Recursive Descent Parser Help (Not reading a text file string) - c++

Question

char input[100]; //Used to check grammar
char *s;
int main(int argc, char *argv[]) {
ifstream fin("input.txt"); //Open input file
while (fin>>input) { //Store text in input[]
s = input; //Point c at the input text
cout<<"String read from file: "<<input<<endl; //Show input text
if (A() && *s == '\0') { //Testing the grammar
cout<<"The string \""<<input<<"\" is in the language."<<endl;
}
else cout<<"The string \""<<input<<"\" is not in the language."<<endl;
cout<<endl; //Formatting for output in console
}
fin.close(); //Close input file
return 0;
}
Can someone tell me what i am doing wrong here. The parser is not reading string from the text file.

Answer 1

Let's treat this like two problems.
Problem 1: Reading in the file.
I've stripped out everything that isn't absolutely essential to reading the data in.
char input[100]; //Used to check grammar
int main(int argc, char *argv[]) {
ifstream fin("input.txt"); //Open input file
while (fin>>input) { //Store text in input[]
cout<<"String read from file: "<<input<<endl; //Show input text
}
return 0;
}
Nothing really wrong here, but you should seriously consider replacing char input[100]; with std::string input; to save you headaches with large tokens. For example, watch what happens here:
int main()
{
stringstream stream("1234567890"); // pack ten characters into stream
// note: no whitespace.
char snookums[] = "snookums";
char array[5];
cout << snookums << endl; // prove contents of snookums
stream >> array; // read up to first whitespace from stream into array
cout << array << endl; // display what we read.
cout << snookums << endl; // oh dear. Now look at poor snookums!
return 0;
}
output:
snookums
1234567890
67890
Despite array being size 5, it contains all 10. Or does it? Nope. Sadly poor snookums got run over. This won't happen with strings.
For all we know fin>>inputjust read 30000 characters from a whitespace-free file, annihilated the rest of your program's memory, and the program died before printing out anything.
Anyway, your code leaves a few questions:
Are you actually able to open the file? You don't know, really. You never checked.
Was the file empty? Also don't know. You didn't tell us. This is one of the things folks are getting on about in the comments.
None of this fixes anything, but hopefully gives you a better idea what's going wrong.
string input; // using string in case the data you're reading is incompatible
//with a 100 character char array.
int main(int argc, char *argv[]) {
ifstream fin("input.txt"); //Open input file
if (fin.is_open()
{
while (fin>>input)
{ //Store text in input
cout<<"String read from file: "<<input<<endl; //Show input text
}
}
else
{
cout << "Failed to open file." << endl;
}
return 0;
}
Once you know if you are actually reading in data, and if not, why not.
Problem 2: language parsing.
We can't help you here. No information was provided, but a few notes on coding style because they will help you ask future questions:
A() is meaningless to everyone but you. Give it a descriptive name so that someone other than you has some hints about what it does. A() takes no parameters. I assume that's because it is operating on input. OK, but why not pass input in? The cost is minimal and it provides more information to readers. Note how commenters zeroed in on A() right away? That's fear. The good kind of fear. We have no [insert expletive here] clue what it is or what it does, so it is instantly scrutinized.
A(input) reads to me as "A does something to input." I don't know what it does, but it does it to input. Unless the writer of the program has a history of doing silly stuff, it probably only does stuff to input and I don't have to fear this function nearly as much.
LanguageInterpreter() tells me that a language interpreter was run. Not much, but if I'm looking tor a bug in the file reading code, I'm not likely to find it in there. Unfortunately it also tells me that LanguageInterpreter is feasting on global data and Crom only knows what sort of side effects it could have on the rest of the program.
LanguageInterpretter(input) tells me a lot. For one thing it tells me that I can get on with my day because it has nothing to do with, or better have nothing to do with, the reading in of the data file. I'll check other places for bugs first.

Answer 2

string *s;
int main()
{
string input;
ifstream fin("input.txt");
if(fin.is_open())
{
while(getline(fin,input))
{
s=&input;
if (A() && *s == '\0') /* error: no match for 'operator==' in '* s == '\000''*/
{ //Testing the grammar
cout<<"The string \""<<*s<<"\" is in the language."<<endl;
}
else cout<<"The string \""<<*s<<"\" is not in the language."<<endl;
cout<<endl;
}
}
fin.close();
return 0;
}