Question

Please write a recursive descent parser (including a lexical analyzer) for the following EBNF in C. Your program codes should be runnable.

<exprs> -> <expr>; {<expr>;}

<expr> -> <term> { (+ | -) <term> }

<term> -> <factor> { (*|/ ) <factor> }

<factor> -> <exp> {^ <exp>}

<exp> -> id | int_lit | real_lit | (<expr>)

where,

^ indicates the power operation, id is a legal identifier name, int_lit represents any positive integer number, and real_lit represents any positive floating-point number.

Input and output examples:

Input:

x ^ (y+1) - x/2.5 + z;

sum + total * 10;

Output:

parsing succeed

Input:

3 + x*y);

Output:

parsing fail

Note 1: Please name your input file as “input.txt”

Note 2: Try adding some error messages in output such as “lack of right parenthesis”.

Note 3: Try to rewrite the grammar to allow negative numbers.

Answer 1

#include<stdio.h>
void E();
void T();
void V();
int flag=0,i,r=1;
char s[100];
void E()
{
   T();
   if(s[i]=='+' || s[i]=='^' || s[i]=='/' || s[i]=='-' || s[i]=='*')
   {
   if(s[i]=='('){
      flag=0;
      i++;
      r++;
      E();
   }
   else if(s[i]==')')
      {
          r--;  
       }
   }
   else if(s[i]==';' && r==1)
   flag=1;
}
void T()
{
   V();
   if(s[i]=='*' || s[i]=='+' || s[i]=='^' || s[i]=='/' || s[i]=='-')
   {
      if(s[i]=='('){
      flag=0;
      i++;
      r++;
      T();
      }
      else if(s[i]==')')
      {
          r--;  
       }
     
   }
   else if(s[i]==';' && r==1)
   flag=1;
}
void V()
{
   if(s[i]>='a' && s[i]<='z')
   i++;
   else
   flag=1;
}
int main()
{
   printf("Enter a valid expression:\n");
   gets(s);
   E();
   if(flag==1)
   printf("parsing succeed");
   else
   printf("parsing fail");
}