使用map實現單詞轉換的實例分析
從map中查找單詞時必須使用find函數，不能使用下表，因為在map中使用下標訪問不存在的元素將導致在map容器中添加一個新的元素，新元素的key即要查找的內容。
代碼如下：
/*****************************************************************************
* Open file
*****************************************************************************/
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
}
/*****************************************************************************
* Word Transform
*****************************************************************************/
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
}
}
