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

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Parse & Syntax Trees Syntax & Semantic Errors Mini‐Lecture

CSC 4181 Compiler Construction

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

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Parse Tree

3 expression Assign‐expression expression = expression Subscript‐expression Additive‐expressive expression [ expression ] expression expression Identifier a Identifier index Number 4 Number 2

• Expression: a[index] = 4 + 2

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Syntax Tree

• Also called “Abstract Syntax Tree” or AST
• Expression: a[index] = 4 + 2
• Condensed version of Parse Tree
• Excludes redundant information

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Syntax Tree

• Expression: a[index] = 4 + 2

5 Assign‐expression Subscript‐expression Additive‐expression Identifier a Identifier index Number 4 Number 2

Syntax Errors

• Syntax error ‐ an error in the syntax (the rules
• f 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

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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");

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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.

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Semantic Errors

• Non‐initialized variable:

int i; i++;

• Type incompatibility:

int a = "hello";

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