RE: [F&O] 7.5.4 fn:substring-before

Another personal response without WG consultation:

If every string contains the zero-length string at the beginning *and* at 
the end (which, by the way, I believe they all do), then I would expect:
    fn:substring-after("Who there?","")
to return "Who there?".  That is, the function finds the first instance of 
the zero-length string, which is the one immediately preceding "W", and 
returns everything following it, which is, of course, the remainder of the 
string, or "Who there?".

By the same reasoning, I would expect:
    fn:substring-before("Baloney!","")
to return "".  My reasoning is that the function finds the first instance 
of the zero-length string, which is the one immediately preceding "B", and 
returns everything preceding it, which is, of course, yet another 
zero-length string, or "".  (I guess that means that I believe that every 
string contains an infinite number of zero-length strings before and after 
the remaining parts of the string.  But I don't want to try to count 
them...)  This particular example uses a string in which no two characters 
are the same character.  But surely fn:substring-before("Who there?","e") 
would return "Who th" instead of "Who ther" (that is, the search always 
starts at the beginning of the string and not at the end).

My reasoning, by the way, for the conclusion in the previous paragraph 
derives from the F&O Last Call WD, Section 7.5.4, "fn:substring-before", in 
which the first paragraph (Summary:) says "Returns the substring of the 
value of $arg1 that precedes in the value of $arg1 the first 
occurrence...".  Therefore, the substring of "Baloney!" that precedes the 
FIRST occurrence of "" (the zero-length string) is "" (the zero-length 
string).  Therefore, even if every string contains the zero-length string 
both at the beginning and the end (which I believe is true), it is only the 
FIRST such zero-length string that affects the result of 
fn:substring-before, not the final such zero-length string.

I thus conclude that Mike Kay's personal response (copied below) is 
accurate and the second example in Section 7.5.4.1, "Examples", is 
incorrect.  That means that I disagree with the suggestion made in Noe 
Michejda's original comment.

But this is my personal opinion, and the WG's must make the final decisions.

Hope this helps,
    Jim


At 08:23 2004-01-16 Friday, Ashok Malhotra wrote:

>Note that one of the examples in 7.5.5 says:
>fn:substring-after("Who there?","") returns "Who there?".
>
>This does assume that each string contains the zero-length string at the
>start.  OTOH, the example
>fn:substring-before("Baloney!","") returns "Baloney!".
>
>Seems to assume that every string contains the zero-length string at the
>end.
>
>We need to decide consciously whether every string implicitly contains a
>zero-length string at the start, at the end or both.
>
>All the best, Ashok
>-----Original Message-----
>From: public-qt-comments-request@w3.org
>[mailto:public-qt-comments-request@w3.org] On Behalf Of Kay, Michael
>Sent: Friday, January 16, 2004 7:07 AM
>To: Noe Michejda; public-qt-comments@w3.org
>Subject: RE: [F&O] 7.5.4 fn:substring-before
>
>
>Thanks. Personal response:
>
>XPath 1.0 compatibility is obviously important here. The XPath 1.0
>specifications are rather terse on these matters, but the only way you
>can interpret them consistently (and the way that implementors have
>consistently interpreted them) is that all the string-matching functions
>behave as if every string (including the zero-length string) contains a
>zero-length string right at the start. This means that the example is
>wrong, and fn:substring-before("Baloney!","") should return "".
>
>Michael Kay
>
> > -----Original Message-----
> > From: public-qt-comments-request@w3.org
> > [mailto:public-qt-comments-request@w3.org] On Behalf Of Noe Michejda
> > Sent: 15 January 2004 21:05
> > To: public-qt-comments@w3.org
> > Subject: [F&O] 7.5.4 fn:substring-before
> >
> >
> >
> > F&O section 7.5.4 (definition of function fn:substring-before) states:
> >
> > "If the value of $arg2 is the zero-length string, then the
> > function returns the zero-length string."
> >
> > But later example is given:
> > "fn:substring-before("Baloney!","") returns "Baloney!". "
> >
> > Then 7.5.5 fn:substring-after states:
> >
> > "If the value of $arg2 is the zero-length string, then the
> > function returns the value of $arg1."
> >
> > I think error in definition of fn:substring-before, behavior
> > should be the same as in fn:substring-after. "If the value of
> > $arg2 is the zero-length string, then the function returns
> > the value of $arg1."
> >
> > If definition is correct and example mistaken, I think it is
> > bad idea to return zero-length string in such case.
> >
> > Best regards,
> > Noe Michejda
> > 7th Portal S.C.
> >
> >
> >

========================================================================
Jim Melton --- Editor of ISO/IEC 9075-* (SQL)     Phone: +1.801.942.0144
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=  Facts are facts.  However, any opinions expressed are the opinions  =
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Received on Friday, 16 January 2004 14:34:02 UTC