Outline of Class 8: The Deadly Conditional Grammar

Held: Wednesday, February 4, 1998

Administrivia

• This Thursday at 3:15, Mike Jipping from Hope college will be speaking on summer research opportunities at Hope college.
• This Thursday at 4:15, selected math and CS faculty will be discussing summer research opportunities. Come hear how you can get both money and credit while participating in interesting projects.
• This Thursday at 7:30, Mike Jipping from Hope college will be speaking on threads in Java. His talk is based in part on work done with undergraduates as part of Hope's REU program.
• Once again, we'll begin today's class with a quiz. The quiz won't be available until class time.
• We may spend some time today on the conditional grammar from the previous class.
• We'll only spend a little time on attribute grammars as they are more appropriately covered in compilers.

A Grammar of Conditional Statements

• Are there common ambiguous grammars? Yes, most people's first attempt at defining a grammar of conditionals ("if statements") is incorrect.
• We'll assume that there are only two kinds of statements:
  • conditional statements
  • everything else (which we'll represent by a terminal)
• What might we use for a grammar of statements?

  Statement ::= Conditional | 'whatever'
  Conditional ::= 'if' test 'then' Statement 'else' Statement
  Conditional ::= 'if' test 'then' Statement
  

• Is this ambiguous? Yes, consider

  if test then if test then whatever else whatever
  

• What do we do?
  • Draw the different trees.
  • Identify which one is "correct" and which one is "incorrect"
  • Identify the problem with the incorrect tree.
  • Update the grammar to fix that problem, possibly creating new symbols.
• We'll do this together.