Why does sum([]) equal to zero?

Executing the following gives zero as the result.

sum([])

I expect an error or an empty matrix as the answer.

Answer 1

[MathWorks Support Team]

A fundamental property of the SUM function is that if A and B are sets of numbers, then

 sum(union(A, B)) = sum(A) + sum(B)

In order to maintain this property, a number must be returned for empty matrices. Thus the number 0 is returned for empty matrix inputs to the SUM function.

Note: the 'union' operation referred to above is a mathematical definition, different from the UNION function in MATLAB.