Question

I am creating a program (In C++) that takes an ASCII file and reads a few values from each line until it reaches the end of the file. I am using ifstream to read the file, and I have never had problems with it stopping when I use the ifstream.eof() method. This time, however, even though it found the eof character in my test case, when I analyzed my other files, it is infinite looping because it never finds the eof character. Is this a coding issue, or an issue with my files?

string line = "";
unsigned long pos = 0;
ifstream curfile(input.c_str());
getline(curfile, line);
int linenumber = 0;
cout<<"About to try to read the file"<<endl;
if (!curfile.good())
    cout<<"Bad file read"<<endl;
while (!curfile.eof())
{

    cout<<"Getting line "<<linenumber<<endl;
    linenumber++;
    pos = line.find_first_of(' ');
    line = line.substr(pos+1, line.size()-1);
    pos = line.find_first_of(' ');
    current.push_back(atof(line.substr(0, pos).c_str()));
    for (int i = 0; i<4; i++)
    {
        pos = line.find_first_of(' ');
        line = line.substr(pos+1, line.size()-1);
    }
    pos = line.find_first_of(' ');
    dx.push_back(atof(line.substr(0, pos).c_str()));
    pos = line.find_first_of(' ');
    line = line.substr(pos+1, line.size()-1);
    pos = line.find_first_of(' ');
    dy.push_back(atof(line.substr(0, pos).c_str()));
    getline(curfile, line);
}

EDIT: When I first run the loop, currentfile.good() returns false…what am I doing that causes it to return that?

Answer 1

First thing is first, you shouldn’t check like that. eof() doesn’t return true until after a failed read. But you can do better (and easier)!

check the stream state with the implicit conversion to void* which can be used in a bool context. Since most of the read operations on streams return a reference to the stream, you can write some very consice code like this:

std::string line;
while(std::getline(currentfile, line)) {
    // process line
}

Basically what it is doing is saying “while I could successfully extract a line from currentfile, do the following”, which is what you really meant to say anyway ;-);

Like I said, this applies to most stream operations, so you can do things like this:

int x;
std::string y;
if(std::cin >> x >> y) {
    // successfully read an integer and a string from cin!
}

EDIT: The way I would rewrite your code is like this:

string line;
unsigned long pos = 0;
int linenumber = 0;

ifstream curfile(input.c_str());

std::cout << "About to try to read the file" << std::endl;
while (std::getline(curfile, line)) {

    std::cout << "Getting line " << linenumber << std::endl;
    linenumber++;

    // do the rest of the work with line
}