Obfuscated SWAP: not portable!

[Norman Diamond]

In article <149 at cpsolv.UUCP>, rhg at cpsolv.UUCP (Richard H. Gumpertz) writes:
>> The exchange of x and y that has been under discussion:
>>     x ^= y ^= x ^= y
>> is non-portable because it depends on right-to-left evaluation which is NOT
>> specified in the C standard.

True, though perhaps a bit too terse for some people.

In article <1400 at levels.sait.edu.au> CCDN at levels.sait.edu.au (DAVID NEWALL) writes:

>I think it is specified.  (Perhaps it depends on whose standard you follow?)
>K&R I, page 19:
>     "The line
>        nl = nw = nc = 0;
>    sets all three variables to zero.  This is not a special case, but a
>    consequence of the fact that an assignment has a value and assignments
>    associate right to left."

OK, let's try to make it perfectly clear.

    x  ^=  y  ^=  x  ^=  y
       [3]    [2]    [1]

The order of evaluation is [1], [2], [3].  But what about the operands?

[1]  middle x   =   old x    ^   old y
[2]  new y      =   old y    ^   middle x
[3]  new x      =   which x  ^   new y

If new x = middle x ^ new y, it works.  But if new x = old x ^ new y,
it doesn't work.  Neither K&R nor the pants say when the operand value
of x is fetched for assignment [3].  It could be either before or after
assignment [1] completes.

--
-- 
Norman Diamond, Sony Corporation (diamond at ws.sony.junet)
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