Why does sum([]) equal to zero?

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MathWorks Support Team
Executing the following gives zero as the result.
sum([])
I expect an error or an empty matrix as the answer.

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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.

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