Compiler Construction Dr. Naveed Ejaz Lecture 5

Lexical Analysis

3 Recall: Front-End  Output of lexical analysis is a stream of tokens scannerparser source code tokens IR errors

4 TokensTokens Example: if( i == j ) z = 0; else z = 1;

5 TokensTokens  Input is just a sequence of characters: if( \b i == j \n\t....

6 TokensTokens Goal:  partition input string into substrings  classify them according to their role

7 TokensTokens  A token is a syntactic category  Natural language: “He wrote the program”  Words: “He”, “wrote”, “the”, “program”

8 TokensTokens  Programming language: “if(b == 0) a = b”  Words: “if”, “(”, “b”, “==”, “0”, “)”, “a”, “=”, “b”

9 TokensTokens  Identifiers: x y11 maxsize  Keywords: if else while for  Integers: L  Floats: e5  Symbols: ( ) + * / { } ==  Strings: “enter x” “error”

10 Ad-hoc Lexer  Hand-write code to generate tokens.  Partition the input string by reading left-to-right, recognizing one token at a time

11 Ad-hoc Lexer  Look-ahead required to decide where one token ends and the next token begins.

12 Ad-hoc Lexer class Lexer { Inputstream s; char next;//look ahead Lexer(Inputstream _s) { s = _s; next = s.read(); }

13 Ad-hoc Lexer class Lexer { Inputstream s; char next;//look ahead Lexer(Inputstream _s) { s = _s; next = s.read(); }

14 Ad-hoc Lexer class Lexer { Inputstream s; char next;//look ahead Lexer(Inputstream _s) { s = _s; next = s.read(); }

15 Ad-hoc Lexer class Lexer { Inputstream s; char next;//look ahead Lexer(Inputstream _s) { s = _s; next = s.read(); }

16 Ad-hoc Lexer class Lexer { Inputstream s; char next;//look ahead Lexer(Inputstream _s) { s = _s; next = s.read(); }

17 Ad-hoc Lexer Token nextToken() { if( idChar(next) ) return readId(); if( number(next) ) return readNumber(); if( next == ‘”’ ) return readString();...

18 Ad-hoc Lexer Token nextToken() { if( idChar(next) ) return readId(); if( number(next) ) return readNumber(); if( next == ‘”’ ) return readString();...

19 Ad-hoc Lexer Token nextToken() { if( idChar(next) ) return readId(); if( number(next) ) return readNumber(); if( next == ‘”’ ) return readString();...

20 Ad-hoc Lexer Token nextToken() { if( idChar(next) ) return readId(); if( number(next) ) return readNumber(); if( next == ‘”’ ) return readString();...

21 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

22 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

23 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

24 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

25 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

26 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

27 Ad-hoc Lexer Token readId() { string id = “”; while(true){ char c = input.read(); if(idChar(c) == false) return new Token(TID,id); id = id + string(c); }

28 Ad-hoc Lexer boolean idChar(char c) { if( isAlpha(c) ) return true; if( isDigit(c) ) return true; if( c == ‘_’ ) return true; return false; }

29 Ad-hoc Lexer Token readNumber(){ string num = “”; while(true){ next = input.read(); if( !isNumber(next)) return new Token(TNUM,num); num = num+string(next); }

30 Ad-hoc Lexer Token readNumber(){ string num = “”; while(true){ next = input.read(); if( !isNumber(next)) return new Token(TNUM,num); num = num+string(next); }

31 Ad-hoc Lexer Token readNumber(){ string num = “”; while(true){ next = input.read(); if( !isNumber(next)) return new Token(TNUM,num); num = num+string(next); }

32 Ad-hoc Lexer Problems:  Do not know what kind of token we are going to read from seeing first character.

33 Ad-hoc Lexer Problems:  If token begins with “i”, is it an identifier “i” or keyword “if”?  If token begins with “=”, is it “=” or “==”?

34 Ad-hoc Lexer  Need a more principled approach  Use lexer generator that generates efficient tokenizer automatically.