[xsl] Answers to review questions in "Beginning XSLT": Chapter 5

["Lars Huttar" <lars_huttar@xxxxxxx>]

OK, here are my answers for review questions from Chapter 5.
Intermingled are questions that I still have.  If you have comments,
I'd be grateful for the feedback.

For now I'm omitting the questions themselves, since I don't yet have
explicit permission to copy them.  (Jeni, would that be OK?)

Answers:

1. No nodes match the select pattern, so no templates get applied,
   so there is no output for that apply-templates.

2. Use one template with match="Description//Film", and another with
   match="Channel/Film".  Or use match="Film" for one of them, and
   the more specific match for the other.

3. This code produces a <div> element with contents as follows:
   if there are Film children of the current node, they are processed
   with xsl:apply-templates; otherwise, the message "No films..."
   is included.

4. Rewriting of preceding code using <xsl:choose> instead of <xsl:if>:

  <div>
    <xsl:choose>
	 <xsl:when test="Film">
        <xsl:apply-templates select="Film" />
      </xsl:when>
      <xsl:otherwise>
        No films showing on this channel.
      </xsl:otherwise>
    </xsl:choose>
  </div>

    The advantage of the latter form is that if there are no Film
    children of the current node, <xsl:apply-templates> doesn't get
    evaluated.  Also, it may be a little more clear to the human reader
    that there are two mutually exclusive possibilities for the <div>
    elements contents: the results of applying templates to "Film" children,
    and the "No films..." message.

    The advantage of the <xsl:if> form is that it is shorter, and for
    that reason perhaps easier to read.

5. The given code inserts a 20x20 icon iff there is a flag attribute
   on the current node.  There is a specific icon for when the value
   of the flag attribute is 'favorite', and another for when it is
   'interesting', and a third for any other value.  If the flag attribute
   doesn't exist, no icon is inserted.

6. Rewriting of the above "as a sequence of <xsl:if> elements" instead of
   using <xsl:choose>:  (One could take "sequence" to mean "no nesting",
   but that would hobble the <xsl:if> and I don't think that was the
   intent of the question, so I'm going to nest them.)

    <xsl:if test="@flag">
      <xsl:if test="@flag = 'favorite'">
        <img src="favorite.gif" alt="[Favorite]" width="20" height="20" />
      </xsl:if>
      <xsl:if test="@flag = 'interesting'">
        <img src="interest.gif" alt="[Interest]" width="20" height="20" />
      </xsl:if>
      <xsl:if test="@flag != 'favorite' and @flag != 'interesting'">
        <img src="flag.gif" alt="[Flagged]" width="20" height="20" />
      </xsl:if>
    </xsl:if>

   The disadvantage of the <xsl:if> form is that if the flag attribute
   exists, all three of the inner tests must be evaluated; whereas
   in an <xsl:choose> element, when one test succeeds, the following
   ones are ignored.  Thus there is no need to repeat the comparisons
   against 'favorite' and 'interesting'.

   The advantage of the <xsl:if> form is that it is shorter.  In this
   case I've wrapped the comparisons of the value of @flag within a
   test for the flag attribute's existence, thus saving work if the
   attribute doesn't exist.  This could also be done with a nested
   <xsl:choose>, but would be more bulky because it would require
   two nested elements, <xsl:choose> and <xsl:when>, as opposed to one
   <xsl:if>.

7. test="Film/@year > 1950"

8. Film[starts-with(Title, 'Romeo')]

9. test="number()"
   This converts the current node's value to a number, which is then
   converted to true if non-zero, false if zero or NaN.

10. To test whether the current node's value is a number,
   use test="number() or number() = 0" which gives the desired result.
   Is there a simpler way?

   I tried test="number() != NaN" but this was always false (Why?  Is
     NaN not recognized as the numeric constant?  No, I just learned that
     it's because NaN is not equal to itself!  See the FAQ,
     http://www.dpawson.co.uk/xsl/sect2/N5846.html);

   test="number() != 'NaN'" was always true;

   test="number() != number('NaN')" also was always true (see above,
     NaN is not equal to itself);

   test="string(number()) != 'NaN'" worked but this is not necessarily
    simpler.  (Btw I see from the FAQ that this is a standard way
    to check whether a node's value is a number.  There Jeni Tennison
    also suggests test="@value &lt;= 0 or @value > 0" or
    test="number(@value) = number(@value)".  However, these both rely
    on properties of NaN that are not covered in ch. 5.)

   Question: is there a way to represent the numeric constant NaN
   in an expression?  Apparently the unquoted NaN is not recognized
   as such; I guess it's interpreted as "the value of the child element
   named NaN."

OK, that's chapter 5.  Thanks for your time and attention.
Feedback is appreciated.

Lars

> -----Original Message-----
> From: owner-xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> [mailto:owner-xsl-list@xxxxxxxxxxxxxxxxxxxxxx]On Behalf Of
> Jeni Tennison
> Sent: Tuesday, March 11, 2003 5:16 AM
> To: Lars Huttar
> Cc: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [xsl] Answers to review questions in "Beginning XSLT"?
>
>
> Hi Lars,
>
> >I've been working through Jeni Tennison's book, "Beginning XSLT". As
> >I answer the review questions at the end of each chapter, it would be
> >comforting to compare them with someone else's to make sure I hadn't
> >missed something important.
> >
> >Does anybody have a collection of answers they've put together that
> >they'd be willing to share?
> >
> >Anyone know of a forum or a web site anywhere where these answers are
> >posted and discussed? Seems like it would be a useful thing to have.
>
> I think that you should feel free to post here, especially if there
> are questions that you're not sure about or want to discuss the answer
> to. Or there's the XSLT list on the p2p.wrox.com site; it looks like
> Mike Kay answers a lot of queries there.
>
> Cheers,
>
> Jeni
>
> ---
> Jeni Tennison
> http://www.jenitennison.com/
>
>
>  XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list
>


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