Hi Peter,

Thanks for the reply.

The thing that was confusing me was that why would we insert a
predicate that starts with / or // in a specifiec location in the
XPath query. Looks like _generally_ they can be anywhere in the
query and yet we will get the same result.

So

//a/b/*/c//d[/a/b/d][/a//b/g][/a/b//g] is equal to
//a[/a/b/d]/b[/a//b/g]/*/c//d[/a/b//g] is equal to
//a[/a/b/d][/a//b/g][/a/b//g]/b/*/c//d

with possible performance differences.

Regards,
Al


On Oct 7, 4:58 am, Peter Flynn <peter.n...@m.silmaril.ie> wrote:
> Altu wrote:
> > Hi all,
>
> > What does something like [/a/b] mean
>
> It creates a dependency for the preceding object reference.
> In this case it specifies that there must exist an element b which is a
> child of the root element a. This condition will be applied to whatever
> precedes the [/a/b] in the statement
>
> > in an XPath query like /c/d[/a/b] ?
>
> This will return or process a child element d of the root element c
> provided that a child element b exists of the root element a. As an XML
> document can only have one root element, this condition can never be
> satisfied, so the statement will always return null or false.
>
> > If I am not wrong, /c/d[./a/b] means return all d elements that their
> > parent is c and their grand parent is the root element and have a
> > child that is a and their a child has a child b.
>
> That's not the query you posted at the start of your message.
> The period introduces a change in scope, so that ./a/b now refers
> to the structure you describe correctly.
>
> > But I don't know when
> > we don't use an expression that starts with ./ or .// in a predicate
> > and instead use an epxression that starts with / or // what it would
> > mean.
>
> Exactly the same as you would when making a directory reference in a
> typed command, eg
>     cd /var/www/html
>     rm -rf /home/peter
> will delete all my personal files irrespective of my current directory.
>     cd /var/www/html
>     rm -rf ./home/peter
> will delete any files in /var/www/html/home/peter
>
> > Does /c/d[/a], for example, mean return all d elements that have a
> > parent (c with a parent root) only if the root has a child a?
>
> Yes, which is an impossibility for the reason already given.
>
> ///Peter