Basic File Input/Output Test

[michaelt]

Hi!

I am trying to test basic file input and output functions. I was trying to write a very simple code to output numbers entered by the user to a file, then read those numbers back in from the file.

I feel like I am missing something simple because it is letting me input numbers, and then after a negative number is entered, responding "unable to open file".

  #include <iostream>
#include <fstream>
using namespace std;

int main () 
{
    int number;
  
    // Output data input by user to file
    ofstream outputfile;
    outputfile.open ("tempfile.txt");
    
    do
    {
        cout << "Enter a positive number ";
        cin >> number;
        outputfile << number << '\n';
    }      
    while (number >= 0);
    outputfile.close();
    
    //Read in data from file and print on screen  
    ifstream infile;
    infile.open ("tempfile.txt");
    
    if (infile.is_open() )
    {
        while ( ! infile.eof() )
            {
                cout << number << '\n';
            }
    infile.close();
    }
    
    else
    
    cout << "Unable to open file";
    

    return 0;
}

[tpb]

I don't know why it's printing "unable to open file", but the input loop isn't reading anything from the file. And you aren't testing for end-of-file properly. Using f.eof() is not really the way to do it. Instead, you should test the input functions themselves. Try this:

#include <iostream>
#include <fstream>
using namespace std;

int main ()  {
    int number;
    ofstream outputfile("tempfile.txt");
    
    while (true) {
        cout << "Enter a positive number ";
        cin >> number;
        if (number <= 0)
            break;
        outputfile << number << '\n';
    }      
    outputfile.close();
    
    ifstream infile("tempfile.txt");
    if (infile.is_open()) {
        while ( infile >> number )
            cout << number << '\n';
        infile.close();
    }    
    else
        cout << "Unable to open file";

    return 0;
}

[zaphraud]

First problem, zero isn't positive. Should be > instead of >=

second problem, the number is output to the file even if its not a positive number, before the loop exits. When I cancel out of the infinite loop and examine the file, a -1 is the last number if thats the last thing I entered.

third problem, you never actually read from the file. So it just loops forever, printing that -1....

(I only fixed the third problem, which verified there was no file not found error. If you actually are getting a file not found error then you have an issue with your OS needing to have its write cache flushed. I ran into something like that coding visual basic scripts to run from an Access database 20 years ago so I know it can happen under some conditions... Not sure how you fix it though, because don't have any idea what your exact situation is.)

$ ./iotest
Enter a positive number 4
Enter a positive number 3
Enter a positive number 4
Enter a positive number -1
4
3
4
-1
-1

  #include <iostream>
#include <fstream>
using namespace std;

int main () 
{
    int number;
  
    // Output data input by user to file
    ofstream outputfile;
    outputfile.open ("tempfile.txt");
    
    do
    {
        cout << "Enter a positive number ";
        cin >> number;
        outputfile << number << '\n';
    }      
    while (number >= 0);
    outputfile.close();
    
    //Read in data from file and print on screen  
    ifstream infile;
    infile.open ("tempfile.txt");
    
    if (infile.is_open() )
    {
        while ( ! infile.eof() )
            {
		infile >> number;
                cout << number << '\n';
            }
    infile.close();
    }
    
    else
    
    cout << "Unable to open file";
    

    return 0;
}

[michaelt]

Thank you for those fixes, but my problem still stands because I am not able to display anything when the program gets to the ifstream portion. it skips to the else statement and won't display anything. I figured it was because the file isn't opening? I'm still lost...

[Thomas1965]

Try using perror maybe this will tell you the problem.
http://www.cplusplus.com/reference/cstdio/perror/

[zaphraud]

Oh wait, you aren't specifying a path for your program.

If its running from somewhere where it doesn't have permission to read and write to, then you will have problems... try running it from within a cmd.exe (Win) or terminal(Mac) window and see what happens.