Question

I have the following code to read a text file.

const string FILENAME = PACKAGES_DIR + pname;
  //the arguments to ifstream is a cstring and hence the conversion must be made
  ifstream freader;
  freader.open(FILENAME.c_str(),ios::in);
  if(freader.is_open())
  {
    while(freader.good())
    {
      string line;
      getline(freader,line);
      cout<<line<<endl;
      if(line.find("PackageId:"))
      {
        cout<<line.substr(11)<<endl;
      }
      else if(line.find("Name:"))
      {
        cout<<line.substr(5)<<endl;
      }
      else if(line.find("Version:"))
      {
        cout<<line.find(8)<<endl;
      }
      else
      {
        cout<<line<<endl;
      }

    }
  }

The contents of the text file in question is

PackageId:994
Name:basket
Version:1.80-1
Deps:kdebase-runtime,libc0.1,libc0.1-udeb,libc6,libc6-udeb,libc6.1,libc6.1-udeb,libgcc1,libgpg-error0,libgpgme11,libkdecore5,libkdeui5,libkfile4,libkio5,libkparts4,libkutils4,libphonon4,libqimageblitz4,libqt4-dbus,libqt4-network,libqt4-qt3support,libqt4-svg,libqt4-xml,libqtcore4,libqtgui4,libstdc++6,libunwind7,libx11-6,phonon

The output that I get is

PackageId:994
geId:994
Name:basket

Version:1.80-1
0-1
Deps:kdebase-runtime,libc0.1,libc0.1-udeb,libc6,libc6-udeb,libc6.1,libc6.1-udeb,libgcc1,libgpg-error0,libgpgme11,libkdecore5,libkdeui5,libkfile4,libkio5,libkparts4,libkutils4,libphonon4,libqimageblitz4,libqt4-dbus,libqt4-network,libqt4-qt3support,libqt4-svg,libqt4-xml,libqtcore4,libqtgui4,libstdc++6,libunwind7,libx11-6,phonon
e-runtime,libc0.1,libc0.1-udeb,libc6,libc6-udeb,libc6.1,libc6.1-udeb,libgcc1,libgpg-error0,libgpgme11,libkdecore5,libkdeui5,libkfile4,libkio5,libkparts4,libkutils4,libphonon4,libqimageblitz4,libqt4-dbus,libqt4-network,libqt4-qt3support,libqt4-svg,libqt4-xml,libqtcore4,libqtgui4,libstdc++6,libunwind7,libx11-6,phonon

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr

The output that I wanted is:

PackageId:994
994
Name:basket
basket
Version:1.80-1
1.80-1
...

What have I done wrong?

Answer 1

Problem 1

while .good or while !.eof is almost always wrong. Throw away whatever book told you to do that, and do this instead.

In this case, the changed code looks a bit like this:

const string FILENAME = PACKAGES_DIR + pname;
//the arguments to ifstream is a cstring and hence the conversion must be made
ifstream freader(FILENAME.c_str(), ios::in);
if (freader) {
   string line;
   while (getline(freader,line)) {  // <-----
      cout << line << endl;

      if (line.find("PackageId:"))
         cout << line.substr(11) << endl;
      else if (line.find("Name:"))
         cout << line.substr(5) << endl;
      else if (line.find("Version:"))
         cout << line.find(8) << endl;
      else
         cout << line << endl;
   }
}

---

Problem 2

You’re not using std::string::find correctly.

line.find("PackageId:") returns either “the position of the first occurrence in the string of the searched content”, or the member value npos if the match is not found.

This combined with not performing bounds checks on the first parameter to std::string::substr is causing issues with your strings.

Instead, write:

if (line.find("PackageId:") != std::string::npos)

---

Problem 3

cout<<line.find(8)<<endl; should say substr, not find.

---

Your code with some of the above fixed:

const string FILENAME = PACKAGES_DIR + pname;
//the arguments to ifstream is a cstring and hence the conversion must be made
ifstream freader(FILENAME.c_str(), ios::in);
if (freader) {
   string line;
   while (getline(freader,line)) {  // <-----
      cout << line << endl;

      if (line.find("PackageId:")    != std::string::npos && line.size() > 11)
         cout << line.substr(11) << endl;
      else if (line.find("Name:")    != std::string::npos && line.size() > 5)
         cout << line.substr(5) << endl;
      else if (line.find("Version:") != std::string::npos && line.size() > 8)
         cout << line.substr(8) << endl;
      else
         cout << line << endl;
   }
}