Parse & Syntax Trees Syntax & Semantic Errors Mini Lecture - PDF document

Parse & Syntax Trees Syntax & Semantic Errors Mini ‐ Lecture CSC 4181 Compiler Construction 1 Parse Tree • Expression: a[index] = 4 + 2 • Identify tokens: a identifier [ left bracket index identifier ] right bracket = assignment operator 4 number + plus sign 2 number 2 2 1

Parse Tree • Expression: a[index] = 4 + 2 expression Assign ‐ expression expression = expression Subscript ‐ expression Additive ‐ expressive expression expression expression + [ expression ] Number 4 Number 2 Identifier a Identifier index 3 3 Syntax Tree • Also called “Abstract Syntax Tree” or AST • Expression: a[index] = 4 + 2 • Condensed version of Parse Tree • Excludes redundant information 4 4 2

Syntax Tree • Expression: a[index] = 4 + 2 Assign ‐ expression Subscript ‐ expression Additive ‐ expression Number Identifier Identifier Number 4 a index 2 5 5 Syntax Errors • Syntax error ‐ an error in the syntax (the rules of formation) of a sequence of characters or tokens that is intended to be written in a particular programming language. • Example is: entering an invalid equation into a calculator, such as opening brackets without closing them, or several decimal points in a number 6 6 3

Syntax Errors • In Java, the following is a syntactically correct statement: System.out.println("Hello World"); • while the following is not: System.out.+println("Hello World"); 7 7 Semantic Errors • Semantic error: Writing a valid programming structure with invalid logic. • The compiler will generate instructions that the computer will execute, because it understands the syntax of the programming statements, but the output will not be correct. 8 8 4

Semantic Errors • Non ‐ initialized variable: int i; i++; • Type incompatibility: int a = "hello"; 9 9 5