解析流

发布于 2024-09-26 10:04:04 字数 687 浏览 7 评论 0原文

我正在解析一个包含字符串和数值的文件。我想逐个字段处理文件，每个字段都由空格或行尾字符分隔。 ifstream::getline() 操作仅允许单个分隔字符。因此，我当前所做的是以字符 ' ' 作为分隔符的 getline，然后如果遇到 '\n' 则手动返回到流中的上一个位置：

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 while (ifs.good())
 {  
  char curField[255];  

  pos = ifs.tellg();
  ifs.getline(curField, 255, ' '); 
  string s(curField);
  if (s.find("\n")!=string::npos) 
  {   
   ifs.seekg(pos); 
   ifs.getline(curField, 255, '\n'); 
   s = string(curField);
  }

 // process the field contained in the string s...
 }

但是，“seekg”似乎定位了流一个字符太晚了（因此我错过了每个换行符之前每个字段的第一个字符）。我知道还有其他方法可以通过逐行扫描等方式来编码这样的解析器，但我真的很想了解为什么这段特定的代码失败...

非常感谢！

原文

I am parsing a file which contains both strings and numerical values. I'd like to process the file field by field, each delimited by a space or an end-of-line character.
The ifstream::getline() operation only allows a single delimiting character. What I currently do is thus a getline with the character ' ' as a delimiter, and then manually go back to the previous position in the stream if a '\n' has been encountered :

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 while (ifs.good())
 {  
  char curField[255];  

  pos = ifs.tellg();
  ifs.getline(curField, 255, ' '); 
  string s(curField);
  if (s.find("\n")!=string::npos) 
  {   
   ifs.seekg(pos); 
   ifs.getline(curField, 255, '\n'); 
   s = string(curField);
  }

 // process the field contained in the string s...
 }

However, the "seekg" seems to position the stream one character too late (I thus miss the first character of each field before each line break).
I know there are other ways to code such a parser, by scanning line by line etc.., but I'd really like to understand why this particular piece of code fails...

Thank you very much!

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风流物 2024-10-03 10:04:04

正如 Loadmaster 所说，可能存在下落不明的字符，或者这可能只是一个相差一的错误。

但这只是必须说的......你可以

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 while (ifs.good())
 {  
  char curField[255];  

  pos = ifs.tellg();
  ifs.getline(curField, 255, ' '); 
  string s(curField);
  if (s.find("\n")!=string::npos) 
  {   
   ifs.seekg(pos); 
   ifs.getline(curField, 255, '\n'); 
   s = string(curField);
  }

 // process the field contained in the string s...
 }

用这个替换这个：

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 string s;
 while (ifs.good())
 {  
   ifs >> s;
   // process the field contained in the string s...
 }

得到你想要的行为。

As Loadmaster said, there may be unaccounted for characters, or this could just be an off-by-one error.

But this just has to be said... you can replace this:

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 while (ifs.good())
 {  
  char curField[255];  

  pos = ifs.tellg();
  ifs.getline(curField, 255, ' '); 
  string s(curField);
  if (s.find("\n")!=string::npos) 
  {   
   ifs.seekg(pos); 
   ifs.getline(curField, 255, '\n'); 
   s = string(curField);
  }

 // process the field contained in the string s...
 }

With this:

 ifstream ifs ( filename , ifstream::in );
 streampos pos;
 string s;
 while (ifs.good())
 {  
   ifs >> s;
   // process the field contained in the string s...
 }

To get the behavior you want.

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