- 90 -
<
!ELEMENT
from (
#PCDATA
)>
<
!ELEMENT
content (
#PCDATA
)>
<
!ATTLIST
letter
priority
ENTITY #REQUIRED
>
]>
&Alex
&Faraz
Hi Alex. Good luck on the exam!
&Alex
&Faraz
How did the exam go?
During parsing, the parser follows all the entity references and pieces the document
together in memory so that (in memory) it now looks like this:
Alex Ceponkus, Canada
Faraz Hoodbhoy, USA
Hi Alex. Good luck on the exam!
Alex Ceponkus, Canada
Faraz Hoodbhoy, USA
How did the exam go?
The key point in this example is that we used entities that
were declared and defined
inside the DTD, in the main XML document.
Internal Parameter Entity Declaration
Parameter Internal Entity declarations are essentially useful for making your DTD more
human readable and accessible. Why should this be an important consideration? It
makes going back and changing your DTD easier (after all, who remembers what they’ve
written months ago?). The argument could be made that
you could just as easily
accomplish this with extensive commenting. Nevertheless, it is a more comprehensive
option than commenting. And like most structural things, it’s
harder to do the first time,
but makes life much easier in the long run.
Let’s say, for example, that you have a DTD of an XML document with a root element of
cars. Each car element has a product_code attribute of string
type that you later decide,
should be of type NMTOKENS (to ensure that each product_code is devoid of white
space—makes comparisons easier for the XML application.) Furthermore, let’s say that
you’ve got several attributes (say delivery_code and distributor_code), all of which are
- 91 -
supposed to have the same format (say string or NMTOKEN).
Instead of changing each
attribute from CDATA to NMTOKEN, you could assign an internal parameter entity called
code_format that is referenced each time for each attribute’s type. Similarly, let’s say you
do the same for each attribute’s default value. By changing the
internal parameter entity
statements, you instantly change the attribute type and default value for all concerned
attributes.
For example:
<
