Using counter from for loop as condition

I am creating a program in C++ that takes a pdf and converts it into a epub. I am having trouble with my function that reads in the chapters and creates different files for each chapter. The input file will look something like this:

Chapter 1

blah blah blah ..... blah

Chapter 2

blah blah blah ..... blah

I am trying to use the for loop to check the string for the beginning of a chapter and if it is, then create a xhtml file with the name of 'chapter#.xhtml' in the correct directory. The problem I am having is checking for the text and creating the respective file.

for(int i=1;i<chapters;i++){
  char j = i;
  string chap = "book/OEBPS/chapters/chapter";
  chap.append(1,j).append(".xhtml");
  if(line == "Chapter "+j){
    out << "</div>\n</body>\n</html>";
    out.close();
    out.open(chap.c_str());
    break;
  }
}

Essentially I am trying to create a counter that either is a string or can be added to a string to use in the if statement conditional and the out.open parameter. Below is the whole function in case that helps.

void readChapters(ofstream& out,int chapters){
  string line,file;
  char letter;
  ifstream in;
  cout << "Enter input file\n";
  cin >> file;
  in.open(file.c_str());
  out.open("junk");
  getline(in,line);
  while(in) {
    for(int i=1;i<chapters;i++){
      char j = i;
      string chap = "book/OEBPS/chapters/chapter";
      chap.append(1,j).append(".xhtml");
      if(line == "Chapter "+j){
        out << "</div>\n</body>\n</html>";
        out.close();
        out.open(chap.c_str());
        break;
      }
    }
    getline(in,line);
    if(line == "\n")
      getline(in,line);
    out << line <<  "\n</p>\n<p>\n";
  }
  out << "</div>\n</body>\n</html>";
  out.close();
  remove("junk");
}