Construction of Lexical Analyzers MCQs
What is the primary purpose of a lexical analyzer?
A) To parse syntax and generate intermediate code
B) To convert a sequence of characters into tokens
C) To optimize memory usage
D) To execute program instructions
Answer: B
Which of the following tools is commonly used to construct lexical analyzers?
A) Lex
B) Yacc
C) GCC
D) GDB
Answer: A
In Lex, what does the %{ %} section typically contain?
A) Code to be included in the generated C file
B) Regular expressions for token definitions
C) Rules for token matching
D) Commands for generating the lexical analyzer
Answer: A
What is the function of the %% symbol in a Lex specification file?
A) Marks the beginning of the rules section
B) Marks the end of the definitions section
C) Marks the end of the rules section
D) Marks the beginning of the definitions section
Answer: C
In Lex, how is a pattern defined?
A) Using regular expressions
B) Using BNF grammar
C) Using C++ classes
D) Using SQL queries
Answer: A
Which function in Lex is automatically generated to handle matched patterns?
A) yyparse()
B) yylex()
C) yyerror()
D) yywrap()
Answer: B
How does Lex handle whitespace and comments in input?
A) Whitespace and comments are ignored by default
B) They are treated as errors
C) They are converted into tokens
D) They are included in the output token stream
Answer: A
Which of the following is a correct Lex rule syntax?
A) pattern { action }
B) pattern -> action
C) action <- pattern
D) pattern : action
Answer: A
What does the yywrap() function do in Lex?
A) It is called when the end of the input file is reached
B) It parses the input file
C) It handles syntax errors
D) It generates code for the lexical analyzer
Answer: A
In Lex, which special character is used to denote the start of a regular expression pattern?
A) |
B) ^
C) *
D) /
Answer: B
What does the -t option do when running Lex?
A) It generates a test file
B) It produces output in the form of a table
C) It prints the generated tokens
D) It displays debugging information
Answer: B
Which of the following is a valid Lex pattern for matching an integer?
A) [0-9]+
B) \d+
C) \int
D) [digit]+
Answer: A
In Lex, what does the -d flag do?
A) It generates a debugging output
B) It disables error reporting
C) It defines a different input file
D) It outputs the generated C code
Answer: A
Which part of a Lex file contains C code that will be included in the generated lexical analyzer?
A) Definitions section
B) Rules section
C) User code section
D) Header section
Answer: C
In Lex, what is the purpose of the | operator in a pattern?
A) It denotes an OR condition between patterns
B) It indicates the start of a new pattern
C) It represents the end of a pattern
D) It matches a literal | character
Answer: A
Which function should be implemented to handle end-of-file (EOF) conditions in Lex?
A) yywrap()
B) yylex()
C) yyparse()
D) yyerror()
Answer: A
How can you include external C functions or variables in a Lex file?
A) By using the %{ %} section
B) By defining them in the rules section
C) By adding them to the definitions section
D) By using Lex’s built-in functions
Answer: A
What is the purpose of the Lex action code?
A) To specify what should happen when a pattern is matched
B) To define new patterns
C) To include external libraries
D) To set up the input stream
Answer: A
Which of the following is a common use case for Lex?
A) Tokenizing and parsing source code
B) Compiling source code into machine code
C) Managing database transactions
D) Performing system-level operations
Answer: A
What does the -o option do in the Lex command?
A) It specifies the output file for the generated C code
B) It overrides default settings
C) It displays output in a different format
D) It specifies the input file for Lex
Answer: A
How do you specify multiple patterns for the same action in Lex?
A) By separating patterns with the | operator
B) By listing patterns one after another
C) By using nested rules
D) By creating multiple actions
Answer: A
What is the output of Lex when it processes a specification file?
A) A C source file that implements the lexical analyzer
B) A compiled executable for lexical analysis
C) A token list in plain text
D) A debug log of the lexical analysis
Answer: A
Which function in Lex is used to report syntax errors?
A) yyerror()
B) yywrap()
C) yylex()
D) yyparse()
Answer: A
How do you define a macro in Lex?
A) Using the %define directive
B) By including it in the definitions section
C) By using #define in the %{ %} section
D) By writing it directly in the rules section
Answer: C
What does the yytext variable represent in Lex?
A) The text matched by the most recent pattern
B) The total length of the input text
C) The current line number in the input
D) The next token to be processed
Answer: A
In Lex, which special variable holds the length of the matched text?
A) yyleng
B) yytext
C) yyinput
D) yychar
Answer: A
What is the function of the <<EOF>> pattern in Lex?
A) It matches the end of the input file
B) It denotes the start of a new pattern
C) It handles input errors
D) It specifies the end of a pattern list
Answer: A
Which of the following is NOT a valid Lex pattern?
A) [a-zA-Z]+
B) \d{2,}
C) [0-9]{2,4}
D) \int
Answer: D
In Lex, how do you specify a pattern to match a single character?
A) By using a regular expression like . (dot)
B) By specifying the character directly
C) By using a pattern with {1}
D) By defining it in the rules section
Answer: A
Which section of a Lex file is used to define regular expressions?
A) Definitions section
B) Rules section
C) User code section
D) Header section
Answer: A
What is the purpose of the -l option when running Lex?
A) It generates a Lex library
B) It produces a listing of the generated C code
C) It specifies the input file for Lex
D) It defines the output format
Answer: B
In Lex, how is an action block defined?
A) Enclosed in { } braces
B) Within parentheses ( )
C) As a separate function
D) Using << >> delimiters
Answer: A
What does the -s option do in Lex?
A) It generates a summary report
B) It strips whitespace from the input
C) It suppresses the generation of a table of patterns
D) It specifies the input file for Lex
Answer: C
What is the function of the yylval variable in Lex?
A) To store the value of the matched token
B) To hold the length of the matched token
C) To indicate the current line number
D) To manage the state of the input stream
Answer: A
How do you define a pattern to match a specific string in Lex?
A) By using the string directly in the pattern
B) By defining it as a regular expression
C) By creating a macro for the string
D) By using a predefined pattern
Answer: A
What is the primary role of the yyinput() function in Lex?
A) To read the next character from the input stream
B) To initialize the Lex environment
C) To handle syntax errors
D) To manage the token stream
Answer: A
Which Lex directive is used to include additional C code in the generated file?
A) %{ %}
B) %include
C) %code
D) %extra
Answer: A
What does the -v option do in Lex?
A) It generates verbose output of the lexical analyzer
B) It specifies a version of Lex to use
C) It validates the syntax of the Lex file
D) It sets the verbosity of the error messages
Answer: A
How can Lex be used in combination with Yacc?
A) Lex generates tokens which are then parsed by Yacc
B) Yacc generates tokens which are then analyzed by Lex
C) Lex and Yacc are used independently for different tasks
D) Lex and Yacc share the same codebase
Answer: A
Which variable is used in Lex to access the current token’s type?
A) yytok
B) yylex
C) yychar
D) yytoken
Answer: C
In Lex, what does the <<EOF>> pattern signify?
A) It matches the end of the file
B) It marks the end of the pattern list
C) It signals the start of a new input block
D) It defines a new token type
Answer: A
How do you specify an optional pattern in Lex?
A) By using parentheses ( ) around the pattern
B) By using the ? operator after the pattern
C) By using the * operator for zero or more occurrences
D) By listing the pattern in an alternative rule
Answer: B
What does the yyless(n) function do in Lex?
A) It sets the current token to be of length n
B) It discards the first n characters of the current token
C) It sets the length of the current token to n
D) It returns the length of the current token
Answer: B
Which of the following is a valid use of the ^ operator in Lex?
A) To match the start of a line
B) To denote the end of a pattern
C) To specify a case-insensitive match
D) To match the end of a line
Answer: A
How does Lex handle overlapping patterns?
A) It uses the longest matching pattern
B) It uses the first matching pattern
C) It merges overlapping patterns
D) It reports an error for overlapping patterns
Answer: A
What does the %option directive do in Lex?
A) It sets options for the lexical analyzer’s behavior
B) It specifies the input file options
C) It configures the output file settings
D) It defines the pattern matching options
Answer: A
In Lex, how is the yytext variable used?
A) To store the matched text from the input stream
B) To indicate the end of the input stream
C) To manage the state of the lexical analyzer
D) To handle syntax errors
Answer: A
What does the -p option do when using Lex?
A) It specifies a prefix for generated identifiers
B) It sets the parser output format
C) It defines the pattern for token matching
D) It specifies the input file for the lexer
Answer: A
Which Lex function is used to initialize the lexical analyzer?
A) yyinit()
B) yylex()
C) yyparse()
D) yysetup()
Answer: B
How do you ensure that Lex generates a specific output file name?
A) By using the -o option followed by the desired file name
B) By specifying the file name in the Lex file header
C) By renaming the output file manually after generation
D) By setting the output file name in the Lex configuration
Answer: A