| ITEC 380 |
| 2009spring |
| ibarland |
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- grammar:
+ terminology: "nonterminal", "production", etc.
+ Recall: From book, p.124 (Example 3.3):
A grammar for arithmetic expressions:
AExpr ::= AExpr + AExpr
| AExpr * AExpr
| (AExpr)
| number
| variable
(We also saw derivations, and parse trees.)
As we saw, this was ambiguous.
+ Here is another grammar for Expr, but this is not ambiguous:
(book example 3.4 p.126)
Expr ::= Expr + Term
| Term
Term ::= Term * Factor
::= Factor
Factor ::= (Expr)
| number
| variable
Make a parse tree for each of
3 + 4 * 5
3 * 4 + 5
Not only is this non-ambiguous, but it has hacked the precedence
rules (semantics) into the syntax-layer.
There are algorithms to tell if simple (context-free) grammars
are non-ambiguous; however, note that in general, context-sensitive
grammars (where the left-hand-side of a production contains
multiple symbols) is is NOT COMPUTABLE whether the grammar is ambiguous.
+ What about a grammar for scheme's arithmetic -- it's very similar
to AExpr. Is that ambiguous? Why not -- what's the difference?
+ A grammar for XML
- programs that handle grammars
Develop XML code (and show)
examples of an internalized form
add a method: toString. We'll make the string "happen" to look
just like the raw input would.
add a method: realLength (the number of *non-markup* characters)
add another class: XEntity
==== For next week:
- writing XML.parse
- map, filter, fold as applied to fire-planes [map-reduce; fold in Perl]
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| ©2009, Ian Barland, Radford University Last modified 2009.Mar.30 (Mon) |
Please mail any suggestions (incl. typos, broken links) to iba�rland  rad�ford.edu |
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