Podcast
Questions and Answers
What is the approach used to recognize the longest possible string in a lexical analyzer?
What is the approach used to recognize the longest possible string in a lexical analyzer?
- Lookahead technique (correct)
- Top-Down parsing
- Dynamic Programming
- Backtracking algorithm
What is the purpose of minimizing a DFA in compiler design?
What is the purpose of minimizing a DFA in compiler design?
- To reduce the number of states in the DFA (correct)
- To simplify the syntax of the language
- To optimize the parser
- To increase the number of states in the DFA
What is the primary function of a lexical analyzer in a compiler?
What is the primary function of a lexical analyzer in a compiler?
- To recognize tokens in the source code (correct)
- To parse the source code
- To generate machine code
- To optimize the code
How does a lexical analyzer typically handle comments in the source code?
How does a lexical analyzer typically handle comments in the source code?
What is the purpose of partitioning in minimizing a DFA?
What is the purpose of partitioning in minimizing a DFA?
What is the purpose of lookahead in a lexical analyzer?
What is the purpose of lookahead in a lexical analyzer?
What is the language denoted by a regular expression called as?
What is the language denoted by a regular expression called as?
What is the result of concatenating the empty string to a string s?
What is the result of concatenating the empty string to a string s?
What is the Kleene closure of a language L?
What is the Kleene closure of a language L?
What is the result of the positive closure of a language L?
What is the result of the positive closure of a language L?
What is the operation that combines two languages L1 and L2 to form a new language that contains all possible concatenations of strings from L1 and L2?
What is the operation that combines two languages L1 and L2 to form a new language that contains all possible concatenations of strings from L1 and L2?
What is the set of all possible identifiers considered in the context of languages?
What is the set of all possible identifiers considered in the context of languages?
What is the purpose of the function nullable(n) in lexical analysis?
What is the purpose of the function nullable(n) in lexical analysis?
What is the set of positions of the last symbols of strings generated by the sub-expression rooted by n called?
What is the set of positions of the last symbols of strings generated by the sub-expression rooted by n called?
How are the followpos of each position computed in the syntax tree?
How are the followpos of each position computed in the syntax tree?
What type of node in the syntax tree has a child node that is a concatenation node?
What type of node in the syntax tree has a child node that is a concatenation node?
What is the purpose of the function firstpos(n) in lexical analysis?
What is the purpose of the function firstpos(n) in lexical analysis?
What is the two-rule definition for the function followpos?
What is the two-rule definition for the function followpos?
Study Notes
Minimizing DFA
- Given two groups: G1 = {2} and G2 = {1,3}, G2 cannot be partitioned because move(1,a)=2 and move(3,a)=2
- Minimized DFA: {1,3} and {2}
- Example 36: groups {1,2} and {3,4}, minimized DFA: {1,2,3} and {4}
Lexical Analyzer
- Recognizes the longest possible string
- Issues:
- What is the end of a token?
- No character marks the end of a token
- Lookahead is needed
- Skipping comments:
- Normally, comments are not returned as tokens
- Lexical analyzer skips comments and returns the next token to the parser
Terminology of Languages
- Alphabet: a finite set of symbols (ASCII characters)
- String: a finite sequence of symbols on an alphabet
- Language: a set of strings over some fixed alphabet
- Operators on Strings:
- Concatenation: xy represents the concatenation of strings x and y
- |s| is the length of string s
- Examples:
- The set of well-formed C programs is a language
- The set of all possible identifiers is a language
Operations on Languages
- Concatenation: L1L2 = {s1s2 | s1 ∈ L1 and s2 ∈ L2}
- Union: L1 ∪ L2 = {s | s ∈ L1 or s ∈ L2}
- Exponentiation: L0 = {ε}, L1 = L
- Kleene Closure: L* = ⋃Li i=0
- Positive Closure: L+ = ⋃Li i=1
- Example: L1 = {a,b,c,d}, L2 = {1,2}
- L1L2 = {a1,a2,b1,b2,c1,c2,d1,d2}
- L1 ∪ L2 = {a,b,c,d,1,2}
- L13 = all strings with length three using a,b,c,d
- L1* = all strings using a,b,c,d and the empty string
- L1+ = doesn't include the empty string
Regular Expressions
- Used to describe tokens of a programming language
- A regular expression is built up of simpler regular expressions
- Each regular expression denotes a language
- A language denoted by a regular expression is called a regular set
- lastpos(n) - the set of positions of the last symbols of strings generated by the sub-expression rooted by n
- nullable(n) - true if the empty string is a member of strings generated by the sub-expression rooted by n
Evaluating firstpos, lastpos, nullable
- nullable(n) - true if the empty string is a member of strings generated by the sub-expression rooted by n
- firstpos(n) = {i} if n is a leaf labeled with position i
- lastpos(n) = {i} if n is a leaf labeled with position i
- firstpos(c1) ∪ firstpos(c2) if c2 is nullable
- lastpos(c1) ∪ lastpos(c2) if c2 is nullable
- firstpos(c) =[firstpos(c1) if nullable(c1) else firstpos(c1)] if c = c1*
- lastpos(c) = [lastpos(c1) if nullable(c2) else lastpos(c2)] if c = c1c2
Evaluating followpos
- Two rules define the function followpos:
- If n is a concatenation-node with left child c1 and right child c2, and i is a position in lastpos(c1), then all positions in firstpos(c2) are in followpos(i)
- If n is a star-node, and i is a position in lastpos(n), then all positions in firstpos(n) are in followpos(i)
- Example: (a|b)*a#
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Description
This quiz focuses on minimizing DFA in lexical analyzer, with examples and explanations of the process. It covers the partitioning of states and the resulting minimized DFA.