COMPILER and its application

Q. What are the phases of analysis and describe them.

Ans: Analysis of the source program: In compiling, analysis consists of three phases: --

• Linear analysis (Lexical Analysis)


• Hierarchical analysis (Syntax Analysis)


• Semantic analysis

Linear (Lexical) analysis: In a compiler, linear analysis is called lexical analysis or scanning. In this case, the stream of characters which make the source program is read from left to right and generates the tokens which are sequences of characters having a collective meaning.

Example: In lexical analysis, the characters in the assignment statement

position:= initial + rate * 60

would be grouped into the following token ---

• The identifier position


• The assignment symbol :=


• The identifier initial


• The plus sign


• The identifier rate


• The multiplication sign


• The number 60

The blanks separating the characters of these tokens would normally be eliminated during lexical analysis.

Hierarchical analysis / Parsing / Syntax analysis: Hierarchical analysis is called parsing or syntax analysis. In this case the tokens of the source program are grouped hierarchically into grammatical parses that are used by the compiler to synthesize output. Usually, the grammatical phrases of the source program are represented by a parse tree as follows: --

The hierarchical structure of a program is usually expressed by recursive rules. The rules of expression are as follows:--

• Any identification is an expression.


• Any number is an expression


• If expression1 and expression2 are expressions then so are

expression1 + expression2

expression1 * expression2

(expression1)

Semantic analysis: The semantic analysis phase checks the source program for semantic errors and gathers type information for the subsequent code – generation phase. It uses the hierarchical structure determined by the syntax analysis phase to identify the operators and operands of expressions and statements. An important component of semantic analysis is type checking.

Q. What is symbol table? What are the functions of symbol table?

Ans: Symbol table: A symbol table is a data structure containing a record for each identifier with fields for the attributes of the identifier. It is generally used to store information about various source language constructs. The information is collected by the analysis phases of the compiler and used by the synthesis phases to generate the target code.

Q. Derive the parse tree from the statement “position: = initial + rate * 60”.

Ans: At first the defined statement should be grouped into tokens as above and then derive the parse tree as follows: --