Important Short Questions and Answers : Principles of Compiler Design - Lexical Analysis

1. What is a Complier?

A Complier is a program that reads a program written in one language-the source language-and translates it in to an equivalent program in another language-the target language . As an important part of this translation process, the compiler reports to its user the presence of errors in the source program

2. State some software tools that manipulate source program?

i. Structure editors

ii. Pretty printers

iii. Static checkers

iv. Interpreters.

3. What are the cousins of compiler?

The following are the cousins of compilers

i. Preprocessors

ii. Assemblers

iii. Loaders

iv. Link editors.

4. What are the main two parts of compilation? What are they performingThe two main parts are

 Analysis part breaks up the source program into constituent pieces and creates an intermediate representation of the source program.

·     Synthesis part constructs the desired target program from the intermediate representation

5. What is a Structure editor?

A structure editor takes as input a sequence of commands to build a source program .The structure editor not only performs the text creation and modification functions of an ordinary text editor but it also analyzes the program text putting an appropriate hierarchical structure on the source program.

6. What are a Pretty Printer and Static Checker?

·     A Pretty printer analyses a program and prints it in such a way that the structure of the program becomes clearly visible.

·     A static checker reads a program, analyses it and attempts to discover potential bugs with out running the program.

7. How many phases does analysis consists?

Analysis consists of three phases

i .Linear analysis

ii .Hierarchical analysis

iii. Semantic analysis

8. What happens in linear analysis?

This is the phase in which the stream of characters making up the source program is read from left to right and grouped in to tokens that are sequences of characters having collective meaning.

9. What happens in Hierarchical analysis?

This is the phase in which characters or tokens are grouped hierarchically in to nested collections with collective meaning.

10. What happens in Semantic analysis?

This is the phase in which certain checks are performed to ensure that the components of a program fit together meaningfully.

11.State some compiler construction tools?

i.Parse Scanner generators

ii.   Syntax-directed translation engines

iii.Automatic code generator

v. Data flow engines.

12. What is a Loader? What does the loading process do?

A Loader is a program that performs the two functions

i. Loading

ii .Link editing

The process of loading consists of taking relocatable machine code, altering the relocatable address and placing the altered instructions and data in memory at the proper locations.

13. What does the Link Editing does?

Link editing: This allows us to make a single program from several files of relocatable machine code. These files may have been the result of several compilations, and one or more may be library files of routines provided by the system and available to any program that needs them.

14. What is a preprocessor? Nov/Dev 2004

A preprocessor is one, which produces input to compilers. A source program may be divided into modules stored in separate files. The task of collecting the source program is sometimes entrusted to a distinct program called a preprocessor.

The preprocessor may also expand macros into source language statements.

15.            State some functions of Preprocessors

i)                   Macro processing

ii)                File inclusion

iii)              Relational Preprocessors

iv)              Language extensions

16.            What is a Symbol table?

A Symbol table is a data structure containing a record for each identifier, with fields for the attributes of the identifier. The data structure allows us to find the record for each identifier quickly and to store or retrieve data from that record quickly.

17.            State the general phases of a compiler

i)                   Lexical analysis

ii)                  Syntax analysis

iii)              Semantic analysis

iv)              Intermediate code generation

v)                Code optimization

vi)              Code generation

18.            What is an assembler?

Assembler is a program, which converts the source language in to assembly language.

19.            What is the need for separating the analysis phase into lexical analysis and parsing? (Or) What are the issues of lexical analyzer?

·     Simpler design is perhaps the most important consideration. The separation of lexical analysis from syntax analysis often allows us to simplify one or the other of these phases.

·     Compiler efficiency is improved.

·     Compiler portability is enhanced.

20.            What is Lexical Analysis?

The first phase of compiler is Lexical Analysis. This is also known as linear analysis in which the stream of characters making up the source program is read from left-to-right and grouped into tokens that are sequences of characters having a collective meaning.

21.            What is a lexeme? Define a regular set.

·     A Lexeme is a sequence of characters in the source program that is matched by the pattern for a token.

·     A language denoted by a regular expression is said to be a regular set

22.            What is a sentinel? What is its usage?

A Sentinel is a special character that cannot be part of the source program. Normally we use ‘eof’ as the sentinel. This is used for speeding-up the lexical analyzer.

23. What is a regular expression? State the rules, which define regular expression?

Regular expression is a method to describe regular language

Rules:

1) ɛ-is a regular expression that denotes { ɛ } that is the set containing the empty string

2) If a is a symbol in ∑,then a is a regular expression that denotes {a}

3)    Suppose r and s are regular expressions denoting the languages L(r ) and L(s) Then,

a)     (r )/(s) is a regular expression denoting L(r) U L(s).

b)    (r )(s) is a regular expression denoting L(r )L(s)

c)     (r )* is a regular expression denoting L(r)*.

d)    (r) is a regular expression denoting L(r ).

24.            What are the Error-recovery actions in a lexical analyzer?

1. Deleting an extraneous character

2. Inserting a missing character

3. Replacing an incorrect character by a correct character

4. Transposing two adjacent characters

25.            Construct Regular expression for the language

L= {w E{a,b}/w ends in abb} Ans: {a/b}*abb.

26. What is recognizer?

Recognizers are machines. These are the machines which accept the strings belonging to certain language. If the valid strings of such language are accepted by the machine then it is said that the corresponding language is accepted by that machine, otherwise it is rejected.