c++ reading undefined number of lines with eof()

问题:

I'm dealing with a problem using eof(). using

string name;
int number, n=0;
while(!in.eof())
{
    in >> name >> number;
    //part of code that puts into object array
    n++;
}

sounds normal to me as it whenever there are no more text in the file. But what I get is n being 4200317. When I view the array entries, I see the first ones ats the ones in the file and other being 0s.

What could be the problem and how should I solve it? Maybe there's an alternative to this reading problem (having undefined number of lines)

回答1:

The correct way:

string name;
int    number;
int    n     = 0;

while(in >> name >> number)
{
    // The loop will only be entered if the name and number are correctly
    // read from the input stream. If either fail then the state of the
    // stream is set to bad and then the while loop will not be entered.

    // This works because the result of the >> operator is the std::istream
    // When an istream is used in a boolean context its is converted into
    // a type that can be used in a boolean context using the isgood() to
    // check its state. If the state is good it will be converted to an objet
    // that can be considered to be true.


    //part of code that puts into object array
    n++;
}

Why your code fails:

string name;
int number, n=0;
while(!in.eof())
{
    // If you are on the last line of the file.
    // This will read the last line. BUT it will not read past
    // the end of file. So it will read the last line leaving no
    // more data but it will NOT set the EOF flag.

    // Thus it will reenter the loop one last time
    // This last time it will fail to read any data and set the EOF flag
    // But you are now in the loop so it will still processes all the
    // commands that happen after this. 
    in >> name >> number;

    // To prevent anything bad.
    // You must check the state of the stream after using it:
    if (!in)
    {
       break;   // or fix as appropriate.
    }

    // Only do work if the read worked correctly.
    n++;
}

回答2:

in << name << number;

This looks like writing, not reading. Am I wrong?

回答3:

int number, n = 0;

You weren't initializing n, and you seem to have a typo.

回答4:

This probably would be more correct

string name;
int number, n = 0;

while (in >> name && in >> number)
{
    n++;
}

The eof is a bad practice.

Note that there is a subtle difference here from your code: your code ended when it encountered an eof or silently looped for infinite time if it found a wrong line (Hello World for example), this code ends when it encounters a non correctly formatted "tuple" of name + number or the file ends (or there are other errors, like disconnecting the disk during the operation :-) ). If you want to check if the file was read correctly, after the while you can check if in.eof() is true. If it's true, then all the file was read correctly.