What are the ways to sum matrix elements in MATLAB?

Question

Given the matrix:

A = [1 2 3; 4 5 6; 7 8 9];

1. How could you use a for loop to compute the sum of the elements in the matrix?
2. Writ

Answer 1

Avoid for loops whenever possible.

sum(A(:))

is great however if you have some logical indexing going on you can't use the (:) but you can write

% Sum all elements under 45 in the matrix
sum ( sum ( A *. ( A < 45 ) )

Since sum sums the columns and sums the row vector that was created by the first sum. Note that this only works if the matrix is 2-dim.