Internal parameter entity

An entity is a part of a document rather than a DTD. Generic object references can only be used in the positions where the document can be a part of the document in the DTD. generic object references cannot be used to insert texts that are only DTD rather than document content. However, entity references in DTD are usually useful. Therefore, XML provides a way to reference parameter entities.
apart from the following two key points, parameter entity references are very similar to common entity references.
(1) the parameter object reference starts with a percent sign (%) instead of an ampersand.
(2) parameter entity reference can only appear in DTD, but not in document content.
the parameter entity reference method declared in the DTD is similar to that of a common object, but the one hundred Semicolon is added before the object name. The syntax structure is as follows:

the abbreviation of an object name is entity content.


when the parameter entity reference is used to replace the generic Entity reference, the abbreviation (# pcdata) that cannot be implemented in the previous article becomes valid.



the real value of parameter entity reference is displayed in the common list of sub-elements and attribute sharing between elements. If the text block to be replaced is large and used frequently, the reference of the just-parameter object is more useful. For example, assume that the DTD Declaration contains many layered elements, such as paragraph, cell, and heading. Each element has a definite number of inline elements, such as internal elements such as person, degree, model, product, animal, and ingredient. The declaration of these elements may be expressed as follows:

1 <! Element praragraph (person | degree | model | product | antimal | ingredient )* >
2 <! Element cell (person | degree | model | product | antimal | ingredient )* >
3 <! Element heading (person | degree | model | product | antimal | ingredient )* >
4

The container elements have the same settings. Assume that a new element, such as equation, CD, or account, must be declared as a child element of all three elements. If you add a new element to the first two elements, but you forget to add it to the first two elements, this will cause a problem. If the number of elements is 30 or 300, but not 3, the problem will multiply.
If a separate sub-element list is not provided for each container element, it is easier to maintain the new DTD. The alternative method is to change the child element list to a parameter object reference, and then apply the parameter object reference to each element. For example:

1 <! Element % inlines "(person | dergee | model | product | antimal | ingredient )*" >
2 <! Element paragraph % inlines; >
3 <! Element cell % inlines; >
4 <! Element heading % inlines; >
5

When adding a new element, you only need to change a parameter object declaration without changing the three element declarations.