End of File in C++ | 易学教程

问题

I have a n X 2 matrix stored in a text file as it is. I try to read it in C++

nb_try=0;
fin>>c_tmp>>gamma_tmp;
while (!fin.eof( ))      //if not at end of file, continue reading numbers
{
  // store
  cs_bit.push_back(c_tmp);
  gammas_bit.push_back(gamma_tmp);
  nb_try++;

  // read
  fin>>c_tmp;
  assert(!fin.fail( )); // fail at the nb_try=n   
  if(fin.eof( ))break;
  fin>>gamma_tmp; // get first number from the file (priming the input statement)
  assert(!fin.fail( ));    

}

The first assert failes, i.e. fin.fail( ) is true, when nb_try==n, which happens when it tries to read the first number which does not exist. But how come fin.eof( ) is not true after reading the last number? Does it mean it becomes true only when reading the first number that does ot exist? Also is it true that fin.fail( ) and fin.eof( ) are becoming true at the same time?

Thanks and regards!

回答1:

This is the wrong way to read a file:

while (!fin.eof( ))
{
      // readLine;
      // Do Stuff
}

The standard pattern is:

while(getlineOrValues)
{
    // Do Stuff
}

So looking at your code quickly I think it would be asier to write it as:

while(fin>>c_tmp>>gamma_tmp)
{
    // loop only eneterd if both c_tmp AND gamma_tmp
    // can be retrieved from the file.

    cs_bit.push_back(c_tmp);
    gammas_bit.push_back(gamma_tmp);
    nb_try++;   
}

The problem is that EOF is only true AFTER you try and read past it. Having no character left in the file to read is not the same as EOF being true. So you read the last line and get values and there is nothing left to read, but EOF is still false so the code re-enter the loop. When it tries and read the c_tmp then EOF gets triggered and your asserts go pear shaped.

The solution is to put the read as the while condition. The result of doing the read is the stream. But when a stream is used in a boolean context (such as a while condition) it is converted into a type that can be used as like a bool (Technically it is a void* but thats not important).

回答2:

IIRC, the eofbit doesn't get set until you actually try to read past the end of the file. That is, once you reach the end of the file, you have to read one more time before that flag will be set.

回答3:

If the text file contains this sequence, without quotes, "12345 67890", then #3 will return false, but #4 will return true because there is no whitespace after the last number:

int i;
bool b;

fin >> i;

b = fin.fail();  // 1
b = fin.eof();   // 2

fin >> i;

b = fin.fail();  // 3
b = fin.eof();   // 4

fin >> i;

b = fin.fail();  // 5
b = fin.eof();   // 6

However, if the sequence is "12345 6789 " (note the space after the last number), then #3 and #4 will both return false, but #5 and #6 will return true.

You should check for both, eof() and fail() and if both are true, you have no more data. If fail() is true, but eof() is false, there's a problem with the file.

来源：https://stackoverflow.com/questions/1494342/end-of-file-in-c

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