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Program semantics
- The meaning of:
- - Expressions
- - Statements
- - Program units
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Program Syntax
Forms and structures of constructs
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How is a programming language defined?
- Language syntax
- Language semantics
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Lexeme
- Lowest syntactic unit of a language
- Described and checked by lexical analyzer
- E.g +, *, =, etc...
- Lexemes are converted to tokens when the code is tokenized
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Tokens
- Categorize lexemes from a program
- E.g new, int, <identifiers>, etc...
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Meta language
Language used to describe another language
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Standard form for describing programming language syntax
Backus-Naur form
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Describe an assignment statement in BNF
<Assignment_Statement> → <var> = <expression>
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Examples of terminals in BNF
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Nonterminal symbols (BNF)
- Symbols that can be replaced
- In the expression,
- <assignment>→<var>=<expr>
- The LHS <assignment> is nonterminal
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Put the following processes in order:
Lexical analyzer
Syntax analyzer
Tokenizer
Parse tree
Code Generator
Assembler
Linker
- Tokenizer
- Syntax analyzer
- Parse tree
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What is a keyword
- Keywords have a special (contextual) meaning
- They can be used as idenfitiers
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What is a reserved word
- Reserved words are reserved
- They cannot be used as identifiers
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BNF for IF statement
<if_statement>→if<expr> then <stmt>
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BNF for IF-ELSE statement
<if_statement> → if <expr> then <statement> else <statement>
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BNF for a list
- <identifier_list>→<identifier>|<identifier_list>→<identifier>,<identifier_list>
- The | is an OR (choose the appropriate definition)
- Recursive
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When is a grammar ambiguous
If two parse trees can be generated from it
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Types of program semantics
- Operational - Compare difference between machine states
- Axiomatic - Define axioms for each statement type
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Conditions in axiomic semantics
- Precondition
- Postcondition
- Weakest precondition
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Precondition
- Before a statement
- Describes relationships and constraints of variables
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Postcondition
- Occurs after a statement
- True after the statement is executed
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Weakest precondition
The least restrictive precondition that garuntees the post condition
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