Change diagonal place

How can I switch places the diagonal and the first column from a square matrix?

[Fangjun Jiang]

You could use an example to explain what you mean.

Answer 1

[bym]

a= magic(3)

a =

       8     1     6
       3     5     7
       4     9     2

>> b=a;
>>  for i =2:size(a,1)
b(i,1:i)= fliplr(a(i,1:i));
end
b

b =

       8     1     6
       5     3     7
       2     9     4

[Paul]

Thanks

[Image Analyst]

proecsm, I know you know better than that, so I assume you just made a careless error. See my solution for the correct answer. Paul, don't use that method if your size is greater than 3 by 3.

[Andrei Bobrov]

b = 1:size(a,1)+1:numel(a)

c = 1:3

a([b,c]) = a([c,b])

Answer 2

[Image Analyst]

This code works of a square matrix of any size. I present both a looping and vectorized way.

a=magic(6)
b=a;
for i =2:size(a,1)
  temp = a(i,i);
  b(i,i)= b(i,1);
  b(i,1) = temp;
end
b

% Loopless way
a=magic(6)
b=a;

diagonalElementsMask = logical(eye(size(a,1)))
% Save diagonal elements, because we're going to replace them.
bDiagonalElements = b(diagonalElementsMask)
% Replace diagonal elements with first column.
b(diagonalElementsMask) = b(:,1);
% Replace first column with diagonal elements.
b(:,1) = bDiagonalElements

a =

      35     1     6    26    19    24
       3    32     7    21    23    25
      31     9     2    22    27    20
       8    28    33    17    10    15
      30     5    34    12    14    16
       4    36    29    13    18    11

b =

      35     1     6    26    19    24
      32     3     7    21    23    25
       2     9    31    22    27    20
      17    28    33     8    10    15
      14     5    34    12    30    16
      11    36    29    13    18     4

[bym]

careless indeed :| +1

[Andrei Bobrov]

b = 1:size(a,1)+1:numel(a)

c = 1:size(a,1)

a([b,c]) = a([c,b])

[Image Analyst]

OK, this one works.