The nonexistent operator (along = v. == lines)

[Richard Barnette]

In article <156 at revcan.UUCP> darren at revcan.UUCP (Darren Morbey) writes:
>I've noticed in my writing C code that there is no such operator
>as ^^ (which would be to ^ as || is to |).  I feel in this case
>I *have* to write a macro for this *nonexistent* operator (you may
>recall I wrote #define XOR(a,b) ((a)^(b)) ).  However...

    It can't be done.  The truth or falseness of a ^ expression
cannot be determined without evaluating all the operands.  Note
the truth tables below:

    A B | A XOR B    A B | A AND B    A B | A AND B
   -----+---------  -----+---------  -----+---------
    F F |    F       F F |    F       F F |    F   
    F T |    T       F T |    T       F T |    F   
    T F |    T       T F |    T       T F |    F   
    T T |    F       T T |    T       T T |    T    

    Given A if F, A AND B is always F; given A is T, A OR B is always
T.  No such statement can be made for A XOR B;  if A is T, A XOR B is
NOT B; if A is F, A XOR B is B.  In either case the value depends on
B regardless of the value of A.

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