C ++使用条件语句读取文本文件

Question

我正在尝试读取文本文件中的行，标记该行，然后继续对开关和中断块中的下一行执行相同的操作，但是在我的程序到达第一个中断后，它退出了循环并忽略文件的其余部分。

ifstream in("test.txt");
    string line,buffer;
    unsigned int firstLetter = 0;
    //istringstream iss;

    if( in.is_open() )
        {
            istringstream iss;
            while(getline(in,line))
            {
                iss.str(line);
                char firstChar = line.at( firstLetter );
                switch ( firstChar )
                {
                    case 'D':
                    while (getline(iss,buffer,'-'))
                    {//print each token, one per line
                        cout<<"buffer: "<<buffer<<" "<<endl;
                    }
                    break;
                    case 'R':
                    while ( getline(iss,buffer,'-') )
                    {cout<<"buffer: "<<buffer<<" "<<endl;}
                    break;
                }
            }               
        }

这是文本文件的内容：

D-Rise of the Titans-25.50-2009-12
D-23 Jump Street-20.00-2013-5
D-Home Alone-24.00-1995-16
D-Twin Warriors-24.00-2011-14
R-XXX-Pierce-Hawthorne-11-13
R-Forbidden Kingdom-Pierce-Hawthorne-13-31
R-23 Jump Street-Troy-Barnes-1-3
R-Home Alone-Shirley-Bennett-1-3
R-Modern Family-Abed-Nadir-10-66

但是当我运行我的代码时，我得到了这个：

buffer: D 
buffer: Rise of the Titans 
buffer: 25.50 
buffer: 2009 
buffer: 12

这是文本文件中第 1 行的内容，我怎样才能转到下一行并一直这样做直到文件末尾？

Answer 1

您正在更改 std::istringstream 对象的内容，但状态标志保持原样（即，您的 eof 位亮起）。一个示例，通过标准输入使用您的输入数据进行了修改：

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
using namespace std;

int main()
{
    istream& in = std::cin;
    string line,buffer;

    istringstream iss;
    while(getline(in,line))
    {
        if (!line.empty())
        {
            iss.str(line);
            std::cout << "line at eof: " << iss.eof() << '\n';

            char firstChar = line.at(0);
            switch ( firstChar )
            {
                case 'D':
                    while (getline(iss,buffer,'-'))
                        cout<<"buffer: "<<buffer<<" "<<endl;
                    break;

                case 'R':
                    while ( getline(iss,buffer,'-') )
                        cout<<"buffer: "<<buffer<<" "<<endl;
                    break;
            }
        }
    }
}

输出

line at eof: 0
buffer: D 
buffer: Rise of the Titans 
buffer: 25.50 
buffer: 2009 
buffer: 12 
line at eof: 1
line at eof: 1
line at eof: 1
line at eof: 1
line at eof: 1
line at eof: 1
line at eof: 1
line at eof: 1

要解决这个问题，要么在每次迭代之前清除流的状态，要么只是将字符串流移动到 while 循环中，使用该行进行初始化。前者在下面完成；我将后者作为练习留给您：

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
using namespace std;

int main()
{
    istream& in = std::cin;
    string line,buffer;

    istringstream iss;
    while(getline(in,line))
    {
        if (!line.empty())
        {
            iss.clear(); // <<=== ADDED HERE
            iss.str(line);
            std::cout << "line at eof: " << iss.eof() << '\n';

            char firstChar = line.at(0);
            switch ( firstChar )
            {
                case 'D':
                    while (getline(iss,buffer,'-'))
                        cout<<"buffer: "<<buffer<<" "<<endl;
                    break;

                case 'R':
                    while ( getline(iss,buffer,'-') )
                        cout<<"buffer: "<<buffer<<" "<<endl;
                    break;
            }
        }
    }
}

输出

line at eof: 0
buffer: D 
buffer: Rise of the Titans 
buffer: 25.50 
buffer: 2009 
buffer: 12 
line at eof: 0
buffer: D 
buffer: 23 Jump Street 
buffer: 20.00 
buffer: 2013 
buffer: 5 
line at eof: 0
buffer: D 
buffer: Home Alone 
buffer: 24.00 
buffer: 1995 
buffer: 16 
line at eof: 0
buffer: D 
buffer: Twin Warriors 
buffer: 24.00 
buffer: 2011 
buffer: 14 
line at eof: 0
buffer: R 
buffer: XXX 
buffer: Pierce 
buffer: Hawthorne 
buffer: 11 
buffer: 13 
line at eof: 0
buffer: R 
buffer: Forbidden Kingdom 
buffer: Pierce 
buffer: Hawthorne 
buffer: 13 
buffer: 31 
line at eof: 0
buffer: R 
buffer: 23 Jump Street 
buffer: Troy 
buffer: Barnes 
buffer: 1 
buffer: 3 
line at eof: 0
buffer: R 
buffer: Home Alone 
buffer: Shirley 
buffer: Bennett 
buffer: 1 
buffer: 3 
line at eof: 0
buffer: R 
buffer: Modern Family 
buffer: Abed 
buffer: Nadir 
buffer: 10 
buffer: 66

Answer 2

问题是您迭代 istringstream 的内容直到失败。然后，您在其余行中重复使用相同的 istringstream。您需要清除其中的 fail/eof 位。

使用 iss.clear() 执行此操作。

#include <iostream>
#include <sstream>
#include <fstream>

int main(){
  ifstream in('test.txt');
  if (in.is_open()){
    string line, buffer;
    istringstream iss;
    while (getline(in, line)){
      iss.str(line);
      iss.clear();
      char c = line[0];
      switch (c){
        case 'D':
          while (getline(iss, buffer, '-')){
            cout << "buffer: " << buffer << endl;
          }
          break;
        case 'R':
          while (getline(iss, buffer, '-')){
            cout << "buffer: " << buffer << endl;
          }
          break;
      }
   }
   return 0;
}

C ++使用条件语句读取文本文件

C++ reading a text file with conditional statements

c++

string

file-io

tokenize

stringstream