file.get() returning random numbers after a loop C++

Question

I am working on a program that reads a series of ints from a text file into a 2D array.
The file contains 40 lines of 81 numbers that have no spaces between them.

The problem is that when I cout the array after the loop has finished, it is outputting 2 random numbers in array[0][0] and array[0][1] before the expected output. I think it has to do with the newline/carriage return characters. The first iteration of the loop runs perfectly. Here is the code:

#include <cstring>
#include <cstdlib>
#include <iostream>
#include <fstream>

using namespace std;

int main()
{
  int array[9][9];

  //Open file:

  fstream ifile;

  ifile.open("numbers.txt", ios::in);
  if (ifile.fail())
    {
      cout << "Could not open numbers.txt" << endl;
      return -1;
    }

   while (!ifile.eof())
    {
      for(int i=0; i<9; i++)
      {   
    for(int j=0; j<9; j++)
    {
       int n = ifile.get();
              if(isdigit(n)) 
            {
                  array[i][j] = n - '0';
        }

          cout<<"array ["<<i<<"]["<<j<<"] is "<<array[i][j]<<endl;
      } 
      } cout<<"This is a test"<<endl;
    }

  return 0;
}

Answer 1

I don't understand the purpose of the outer loop at all. First, file will never be equal to eof(), or... What is eof(), any way? Second, if you've actually written while ( !file.eof() ), this could explain the fact that some of the elements get overwritten. There will likely be some trailing characters (a new line, at least) after the last digit, so you'll reenter the loop again.

And you're incrementing the indexes even when the character you read is not a digit. If the data is 9 lines of 9 digits, you'll end up with 9 cells in grid which haven't been initialized, and 9 characters that haven't been read from the file once you've finished the inner two iterations. So you'll enter the outer loop again, reading these characters. Some of which will be digits, so you'll end up overwriting cells in grid that you've already written—this is probably the effect you're observing. Also that once you reach the end of the file, file.get() will start returning EOF—typically -1. That's doubtlessly why your tests for '\n' and '\r' didn't work.

And these are just the problems with a correctly formatted file. For a correctly formatted file, just using file >> n, with char n; would almost work; operator>> skips whitespace. But you'd still enter the outermost loop a second time, since file.eof() won't be reliable until an input has failed. You say "I have to use this", but your code can't be made to work unless you change it.

Personally, I favor robust solutions, with lots of error checking. I'd use std::getline(), and I'd verify that each line contained exactly 9 digits. Something like:

std::string line;
int i = 0;
while ( i < 9 && std::getline( file, line ) ) {
    if ( line.size() != 9 ) {
        throw FormatError( "wrong line length" );
    }
    for ( int j = 0; j != 9; ++ j ) {
        if ( ! isdigit( static_cast<unsigned char>( line[j] ) ) ) {
            throw FormatError( "illegal character" );
        }
        grid[i][j] = line[i] - '0';
    }
}
if ( i != 9 || std::getline( file, line ) ) {
    throw FormatError( "wrong line count" );
}

It wouldn't be too hard to use file.get(), and read one character at at time, but you'd still want to check for EOF after each read:

for ( int i = 0; i != 9; ++ i ) {
    for ( int j = 0; j != 9; ++ j ) {
        int ch = file.get();
        if ( ch == EOF ) {
            throw FormatError( j == 0 
                                ? "line too short" 
                                : "too few lines" );
        }
        if ( !isdigit( ch ) ) {
            throw FormatError( "illegal character" );
        }
        grid[i][j] = ch - '0';
    }
    int ch = file.get();
    if ( ch != '\n' ) {
        throw FormatError( ch == EOF ? "too few lines" : "line too long" );
    }
}
if ( file.get() != EOF ) {
    throw FormatError( "too many lines" );
}

Answer 2

The random numbers appear because you increment j regardless of whether you write to grid[i][j].

Try replacing your inner loop with:

for(int j=0; j<9; )
{
  int n = file.get();
  if(!file) break;
  if(isdigit(n))
  {
    array[i][j] = n - '0';
    cout<<"array ["<<i<<"]["<<j<<"] is "<<array[i][j]<<endl;
    j++;
  }
  cout<<"grid ["<<i<<"]["<<j<<"] is "<<grid[i][j]<<endl;
} 

Answer 3

eof isn't set when you reach the end of the file, it's set after a read fails. And the data from that last read is of course invalid, because it failed. Your current code uses the invalid data...

---

Beyond that, file != eof() is all kinds of wrong. It shouldn't even compile, because there is no ::eof() function, and iostream::eof() needs an object. file != EOF might compile, but then file will be converted to bool and promoted to int (either 0 or 1), it will never be equal to EOF (-1). What you meant was !file.eof(), but that's also wrong for the reason given above.