Question

This is some code I wrote to check a string's presence in a file:

bool aviasm::in1(string s)
{
ifstream in("optab1.txt",ios::in);//opening the optab
//cout<<"entered in1 func"<<endl;
char c;
string x,y;
while((c=in.get())!=EOF)
{
    in.putback(c);
    in>>x;
    in>>y;
    if(x==s)
    return true;
}
return false;
}

it is sure that the string being searched lies in the first column of the optab1.txt and in total there are two columns in the optab1.txt for every row.
Now the problem is that no matter what string is being passed as the parameter s to the function always returns false. Can you tell me why this happens?

Answer 1

What a hack! Why not use standard C++ string and file reading functions:

bool find_in_file(const std::string & needle)
{
  std::ifstream in("optab1.txt");
  std::string line;

  while (std::getline(in, line))  // remember this idiom!!
  {
    // if (line.substr(0, needle.length()) == needle)  // not so efficient
    if (line.length() >= needle.length() && std::equal(needle.begin(), needle.end(), line.begin())) // better
    // if (std::search(line.begin(), line.end(), needle.begin(), needle.end()) != line.end())  // for arbitrary position
    {
      return true;
    }
  }
  return false;
}

You can replace substr by more advanced string searching functions if the search string isn’t required to be at the beginning of a line. The substr version is the most readable, but it makes a copy of the substring. The equal version compares the two strings in-place (but requires the additional size check). The search version finds the substring anywhere, not just at the beginning of the line (but at a price).