How to Sum matrix diagonals

If I have the following matrix

a = [1 2 3; 4 5 6; 7 8 10
a =

     1     2     3
     4     5     6
     7     8    10

How do I sum the diagonal of 7,5,3? I know that to sum 1,5,10 I use

sum(diag(a))

But when I try

sum(diag(a,2))

The answer is 3. Is there a built in function that calculates the sum, or do I have to make a while loop?

[Tia Miller]

sum(diag(a,2)) is summing the diagonal 2 away from the main diag, which has one entry, 3.

sum(diag(flip(a))) will sum 3, 5, 7, or the main diag after the matrix has been flipped

Answer 1

[Andrei Bobrov]

sum(diag(a(:,end:-1:1)));

or summing all diagonals:

sum(spdiags(rot90(a)));

Answer 2

[John D'Errico]

sum(diag(flip(A)))

should work.

Answer 3

[Ahmed Diaa]

x=sum(diag(E));% calculating the normal diagonal
y=sum(diag(flip(E)));% calculating the inverse diagonal 
z=[];% subtracting the common element 
s=y+x-z ;% the sum