Expression sequencing query

Mike Meyer mwm at
Fri Sep 26 19:27:59 AEST 1986

In article <353 at cullvax.UUCP> drw at cullvax.UUCP (Dale Worley) writes:
>> In article <760 at oakhill.UUCP> tomc at oakhill.UUCP (Tom Cunningham) writes:
>> >	/* a = b + b + b */
>> >	a = ((b=1),b) + ((b=2),b) + ((b=3),b)
>> >
>> >I expected the result to be 6.  With the Microsoft C compiler and the
>> >compiler on the Sun 3, the result is 9.  Apparently the parenthetical
>> >assignments are all getting done before the comma and addition.  Any
>> >thoughts on this?
>Harbison&Steele (7.11) makes it clear that an implementation must
>evaluate one argument of a binary operator completely before starting
>evaluation of the other argument.  Thus, the result should be 6.  I
>don't know what the ANSI standard says.

My reading of the ANSI hardcopy is that it doesn't say. I thought H&S
was a description, not a definition.

>Dec VAX Ultrix gives 9.

As do VAX 4.2 and 4.3.

>Lattice C 3.00 for MS-DOS gives 7!!!  (Yes, that's "7", not a typo!)

Some messy - and almost believable - arguments can be made that the
value of that expression is "implementation dependent". Rather than do
that, I'll just point out that *ANY* time++ you have a single
statement that changes a variable, then uses the variable in a
different place, you're asking for trouble. For that particular
expression, I'd expect the following, with increasing surprise as you
move down the list:

	3, 9
	4, 5, 7, 8
	other integers representable on the machine
	NANs of various flavors
	dropped cores.


++ Except for those cases that are in different operands of a logical
operator, as the evaluation order on those is known.

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