Re: Baby X is bor nagain

Ben Bacarisse <ben@bsb.me.uk> writes:

Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
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Ben Bacarisse <ben@bsb.me.uk> writes:
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:
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Ben Bacarisse <ben@bsb.me.uk> writes:
[...]
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On a C language point, I don't think the standard says anything
about sorting with non-order functions like the one above.  Is
an implementation of qsort permitted to misbehave (for example
by not terminating) when the comparison function does not
implement a proper order relation?

>
N1570 7.22.5p4 (applies to bsearch and qsort):
"""
When the same objects (consisting of size bytes, irrespective of
their current positions in the array) are passed more than once
to the comparison function, the results shall be consistent with
one another.  That is, for qsort they shall define a total
ordering on the array, and for bsearch the same object shall
always compare the same way with the key.
"""
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That's a "shall" outside a constraint, so violating it results in
undefined behavior.

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I think it should be clearer.  What the "that is" phrase seems to
clarify in no way implies a total order, merely that the repeated
comparisons or the same elements are consistent with one another.
That the comparison function defines a total order on the elements
is, to me, a major extra constraint that should not be written as
an apparent clarification to something the does not imply it:
repeated calls should be consistent with one another and, in
addition, a total order should be imposed on the elements present.

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I think you're misreading the first sentence.

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Let's hope so.  That's why I said it should be clearer, not that it
was wrong.
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Suppose we are in
court listening to an ongoing murder trial.  Witness one comes in
and testifies that Alice left the house before Bob.  Witness two
comes in (after witness one has gone) and testifies that Bob left
the house before Cathy.  Witness three comes in (after the first
two have gone) and testifies that Cathy left the house before
Alice.  None of the witnesses have contradicted either of the
other witnesses, but the testimonies of the three witnesses are
not consistent with one another.

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My (apparently incorrect) reading of the first sentence is that
the consistency is only required between the results of multiple
calls between each pair.  In other words, if the witnesses are
repeatedly asked, again and again, if Alice left before Bob and/or
if Bob left before Alice the results would always be consistent
(with, of course, the same required of repeatedly asking about the
other pairs of people).

Let me paraphrase that.  When the same pair of objects is passed
more than once to individual calls of the comparison function, the
results of those different calls shall each be consistent with
every other one of the results.

To paraphrase my reading, when some set of "same" objects is each
passed more than once to individual calls of the comparison
function, the results of all of those calls taken together shall
not imply an ordering contradiction.

Are the last two paragraphs fair restatements of our respective
readings?  Is the second paragraph plain enough so that you
would not misconstrue it if read in isolation?  Or if not, can
you suggest a better phrasing?

Try a web search
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    "consistent with" definition
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for more explanation.

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Seriously?

Yes, it's a serious suggestion, and I'm sorry if it came across as
condescending.  I did this search myself, and learned something from
it.  The important point is the "consistent with" is something of an
idiomatic phrase, and it doesn't mean "equivalent to" or "the same
as".  Maybe you already knew that, but I didn't, and learning it
helped me see what the quoted passage is getting at.

Also, for "one another", if we say the
children in the Jones family get along with one another, we don't
mean that each child gets along with at least one of the others,
but instead mean that every child gets along with every other
child, that is, that they all get along with each other.

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The sentence in question has, to my mind, already stated what the
"one another" refers to -- the multiple calls between pairs
containing the same objects.  I get you think that's not the
intended meaning, but I get my reading so strongly that I struggle
to see the other.

Yes, I got that.  The incongruity between the first sentence and the
second sentence prompted me to re-examine the entire paragraph,
which is what eventually led me to my current reading.

Whether
or not some other reading (of that problem sentence in the C
standard) is sensible, surely the reading I have suggested is a
plausible one.  Do you agree?  It seems clear, given how the
second sentence is phrased, that this suggested reading is what
was intended.

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I still can't read it the way you do.  Every time I try, I find
the consistency is to be taken as applying to the results of the
multiple calls between pairs of the same objects.  Nothing more.
It starts with "When the same objects".  It seems so clear that
the consistency is all about the multiple calls with these same
objects.  I keep trying to see your reading of it, but I can't.

Yes, the phrase "the same objects" starts one down a wrong path.
What I think is meant is that "sameness" applies to objects
individually, without regard to what the object is being compared
to.  It's a tricky point because it isn't literally the same object:
what is meant is the same "logical" object, not the same physical
object.  If you think of "the same objects" as meaning a set of
individual logical objects, rather than pairs of logical objects,
that might be a way to dislodge the (unfortunately all too easy
to fall into) initial impression.

I don't mean to defend the quality of writing in this passage.
Certainly it would be nice if the meaning could have been stated
more plainly.  But I think it's an overstatement to say that the
first sentence in no way implies a total order.

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I have a second objection that promoted that remark.  If I take the
(apparently) intended meaning of the first sentence, I think that
"consistent" is too weak to imply even a partial order.  In dog club
tonight, because of how they get on, I will ensure that Enzo is
walking behind George, that George is walking behind Benji, Benji
behind Gibson, Gibson behind Pepper and Pepper behind Enzo.  In what
sense is this "ordering" not consistent?  All the calls to the
comparison function are consistent with each other.

I understand the objection, and this is the point I was trying to
make in the paragraph about children in the Jones family.  The
phrase "one another" in "the results shall be consistent with one
another" is meant to be read as saying "all the results taken
together".  It is not enough that results not be contradictory taken
two at a time;  considering all the results at once must not lead to
an ordering contradiction.

Hopefully this has been helpful for you.  If it hasn't I'd like to
hear where the sticking points are.