Theoretical Part. Chapter one:- - What are the Phases of compiler? Answer:

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1 Theoretical Part Chapter one:- - What are the Phases of compiler? Six phases Scanner Parser Semantic Analyzer Source code optimizer Code generator Target Code Optimizer Three auxiliary components Literal table Symbol table Error Handler

2 - What is the difference between : parse tree and syntax tree Or - Syntax tree and parse tree are all major data structures of any compiler. Differentiate between them. The Syntax tree: Syntax trees represent abstractions of the actual source code token sequences, The token sequences cannot be recovered from them (unlike parse trees). Nevertheless they contain all the information needed for translation, in a more efficient form than parse trees. The Parse tree: A parse tree corresponding to a derivation is a labeled tree. The interior nodes are labeled by non-terminals, the leaf nodes are labeled by terminals; And the children of each internal node represent the replacement of the associated non-terminal in one step of the derivation. - Syntax error and semantic errors are types of errors that a compiler must deal with. Explain each one and give examples from your popular language. A syntax error : o Occurs when you write code that violates the rules of grammar of the programming language. o Syntax errors are detected by the compiler Example: X=y+2 missing semi colon Semantic error : o Means writing a valid programming structure with invalid logic. Semantic errors can be broken down into o Static semantic errors: Which can be detected by the compiler o Run time errors: Runtime errors cause the program to crash or abort in some way o Logic errors: Cause a program to run to completion,but produce the incorrect output or result Those errors are not detected by the compiler Example: Memory overflow

3 Chapter two:- - Explain the function of lexical analyzer. Or - What is the purpose of lexical analysis? The job of the lexical analyzer, or scanner, is to transform a stream of characters into a stream of tokens. - What is meant by Regular Expression? The single characters from alphabet matching themselves A matches the character a by writing L (a) = {a} ε denotes the empty string, by L(ε)={ε} {} or Φ matches no string at all, by L (Φ) = { } - What is the purpose of Regular Expression? - To represent patterns of strings of characters. - Define DFA. Definition of DFA: A DFA (Deterministic Finite Automation) M consist of (1) An alphabet, (2) A set of states S, (3) A transition function T: S S, (4) A start state s0 S,

4 (5)And a set of accepting states A S - What is the purpose of DFA? Describing particular kinds of algorithms, using Automata where the next state is uniquely given by the current state and the current input character. - What is Transition table of DFA? - Is a data structure which is two-dimensional array, indexed by state and input character that expresses the values of the transition function T

5 Chapter three (part one):- - Explain the Purpose of syntax analysis. - Determining the syntax, or structure, of a program. - Describe the parsing process. Or - What is the function of the parser? Function of a Parser Takes the sequence of tokens produced by the scanner as its input and produces the syntax tree as its output. Parser Sequence of tokens > Syntax-Tree - What are the Rules of CFG and its representation BNF? Grammars Rules Vertical bar appears as meta-symbol for choice. Concatenation is used as a standard operation. No meta-symbol for repetition (like the * of regular expressions) Use the arrow symbol instead of equality to express the definitions of names Names are written in italic( in a different font) Grammar rules use regular expressions as components The notation was developed by John Backus and adapted by Peter Naur for the Algol60 report Grammar rules in this form are usually said to be in Backus-Naur form, or BNF Describe the derivation process.. Derivations Grammar rules determine the legal strings of token symbols by means of derivations A derivation is a sequence of replacements of structure names by choices on the right-hand sides of grammar rules

6 A derivation begins with a single structure name and ends with a string of token symbols At each step in a derivation, a single replacement is made using one choice from a grammar rule

7 Chapter three (part two):- (Parse tree. Answered in chapter one.) What is Ambiguity Parse trees and syntax trees uniquely express the structure of syntax But it is possible for a grammar to permit a string to have more than one parse tree What are the Two Basic Methods dealing with Ambiguity One is to state a rule that specifies in each ambiguous case which of the parse trees (or syntax trees) is the correct one, called a disambiguating rule. The advantage: it corrects the ambiguity without changing (and possibly complicating) the grammar. The disadvantage: the syntactic structure of the language is no longer given by the grammar alone. Change the grammar into a form that forces the construction of the correct parse tree, thus removing the ambiguity. Of course, in either method we must first decide which of the trees in an ambiguous case the correct one is. - What is Precedence and Associativity? - And how can we solve it? Precedence: We can solve it by: Grammar that have operators with different precedence Such as multiplication and addition Grouping operators into different precedence levels. Cascade is a standard method in syntactic specification using BNF.. Make the lower precedence to be evaluated first Associativity: Grammar that have operators with same precedence and not know which operator will be evaluated first. We ca solves it by: Change the grammar rules to force one way recursion. not in both sides

8 - What is dangling else problem and how can we solve it?. Dangling else problem Which tree is correct depends on associating the single else-part with the first or the second if-statement. The first associates the else-part with the first if-statement; The second associates it with the second if-statement. This ambiguity called dangling else problem We can solve it by Using a disambiguating rule is the most closely nested rule Implies which parse tree is the correct one. Require the presence of the else-part, and this method has been used in LISP and other functional languages (where a value must also be returned). Uses a bracketing keyword for the if-statement languages that use this solution include Algol68 and Ada. - Define EBNF and Syntax Diagram. - EBNF: Is a grammar that has its own notation such as the asterisk * for representing repetition the [] to represent optional - Is an extended grammar of BNF grammar?