#include<stdio.h>
#include<ctype.h>
#include<string.h>
#include<stdlib.h>
#define SIZE 128
#define NONE -1
#define EOS '\0'
#define NUM 257
#define KEYWORD
258
#define ID 259
#define DONE 260
#define MAX 999
char lexemes[MAX];
char buffer[SIZE];
int lastchar=-1;
int lastentry=0;
int tokenval=DONE;
int lineno=1;
int lookahead;
struct entry
{
char *lexptr;
int token;
}symtable[100];
struct entry
keywords[]={"if",KEYWORD,"else",KEYWORD,"for",KEYWORD,"int",KEYWORD,
"float",KEYWORD,"double",KEYWORD,"char",KEYWORD,"struct",KEYWORD,"ret
urn",KEYWORD,0,0};
void Error_Message(char
*m)
{
fprintf(stderr,"line
%d, %s \n",lineno,m);
exit(1);
}i
nt look_up(char s[
])
{
int k;
for(k=lastentry;k>0;k--)
if(strcmp(symtable[k].lexptr,s)==0)
return k;
return 0;
}i
nt insert(char s[
],int tok)
{
int
len;
len=strlen(s);
if(lastentry+1>=MAX)
Error_Message("Symbpl
table is full");
if(lastchar+len+1>=MAX)
Error_Message("Lexemes
array is full");
lastentry=lastentry+1;
symtable[lastentry].token=tok;
symtable[lastentry].lexptr=&lexemes[lastchar+1];
lastchar=lastchar+len+1;
strcpy(symtable[lastentry].lexptr,s);
return lastentry;
}/
*void Initialize()
{
struct entry *ptr;
for(ptr=keywords;ptr->token;ptr+1)
insert(ptr->lexptr,ptr->token);
}*/
int lexer()
{
int t;
int val,i=0;
while(1)
{
t=getchar();
if(t=='
'||t=='\t');
else if(t=='\n')
lineno=lineno+1;
else
if(isdigit(t))
{
ungetc(t,stdin);
scanf("%d",&tokenval);
return NUM;
}
else
if(isalpha(t))
{
while(isalnum(t))
{
buffer[i]=t;
t=getchar();
i=i+1;
if(i>=SIZE)
Error_Message("Compiler
error");
}
buffer[i]=EOS;
if(t!=EOF)
ungetc(t,stdin);
val=look_up(buffer);
if(val==0)
val=insert(buffer,ID);
tokenval=val;
return
symtable[val].token;
}
else if(t==EOF)
return DONE;
else
{
tokenval=NONE;
return t;
}
}
}
void Match(int t)
{
if(lookahead==t)
lookahead=lexer();
else
Error_Message("Syntax
error");
}
void display(int
t,int tval)
{
if(t=='+'||t=='-'||t=='*'||t=='/')
printf("\nArithmetic
Operator: %c",t);
else if(t==NUM)
printf("\n
Number: %d",tval);
else if(t==ID)
printf("\n
Identifier: %s",symtable[tval].lexptr);
else
printf("\n
Token %d tokenval %d",t,tokenval);
}
void F()
{
//void E();
switch(lookahead)
{
case '(' :
Match('(');
E();
Match(')');
break;
case NUM :
display(NUM,tokenval);
Match(NUM);
break;
case ID :
display(ID,tokenval);
Match(ID);
break;
default :
Error_Message("Syntax error");
}
}
void T()
{
int t;
F();
while(1)
{
switch(lookahead)
{
case '*' :
t=lookahead;
Match(lookahead);
F();
display(t,NONE);
continue;
case '/' :
t=lookahead;
Match(lookahead);
display(t,NONE);
continue;
default : return;
}
}
}
void E()
{
int t;
T();
while(1)
{
switch(lookahead)
{
case '+' :
t=lookahead;
Match(lookahead);
T();
display(t,NONE);
continue;
case '-' :
t=lookahead;
Match(lookahead);
T();
display(t,NONE);
continue;
default
: return;
}
}
}
void parser()
{
lookahead=lexer();
while(lookahead!=DONE)
{
E();
Match(';');
}
}
main()
{
char ans[10];
printf("\n
Program for recursive decent parsing ");
printf("\n
Enter the expression ");
printf("And
place ; at the end\n");
printf("Press
Ctrl-Z to terminate\n");
parser();
}
Output:
Program for recursive decent parsing
Enter the expression And place ; at the
end
Press Ctrl-Z to terminate
a+b*c;
Identifier: a
Identifier: b
Identifier: c
Arithmetic Operator: *
Arithmetic Operator: +
2*3;
Number: 2
Number: 3
Arithmetic Operator: *
+3;
line 5,Syntax error
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