif
endfunction

[Natasha St Hilaire]

What is "menor" short for?

[Walter Roberson]

@Natasha St Hilaire

I suspect that the English word would be "minor". The Spanish word "menor" can be translated as English "minor" in some situations.

Answer 1

[Walter Roberson]

http://www.mathworks.com/matlabcentral/fileexchange/2284-introductory-linear-algebra-with-applications/content/kolman/ILA6/cofactor.m

[Mariana]

Walter, thanks for your answer.. Where I have to save the function (.m) and what name i must to give to the file (.m)?

[Walter Roberson]

Save the file in any directory on your matlab path, or save it in a directory and use the path tool to add that directory to your matlab path.

Name the file anything that is convenient and which is a valid matlab variable name (starts with a letter, letters or digits or underscore after, 63 characters max, is not a reserved word). Naming it the same thing as another existing MATLAB routine would usually cause Problems.

As there is no MATLAB cofactor command, naming it cofactor.m would seem appropriate.

[Mariana]

Walter, when I save the file appear a message saying that I don't have permissions to do that.. and when I try to add it appear a message too and don't let me add. Thank you!

[Mariana]

But anyway the code is working and given the cofactor without saving.. I just wanna know why I can't save the function, thanks

[Walter Roberson]

Sounds like you might be using Windows 7, and that its brilliant new UAC features are "helping" you.

http://www.mathworks.com/support/solutions/en/data/1-36TXZP/index.html?product=ML&solution=1-36TXZP

http://www.mathworks.com/support/solutions/en/data/1-9574H9/index.html?solution=1-9574H9

[Mariana]

Walter,

I get it, Im using Windows 7 and a Matlab 7.12, I just click in a green icon on the Editor that say RUN, and after appear a windows asking to add to a PATH.. and say YES ... I thing the code is in the livrary now.. and it's working so fine.. thanks a lot

[Mariana]

Yes.. this work, but when I close Matlab and open again.. not work

[Walter Roberson]

Did you try the solutions in the second link?

[Mariana]

Yes, I right-click on the shortcut and select to run as administrator. And I save the function on the lib file, and the function work with matrix.. But when I close and open again the function when I try to use the function a message say that it is Undefined..

[Mariana]

Walter,

I thing I get it.. I forget to add the function to the PATH through the SET PATH in the menu file.. Thank you very much for all..

Just one question more.. Matlab run in linux? what distribution is better?

Answer 2

[Dr. Murtaza Khan]

A = [
    2 4 1
    4 3 7
    2 1 3
    ]
detA = det(A)

invA = inv(A)

cofactorA = transpose(detA*invA)

[Franco Salcedo Lópezz]

Here I leave this code, I hope it helps. Regards

function v = adj(M,i,j)
  t=length(M);
  v=zeros(t-1,t-1);
ii=1;
ban=0;
for k=1:t
   jj=1;
  for m=1:t
    if ( (i~=k)&&(j~=m) )
      v(ii,jj)=M(k,m);
      jj++;
      ban=1;
    endif 
  endfor
   if(ban==1)ii++;ban=0;endif
endfor

[Walter Roberson]

This is not MATLAB code. It might be Octave.

Answer 3

[Francisco Trigo]

The matrix confactor of a given matrix A can be calculated as det(A)*inv(A), but also as the adjoint(A). And this strange, because in most texts the adjoint of a matrix and the cofactor of that matrix are tranposed to each other. But in MATLAB are equal. I found a bit strange the MATLAB definition of the adjoint of a matrix.

[Zuhri Zuhri]

adjoint matrix is ​​the transpose of the cofactor matrix so the above result is correct