Re: Understanding Identity Transformations

What Mike is explaining here is something crucial.

Not understanding the difference in the behaviour on node-sets of the
"!="operator and the not() function causes enormous confusion to a
lots of XSLT programmers.

One of the most valuable advice from even the first edition of "XSLT
Programmer's Reference" is always to try to use "not()" and to use
"!=" only when it is really necessary.


Cheers,
Dimitre Novatchev


On Sat, 12 Feb 2005 22:00:49 -0000, Michael Kay <mike@xxxxxxxxxxxx> wrote:
> > > Wow.. that is easy except that I do not understand your notice
> > > explanation... the difference between your not( ... ) and your !=.
> >
> > "@cat != 'BLUE'": will be true if the context node has a
> > 'cat' attribute
> > *and* if it has a 'cat' attribute not equal to 'BLUE'. So
> > it's the same as
> > "boolean(@cat) and not(@cat='BLUE')"
> >
> > "not(@cat='BLUE')": will be true if the context node has no 'cat'
> > attribute *or* if it has a 'cat' attribute not equal to
> > 'BLUE'. So it's
> > equal to "not(@cat) or not(@cat='BLUE')"
> 
> Let's try to phrase it a different way. An expression such as X=Y in XPath
> is shorthand for
> 
> some $x in X, $y in Y satisfies $x=$y
> 
> in other words, it's true if there's some pair of values from the two sets
> that are equal.
> 
> Similarly, X!=Y is short for
> 
> some $x in X, $y in Y satisfies $x!=$y
> 
> which is true if there's some pair that are not equal.
> 
> This means that if X is an empty set, then X=3 and X!=3 must both be false.
> If you're testing an attribute, @A=3, then @A is a set that's either empty
> or contains one node. If there's no A attribute, then @A!=3 is false
> (because there's no A that's not equal to 3), but not(@A=3) is true (because
> it's not true that there's an A that's equal to 3).
> 
> Michael Kay
> http://www.saxonica.com/