Example analysis of word transformation using map

Example analysis of word transformation using map
You must use the find function when looking up words from a map, not the following table, because using subscripts in a map to access nonexistent elements will cause a new element to be added to the map container, with the key of the new element being what you are looking for.

ifstream& open_file(ifstream &in, const string &file)
{
 in.close();  // close in case it was already open
 in.clear();  // clear any existing errors
 // if the open fails, the stream will be in an invalid state
 in.open(file.c_str()); // open the file we were given
 return in; // condition state is good if open succeeded
}

void WordTransform(const string rule, const string infile)
{
 if (rule.empty() || infile.empty())
 {
  return;
 }
 map<string ,string> trans_map;
 string key, value;
 // Open transformation file and check that open succeeded
 ifstream map_file;
 if (!open_file(map_file, rule))
 {
  throw runtime_error("No transformation file.");
 }
 // Read the transformation map and build the map
 while (map_file >> key >> value)
 {
  trans_map.insert(make_pair(key, value));
 }
 // Open the input file and check that the open succeeded
 ifstream input;
 if (!open_file(input, infile))
 {
  throw runtime_error("No input file.");
 }
 string line; // Hold each line from the input

 // Read the text to transform it a line at a time
 while (getline(input, line))
 {
  istringstream stream(line); // Read the line a word at a time
  string word;
  bool bFirstWordFlg = true; // Controls whether a space is printed
  while (stream >> word)
  {
   // ok: the actual mapwork, this part is the heart of the program
   map<string, string>::const_iterator map_it = trans_map.find(word);
   // If this word is in the transformation map
   if (map_it != trans_map.end())
   {
    // Replace it by the transformaion value in the map
    word = map_it->second;
   }
   if (bFirstWordFlg)
   {
    bFirstWordFlg = false;
   }
   else
   {
    cout << " "; // Print space between words
   }
   cout << word;
  }
  cout << endl; // Done with this line of input
 }
}