get a diagonal line in matrix

Hi, please help me get a diagonal line in matlab. Example, I have a matrix: a = [1 2 3; 4 5 6; 7 8 9] I want to get vector b =[3 5 7] which is a diagonal line of matrix a. This is my code but it doesn't work

 a=[1 2 3; 4 5 6; 7 8 9];
 b = zeros(1,3);
for i=1:3
    for i=1:3
       b = a(i,j);
    end
end

Answer 1

[Jan]

Or:

a     = [1 2 3; 4 5 6; 7 8 9];
s     = size(a);
index = (numel(a) - s(1) + 1):(1 - s(1)):s(2);
b     = a(index)

[EDITED, works with non square matrices now]

[dao]

Thank you so much Jan Simon and Star Strider for the answer.

But how could I use the two for loop to this work ? I am a newbie and I want to get used to with the loop in matlab. Thank you.

[Jan]

If you really want a loop:

b = zeros(1,3);
for i = 1:3
   b(i) = a(i, 4 - i);
end

[dao]

Thank you for your answer, it is neat. in my case, the column and row are equal. How about they are different. Example, I have a matrix 3x4 ? Do I need to use two loop ?