CS320/08 Expressions and Assignment Statements
- expression::= simple_expression #( operator simple_expression),
- operator::= "+" | "*".
But this does not show the relative precedence of "+" and "*".
We show the precedence of "*" and "+" in algebra by replacing the above by:
- expression::= term #( "+" term),
- term::= simple_expression #( "*" simple_expression).
To express associativity we can replace the '#' by recursion. For example:
- expression::= term | expression "+" term, -- left associative.
- term::= simple_expression| simple_expression "*" term, -- right
associative.
Warning: it is unusual(and confusing) to have different associativities for multiplication and addition in a real language.
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Preparation
Study chapter 7. This chapter is about expressions and assignments. See the UML diagram.
Answer review questions on chapter 7.
Hand in answers to 3 or more review questions.
Note 1: Expressions are not Statements
Be careful to distinguish Expressions from Statements. Expressions are evaluated and return Values. Statements are executed and return a new State. Most languages have both.
Note 2: Precedence and Associativity
You may want to express precedence and associativity in your project.
These are syntax not semantics. Express precedence and associativity in BNF/EBNF/XBNF not in the UML. It takes some extra definitions. See sections 3.3.1.7, 3.3.1.8 and 3.3.1.9 in the book.
For example, suppose we might define