c++读取文件太慢

Question

我正在尝试读取 ~36KB，完成这个循环需要 ~20 秒:

ifstream input_file;

input_file.open("text.txt");
if( !(input_file.is_open()) )
{
    cout<<"File not found";
    exit(1);
}

std::string line;
stringstream line_stream;   //to use << operator to get words from lines

int lineNum=1;

while( getline(input_file,line) )   //Read file line by line until file ends
{
    line_stream.clear();    //clear stream
    line_stream << line;    //read line
    while(line_stream >> word)  //Read the line word by word until the line ends
    {
        //insert word into a linked list...
    }
    lineNum++;
}
input_file.close();

如有任何帮助，我们将不胜感激。

Answer 1

stringstream::clear() 不会清除其中的所有上下文。它只重置错误和 EOF 标志，参见 http://en.cppreference.com/w/cpp/io/basic_ios/clear .

结果是您的 line_stream 累积所有先前的行，并且内部循环将一次又一次地在所有累积的行上运行单词。

因此，与您预期的 O(n) 相比，您花费的总时间约为 O(n^2)。

您可以在 while 循环中定义新的 line_stream 实例，而不是在每一行中使用相同的对象，以拥有一个全新的空实例。像这样:

fstream input_file;

input_file.open("text.txt");
if( !(input_file.is_open()) )
{
    cout<<"File not found";
    exit(1);
}

std::string line;

int lineNum=1;

while( getline(input_file,line) )   //Read file line by line until file ends
{
    stringstream line_stream;   // new instance, empty line.
    line_stream << line;    //read line
    while(line_stream >> word)  //Read the line word by word until the line ends
    {
        //insert word into a linked list...
    }
    lineNum++;
}
input_file.close();