Lexical Analyzer(C)

/* Program on lexical analysis */

#include
#include
#define MAX 30

void main()
{
char str[MAX];
int state=0;
int i=0, j, startid=0, endid, startcon, endcon;

clrscr();

for(j=0; j str[j]=NULL; //Initialise NULL

printf("*** Program on Lexical Analysis ***");
printf("


Enter the string: ");
gets(str); //Accept input string
str[strlen(str)]=' ';

printf("

Analysis:
");

while(str[i]!=NULL)
{
while(str[i]==' ') //To eliminate spaces
i++;
switch(state)
{
case 0: if(str[i]=='i') state=1; //if
else if(str[i]=='w') state=3; //while
else if(str[i]=='d') state=8; //do
else if(str[i]=='e') state=10; //else
else if(str[i]=='f') state=14; //for
else if(isalpha(str[i]) || str[i]=='_')
{
state=17;
startid=i;
} //identifiers

else if(str[i]=='<') state=19;
//relational '<' or '<='

else if(str[i]=='>') state=21;
//relational '>' or '>='

else if(str[i]=='=') state=23;
//relational '==' or assignment '='

else if(isdigit(str[i]))
{
state=25; startcon=i;
}
//constant

else if(str[i]=='(') state=26;
//special characters '('

else if(str[i]==')') state=27;
//special characters ')'

else if(str[i]==';') state=28;
//special characters ';'

else if(str[i]=='+') state=29;
//operator '+'

else if(str[i]=='-') state=30;
//operator '-'

break;

//States for 'if'
case 1: if(str[i]=='f') state=2;
else { state=17; startid=i-1; i--; }
break;
case 2: if(str[i]=='(' || str[i]==NULL)
{
printf("
if : Keyword");
state=0;
i--;
}
else { state=17; startid=i-2; i--; }
break;

//States for 'while'
case 3: if(str[i]=='h') state=4;
else { state=17; startid=i-1; i--; }
break;
case 4: if(str[i]=='i') state=5;
else { state=17; startid=i-2; i--; }
break;
case 5: if(str[i]=='l') state=6;
else { state=17; startid=i-3; i--; }
break;
case 6: if(str[i]=='e') state=7;
else { state=17; startid=i-4; i--; }
break;
case 7: if(str[i]=='(' || str[i]==NULL)
{
printf("
while : Keyword");
state=0;
i--;
}
else { state=17; startid=i-5; i--; }
break;

//States for 'do'
case 8: if(str[i]=='o') state=9;
else { state=17; startid=i-1; i--; }
break;
case 9: if(str[i]=='{' || str[i]==' ' || str[i]==NULL || str[i]=='(')
{
printf("
do : Keyword");
state=0;
i--;
}
break;

//States for 'else'
case 10: if(str[i]=='l') state=11;
else { state=17; startid=i-1; i--; }
break;
case 11: if(str[i]=='s') state=12;
else { state=17; startid=i-2; i--; }
break;
case 12: if(str[i]=='e') state=13;
else { state=17; startid=i-3; i--; }
break;
case 13: if(str[i]=='{' || str[i]==NULL)
{
printf("
else : Keyword");
state=0;
i--;
}
else { state=17; startid=i-4; i--; }
break;

//States for 'for'
case 14: if(str[i]=='o') state=15;
else { state=17; startid=i-1; i--; }
break;
case 15: if(str[i]=='r') state=16;
else { state=17; startid=i-2; i--; }
break;
case 16: if(str[i]=='(' || str[i]==NULL)
{
printf("
for : Keyword");
state=0;
i--;
}
else { state=17; startid=i-3; i--; }
break;

//States for identifiers
case 17:

if(isalnum(str[i]) || str[i]=='_')
{
state=18; i++;
}
else if(str[i]==NULL||str[i]=='<'||str[i]=='>'||str[i]=='('||str[i]==')'||str[i]==';'||str[i]=='='||str[i]=='+'||str[i]=='-') state=18;
i--;
break;

case 18:

if(str[i]==NULL || str[i]=='<' || str[i]=='>' || str[i]=='(' || str[i]==')' || str[i]==';' || str[i]=='=' || str[i]=='+' ||str[i]=='-')
{
endid=i-1;
printf("
");
for(j=startid; j<=endid; j++)
printf("%c", str[j]);
printf(" : Identifier");
state=0;
i--;
}
break;

//States for relational operator '<' & '<='
case 19: if(str[i]=='=') state=20;
else if(isalnum(str[i]) || str[i]=='_')
{
printf("
< : Relational operator");
i--;
state=0;
}
break;
case 20: if(isalnum(str[i]) || str[i]=='_')
{
printf("
<= : Relational operator");
i--;
state=0;
}
break;

//States for relational operator '>' & '>='
case 21: if(str[i]=='=') state=22;
else if(isalnum(str[i]) || str[i]=='_')
{
printf("
> : Relational operator");
i--;
state=0;
}
break;
case 22: if(isalnum(str[i]) || str[i]=='_')
{
printf("
>= : Relational operator");
i--;
state=0;
}
break;

//States for relational operator '==' & assignment operator '='
case 23: if(str[i]=='=') state=24;
else
{
printf("
= : Assignment operator");
i--;
state=0;
}
break;
case 24: if(isalnum(str[i]))
{
printf("
== : Relational operator");
state=0;
i--;
}
break;

//States for constants
case 25: if(isalpha(str[i]))
{
printf("

*** ERROR ***
");
puts(str);
for(j=0; j printf(" ");
printf("^");
printf("
Error at position %d
Alphabet cannot follow digit", i);
state=99;
}
else if(str[i]=='(' || str[i]==')' || str[i]=='<' || str[i]=='>' || str[i]==NULL || str[i]==';' || str[i]=='=')
{
endcon=i-1;
printf("
");
for(j=startcon; j<=endcon; j++)
printf("%c", str[j]);
printf(" : Constant");
state=0;
i--;
}
break;

//State for special character '('
case 26: printf("
( : Special character");
startid=i;
state=0;
i--;
break;

//State for special character ')'
case 27: printf("
) : Special character");
state=0;
i--;
break;

//State for special character ';'
case 28: printf("
; : Special character");
state=0;
i--;
break;

//State for operator '+'
case 29: printf("
+ : Operator");
state=0;
i--;
break;

//State for operator '-'
case 30: printf("
+ : Operator");
state=0;
i--;
break;

//Error State
case 99: goto END;
}
i++;
}
printf("

End of program");
END:
getch();
}

/* Output

Correct input
-------------

*** Program on Lexical Analysis ***


Enter the string: for(x1=0; x1<=10; x1++);


Analysis:

for : Keyword
( : Special character
x1 : Identifier
= : Assignment operator
0 : Constant
; : Special character
x1 : Identifier
<= : Relational operator
10 : Constant
; : Special character
x1 : Identifier
+ : Operator
+ : Operator
) : Special character
; : Special character

End of program



Wrong input
-----------

*** Program on Lexical Analysis ***


Enter the string: for(x1=0; x1<=19x; x++);


Analysis:

for : Keyword
( : Special character
x1 : Identifier
= : Assignment operator
0 : Constant
; : Special character
x1 : Identifier
<= : Relational operator

Token cannot be generated
*/