Why Does My C Code Read the Last Line of a Text File Twice When Checking for EOF?

Troubleshooting EOF Reading from Text Files

When reading from a text file until end-of-file (EOF) is reached, it's important to understand the behavior of the input stream to avoid duplicating lines.

In C , the ifstream object is used for reading from text files. The code provided illustrates a common approach to read integers from a text file, but it encounters an issue where the last line is read twice.

Understanding the Issue

The problem arises because the eof() function checks for EOF based on the stream's internal pointer, which is positioned after the last character of the file when EOF is reached. The code reads the last line once when it encounters the integer and again when it checks for EOF within the loop.

Solution

To fix this, it's advisable to check for EOF before reading the integer within the loop. This ensures that the last integer is read only once:

while (!iFile.eof())
{
    if (iFile.peek() != EOF)
    {
        int x;
        iFile >> x;
        cerr << x << endl;
    }
}

Here, iFile.peek() checks the next character in the stream without actually reading it. If it's not EOF, then the integer is read and output.

Alternative Approach

Additionally, the code can be rewritten using the getline function to read entire lines from the file and then parse the integers manually. This approach avoids the EOF issue altogether:

ifstream iFile("input.txt");

while (getline(iFile, line))
{
    istringstream ss(line);
    int x;
    ss >> x;
    cerr << x << endl;
}

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