A=[3 4 2 3; 4 2 1 5; 9 1 1 10]


for r=1:3
  pv=A(r, r);
  for c=1:4
    A(r, c)=A(r, c)/pv;
  endfor

  for i=1:3
   if i==r
     continue;
    endif

    pv=A(i, r);
    for j=1:4
      A(i, j)=A(i, j)-pv*A(r, j);
    endfor


  endfor

  A

endfor
A