Why does setdiff answer depend on order of arguments?

>>bob = {'a','b','c'};
>>bill = {'a','b','c', 'd','e'}

As expected,

>>A = setdiff(bill,bob)
A = 
    'd'    'e'

BUT

>> B = setdiff(bob,bill)
B = 
   Empty cell array: 1-by-0

WHY??

Answer 1

[Steven Lord]

Were you expecting the output to contain elements that are only in one of the inputs but not both? That's not what setdiff does. If that is what you want to do, use setxor instead.

[Brian Wilson]

Thanks, yes. i was expecting it to tell me the "difference between the sets" i.e., which elements are not in both , but i see now, its computing A \ B.

[Stephen23]

It is worth nothing that MATLAB setdiff follows the standard mathematical definition of "set difference", which is defined as A\B (i.e the elements of A that are not in B):

https://www.basic-mathematics.com/difference-of-sets.html

http://mathworld.wolfram.com/SetDifference.html

https://proofwiki.org/wiki/Definition:Set_Difference

http://web.mnstate.edu/peil/MDEV102/U1/S6/Complement3.htm

etc.

The setdiff documentation states "setdiff(A,B) returns the data in A that is not in B"

Although this is the accepted mathematical definition, the term "set difference" is rather ambiguous in common english. It would be nice if it used terms whose meaning was obvious.

Answer 2

[madhan ravi]

bob not in bill (nothing unique all elements of bob belong to bill)

help setdiff

[Brian Wilson]

Thanks madhan, I was confusing the functionality with setxor.

Answer 3

[Walter Roberson]

setdiff implements set subtraction A \ B which is not commutative

[Brian Wilson]

thanks Walter, I was expecting it to do setxor.