use this lex$()$ program token.h

#ifndef TOKEN$\_$H

#define TOKEN$\_$H

$//$ Token types

enum $\{$

IDENT $=11,$

INT$\_$LIT $=10,$

FLOAT$\_$LIT $=12,$

ADD$\_$OP $=21,$

SUB$\_$OP $=22,$

MULT$\_$OP $=23,$

DIV$\_$OP $=24,$

LEFT$\_$PAREN $=25,$

RIGHT$\_$PAREN $=26,$

ASSIGN$\_$OP $=27,$

LT$\_$OP $=28,$

GT$\_$OP $=29,$

EQ$\_$OP $=30,$

NEQ$\_$OP $=31,$

FOR$\_$CODE $=32,$

IF$\_$CODE $=33,$

ELSE$\_$CODE $=34,$

WHILE$\_$CODE $=35,$

DO$\_$CODE $=36,$

INT$\_$CODE $=37,$

FLOAT$\_$CODE $=38,$

SEMICOLON $=39,$

LEFT$\_$BRACE $=40,$

RIGHT$\_$BRACE $=41,$

MOD$\_$OP $=42,$

NOT$\_$EQ $=43,$

EOF$\_$TOKEN $=-1//$ Ensure EOF$\_$TOKEN is defined

$\}$;

$//$ Global variables

extern char lexeme$[]$;

extern int nextToken;

extern int lexLen;

extern int token;

extern int charClass;

extern FILE $*$in$\_$fp;

$//$ Function prototypes

void addChar$()$;

void getChar$()$;

void getNonBlank$()$;

int lookup$($char ch$)$;

int lex$()$;

int isKeyword$($char $*$lexeme$)$;

#endif $//$ TOKEN$\_$H

lexer.c

#include

#include

#include

#include "token.h$"$

$//$ Global variables

char lexeme$[100]$;

int lexLen;

int token;

int nextToken;

int charClass;

FILE $*$in$\_$fp;

$//$ Character classes

#define LETTER $0$

#define DIGIT $1$

#define UNKNOWN $99$

void addChar$()\{$

if $($lexLen $<=98)\{$

lexeme$[$lexLen$++]=($char$)$token;

lexeme$[$lexLen$]=0$;

$\}$ else

printf$("$Error: lexeme is too long

$")$;

$\}$

void getChar$()\{$

if $(($token $=$ getc$($in$\_$fp$))!=$ EOF$)\{$

if $($isalpha$($token$))$

charClass $=$ LETTER;

else if $($isdigit$($token$))$

charClass $=$ DIGIT;

else

charClass $=$ UNKNOWN;

$\}$ else

charClass $=$ EOF$\_$TOKEN;

$\}$

void getNonBlank$()\{$

while $($isspace$($token$))$

getChar$()$;

$\}$

int lookup$($char ch$)\{$

switch $($ch$)\{$

case $\text{'}(\text{'}$: addChar$()$; nextToken $=$ LEFT$\_$PAREN; break;

case $\text{'})\text{'}$: addChar$()$; nextToken $=$ RIGHT$\_$PAREN; break;

case $\text{'}+\text{'}$: addChar$()$; nextToken $=$ ADD$\_$OP; break;

case $\text{'}-\text{'}$: addChar$()$; nextToken $=$ SUB$\_$OP; break;

case $\text{'}*\text{'}$: addChar$()$; nextToken $=$ MULT$\_$OP; break;

case $\text{'}/\text{'}$: addChar$()$; nextToken $=$ DIV$\_$OP; break;

case $\text{'}=\text{'}$: addChar$()$; nextToken $=$ ASSIGN$\_$OP; break;

case $\text{'}<\text{'}$: addChar$()$; nextToken $=$ LT$\_$OP; break;

case $\text{'}>\text{'}$: addChar$()$; nextToken $=$ GT$\_$OP; break;

case $\text{'}$;$\text{'}$: addChar$()$; nextToken $=$ SEMICOLON; break;

case $\text{'}\{\text{'}$: addChar$()$; nextToken $=$ LEFT$\_$BRACE; break;

case $\text{'}\}\text{'}$: addChar$()$; nextToken $=$ RIGHT$\_$BRACE; break;

case $\text{'}\%\text{'}$: addChar$()$; nextToken $=$ MOD$\_$OP; break;

default: addChar$()$; nextToken $=$ EOF$\_$TOKEN; break;

$\}$

return nextToken;

$\}$

int isKeyword$($char $*$lexeme$)\{$

if $($strcmp$($lexeme$,$ "for"$)==0)$ return FOR$\_$CODE;

if $($strcmp$($lexeme$,"$if$")==0)$ return IF$\_$CODE;

if $($strcmp$($lexeme$,$ "else"$)==0)$ return ELSE$\_$CODE;

if $($strcmp$($lexeme$,$ "while"$)==0)$ return WHILE$\_$CODE;

if $($strcmp$($lexeme$,"$do$")==0)$ return DO$\_$CODE;

if $($strcmp$($lexeme$,$ "int"$)==0)$ return INT$\_$CODE;

if $($strcmp$($lexeme$,$ "float"$)==0)$ return FLOAT$\_$CODE;

return IDENT;

$\}$

int lex$()\{$

lexLen $=0$;

getNonBlank$()$;

switch $($charClass$)\{$

case LETTER:

addChar$()$;

getChar$()$;

while $($charClass $==$ LETTER $||$ charClass $==$ DIGIT$)\{$

addChar$()$;

getChar$()$;

$\}$

nextToken $=$ isKeyword$($lexeme$)$;

break;

case DIGIT:

addChar$()$;

getChar$()$;

while $($charClass $==$ DIGIT$)\{$

addChar$()$;

getChar$()$;

$\}$

if $($token $==\text{'}.\text{'})\{$

addChar$()$;

getChar$()$;

while $($charClass $==$ DIGIT$)\{$

addChar$()$;

getChar$()$;

$\}$

nextToken $=$ FLOAT$\_$LIT;

if $($token $==\text{'}$e$\text{'}||$ token $==\text{'}$E$\text{'})\{$

addChar$()$;

getChar$()$;

if $($token $==\text{'}+\text{'}||$ token $==\text{'}-\text{'})\{$

addChar$()$;

getChar$()$;

$\}$

while $($charClass $==$ DIGIT$)\{$

addChar$()$;

getChar$()$;

$\}$

nextToken $=$ FLOAT$\_$LIT;

$\}$

$\}$ else $\{$

nextToken $=$ INT$\_$LIT;

$\}$

break;

case UNKNOWN:

lookup$($token$)$;

getChar$()$;

break;

case EOF$\_$TOKEN:

nextToken $=$ EOF$\_$TOKEN;

lexeme$[0]=\text{'}$E$\text{'}$;

lexeme$[1]=\text{'}$O$\text{'}$;

lexeme$[2]=\text{'}$F$\text{'}$;

lexeme$[3]=0$;

break;

$\}$

printf$("$Next token is: $\%$d$,$ Next lexeme is $\%$s

$",$ nextToken, lexeme$)$;

return nextToken;

$\}$

main.c

#include

#include "token.h$"$

int main$()\{$

if $(($in$\_$fp $=$ fopen$("$front$.$in$","$r$"))==$ NULL$)\{$

printf$("$ERROR $-$ cannot open front.in

$")$;

return $1$;

$\}$ else $\{$

getChar$()$;

do $\{$

lex$()$;

$\}$ while $($nextToken $!=$ EOF$\_$TOKEN$)$;

$\}$

return $0$;

$\}$

front.in

for $($int i $=0$; i $<10$; i$++)\{$

if $($i $\%2==0)\{$

float x $=3\mathrm{.}14$;

$\}$ else $\{$

while $($x $<10)\{$

x $+=1\mathrm{.}0$e$+2$;

$\}$

$\}$

$\}$

do $\{$

int y $=5$;

$\}$ while $($y $<10)$;to create a recursive descent parser as follows:

Modify$/$Extend the recursive descent parser given in Section $4\mathrm{.}4\mathrm{.}1($which recognises $,,$ and $),$ such that it ALSO recognises the following language constructs as well $($follow C$-$like or Java$-$like syntax$)$:

The $($for$)$ statement.

The simple $($if$)$ and the $($if$-$else$)$ statements.

The $($while$)$ statement

The $($do$-$while$)$ statement