Re: Last tango in Paris

[Nigel S. <nig...@us...>]

(Really old thread alert...)

> This is not the same thing. This is the last employee in each parent node
if
> male (or in my example the last of each tango which is in Paris, i.e. all
> tangos in Paris). What I want is the last instance of all male employees,
> i.e.
>
> //employee[@gender='m'][last()]
>
> should find a1010 and gender = 'f' should find a1006.
>
> Incidentally, if I try
>
> //employee[(@gender='m') and (last())]
>
> I get a different answer to your example. What is this doing? 'Position()
=
> last()' and 'last()' should surely be identical.

    //employee[(@gender='m') and (last())]

And

    //employee[(@gender='m') and (position() = last())]

Are NOT the same.  Because of the "and" operator.  what happens is that
last() returns a number.  The "and" takes two operands that need to be
bools, so it has to convert the number from last() into a bool.  If you
think about it, last() always returns a non zero number, which always
converts to TRUE.

So for example I got confused by a test that I had that was //*[1 and
last()] and was surprised to see that this returned different results to
//*[1 and (position() = last())].  //*[1 and last()] actually matched EVERY
node in the document.  This is because:

http://www.w3.org/TR/xpath#section-Node-Set-Functions
"The last function returns a number equal to the context size from the
expression evaluation context.The last function returns a number equal to
the context size from the expression evaluation context."

So last returns a non zero number, because context size >= 1.  When you
convert a non zero number to a bool, you get TRUE.  So basically I was
doing:  //*[1 and TRUE].  So now it's not really surprising that it returned
all the nodes.

On the other hand, //*[1 and (position() = last())] must now evaluate the
"position() = last()" as a bool, which only returns TRUE for the last node
in the context.  And hence //*[1 and (position() = last())] is the same as
//*[(position() = last())] is the same as //*[last()], but is not the same
as //*[1 and last()].

Seems a bit bizzare....but means we have less bugs than we'd thought :o)

Nigel