Lecture 4

Question 1

Lexical structures

Answer 1

Lexical structures, which concerns the forms of its individual symbols.

e.g := (keywords, identifiers)

Question 2

Syntax

Answer 2

Defines structure of components of language eg structures of programs, statements, expressions, terms etc.

Question 3

Semantics

Answer 3

Defines the meanings and usage of structures and requirements that cannot be described by grammar.

Checking type consistency, applying + operator to two numerical values is to evaluate the arithmetic sum.

Question 4

Semantic Description problem

Answer 4

SEMANTICS MORE DIFFIICULT TO DEFINE than syntax and not well accepted methods for semantic definition.

Question 5

Languages Analysis

Answer 5

Language implementation must analyse source code - its lexical and syntax structure.

Question 6

Language Analysis two parts.

Answer 6

Low level is lexical analyzer, mathematically finite automation based on regular grammar.

High level is syntax analyser, or parser whcih is mathematically a push down automation based context free grammar.

Question 7

lexical analyser (a.k.a. scanner

Answer 7

Reads source code character-by-character

Groups characters into tokens

Removes whitespace and comments

Sends tokens to the parser (next phase)

substrings of source program as lexemes.

sum is a lexeme; its token may be IDENT.

Question 8

Lexical Analysis ( Alphabet )

Answer 8

An alphabet Σ is a finite non-empty set (of symbols).

the set Σab = {a, b} is an alphabet comprising symbols a and b;

– the set Σaz = {a, …, z} is the alphabet of lowercase English letters;

– the set Σasc of all ASCII characters is an alphabet.

Question 9

Lexical Analysis (String)

Answer 9

A string or word over an alphabet Σ is a finite concatenation (or juxtaposition) of symbols from
Σ. For example,

• abba, aaa and baaaa are strings over Σab;
  – hello, abacab, and baaaa are strings over Σaz;
  – h$(e′lo, PjM#;, and baaaa are strings over Σasc.
• The length of a string w (that is, the number of
  symbols it has) is denoted |w|. E.g., |abba| = 4.
• The empty or null string is denoted ε, and so |ε|
  = 0.

Question 10

Set of all strings…

Answer 10

set of all strings over Σ is denoted Σ∗

E.g., Σ ∗ ab = {ε, a, b, aa, ab, ba, bb, aab,…}

For any symbol or string x, x n denotes the string of the concatenation of n copies of x.

E.g.

a^4 = aaaa
(ab)^4 = abababab

Question 11

Regular Expressions

Answer 11

specifiy parterns of string or symbols.

r matches or is matched by set of strings if the patterns of the strings are specified by the Reg Ex.

Set of strings matched by RE r is denoted by L(r) ⊆ Σ∗ ( if strings belong to alphbet Σ) is called by language determined by r

Question 12

Regular Expression Definiton

Answer 12

∅ (empty set symbol) is regular expression. not useful.

ε (the empty string symbol) is a regular
expression. This matches just the empty string ε.

Question 13

∅ and ε

Answer 13

empty string ε should not be confused with the empty language ∅

∅ is a formal language (i.e., a set of strings) that contains no strings, not even the empty string. ε

The empty string is a string that has the
properties:

ε +s =s +ε =s i.e., the empty string is the identity element of the concatenation operation

ε |=0. i.e., its length is zero.