So I'm working on a project for a class in which we have to create a parser for a made up grammar. The input is read from a file. When I use input that is in the grammar, the program works. For certain input not in the grammar such as 9=b+a (which is not in the grammar) the program says it is not in the grammar. However, when I test other input that is not in the grammar such as b=g**h (basically whenever I use two operators (+ - / *) in a row, the program crashes with I believe a segmentation fault. Any ideas why this is happening? Thanks in advance for your help. Here is my code with the grammar at the top commented out.

/*
A -> I = E
E -> T + E | T - E | T
T -> P * T | P / T | P
P -> I | L | (E)
I -> a | b | ... | y | z
L -> 0 | 1 | ... | 8 | 9
*/

#include <iostream>
#include <fstream>
#include <string>

using namespace std;


bool literal(char *);
bool integer(char *);
bool primary(char *);
bool term(char *);
bool expr(char *);
bool assign(char *);

int main ()
{
    ifstream inFile;
    inFile.open("experiment.txt");
    
    string line;
    inFile >> line;

    
    char *c;
    c = &line[0];
    
    cout << "String read from file: " << line << endl;

    
    if (assign(c))
       cout << "The string: '" << line << "' is in the language" << endl;
    else
       cout << "The string: '" << line << "' is not in the language" << endl;
    
    

 system("Pause");
    return 0;
}
bool assign (char *c)
{
    if (integer(c))
    {        
         
         ++c;  
         if (*c == '=')
         {             
             ++c;              
             if (expr(c))
             {                
                ++c; 
                return true;       
             }
         }
     }
     else
         return false;
}

bool literal(char *c)
{
     if (*c == 0 || *c == 1 || *c == 2 || *c == 3 || *c == 4 || *c == 5 || *c == 6 || *c == 7 || *c == 8 || *c == 9)
     {   
        
        return true;
     }
     return false;
}

bool integer (char *c)
{
     cout << "in integer" << endl;
     if (*c == 'a' || *c == 'b' || *c == 'c' || *c == 'd' || *c == 'e' || *c == 'f' || *c == 'g' || *c == 'h' || *c == 'i' || *c == 'j' || *c == 'k' || *c == 'l' || *c == 'm' || *c == 'n' || *c == 'o' || *c == 'p' || *c == 'q' || *c == 'r' || *c == 's' || *c == 't' || *c == 'u' || *c == 'v' || *c == 'w' || *c == 'x' || *c == 'y' || *c == 'z')
     {
        return true;
     }
     else
         return false;
}

bool primary (char *c)
{
     if(integer(c))
        return true;
     else if (literal(c))
          return true;
     else if (*c == '(')
          ++c;
          if (expr(c))
          {
             ++c;
             if (*c == ')')
             {
                ++c;
                return true;
             }
             else
                 return false;
          }
          else
              return false;
}

bool term (char *c)
{
     if (primary(c))
     {
          ++c;
          if (*c == '*' || *c == '/')
          {
             ++c;
             if (term(c))
             {
                return true;
             }
             else
                 return false;
          }
          return true;
     }
     else
         return false;
}

bool expr (char *c)
{
     if (term(c))
     {
          ++c;          
          if (*c == '+' || *c == '-')
          {
             ++c;
             if (expr(c))
             {                
                return true;
             }
          }
          return true;
     }
     else
         return false;
}
Comments
nice project

lines 33 and 34:
>>char *c;
>>c = &line[0];

That is not necessary. Just pass line.c_str() to the function on line 39

line 71: all you have to do is use isdigit() function if( isdigit(*c)) line 82: get rid of all that crap and use isalpha() if( isalpha(*c) ) The program is missing a set of { and }

bool primary (const char *c)
{

     if(integer(c))
        return true;
     else if (literal(c))
          return true;
     else if (*c == '(')
     {
          ++c;
          if (expr(c))
          {
             ++c;
             if (*c == ')')
             {
                ++c;
                return true;
             }
             return false;
          }
     }
     return false;
}

Edited 7 Years Ago by Ancient Dragon: n/a

Thank you so much for your help!!! My program works now !! I found the missing brackets and also added else return false to every if in the assign function. Just to make sure I understand, the error was simply in the missing brackets in the primary method?

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