Recursive Predictive Parser - Compiler Lab - C Program

Aim:
Parse the following grammar.


E --> E + T / T
T --> T * F / F
F --> ( E ) / id


Program:
#include<stdio.h>
#include<ctype.h>
#include<stdlib.h>
#include<string.h>
void err(const char* s)
{
perror(s);
exit(0);
}
int factor()
{
int val,i;
char ch[0];
scanf("%s",ch);
switch(ch[0])
{
case '(':
val=expr();
scanf("%s",ch);
if(ch[0]!=')')
err("Missing closing paranthesis in factor.");
break;
default :{
for(i=0;i<strlen(ch);i++)
{
if((ch[i]>'0')&&(ch[i]<='9'))
continue;
else
err("Illegal character sequence in place of factor.");
}
val=atoi(ch);
}
}
return val;
}
int term()
{
int val;
char ch[10];
val=factor();
while(1)
{
scanf("%s",ch);
if(ch[0]=='*')
{
val=val*factor();
}
else
break;
}
ungetc(ch[0],stdin);
return val;
}
int expr()
{
int val;
char ch[10];
val=term();
while(1)
{
scanf("%s",ch);
if(ch[0]=='+')
val=val+term();
else
break;
}
ungetc(ch[0],stdin);
return val;
}
main()
{
printf("\nEnter the expression: ");
printf("\n Result: %d\n",expr());
}

       

Output:
nn@linuxmint ~ $ gcc rec.c
nn@linuxmint ~ $ ./a.out
Enter the expression: 5 * ( 3 + 1 )
;
Result: 20
nn@linuxmint ~ $