Problem writing to external file

This is a discussion on Problem writing to external file within the C++ Programming forums, part of the General Programming Boards category; I am attempting to write a program for an assignment. I have to use several functions to open and then ...

  1. #1
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    Mar 2006
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    Problem writing to external file

    I am attempting to write a program for an assignment. I have to use several functions to open and then write some data to an external file. I have to use one function just to open the external file and then another function to write a header with columns to the file. It is a requirement that we allow the user to name the file. Here is my difficulty; I was able to allow the user to name the file and then open it, but it doesn't write the data to the file. Here is my code.
    Code:
    
    #include <iostream>
    #include <iomanip>
    #include <fstream>
    #include <cmath>
    #include <cstdlib>     
    #include <fstream>     
    #include <iostream>
    
    using namespace std;
    
    int openOutputFile(char n[]);
    void writeHeaderLinesToOutputFile();
    int createAndSaveLoanData();
    double calculateOneLineOfLoanResults();
    double calculateMonthlyPayment(double r, double expm, double yrs);
    double calculateTotalPayment(double mPmt);
    int writeLineOfResultsToOutputFile();
    
    int main()
    {
        
       
        ofstream outFile;    
        char outputName[50];
       
        cout << "Please type the name of the output file, including the extension: ";
        cin >> outputName;
        openOutputFile(outputName);
        
        cin.get();
        return 0;
    }
    int openOutputFile(char name[50])
    {
        
        ofstream outFile; 
        
        outFile.open(name);
            if (outFile.fail())
            {
                cerr << "Could not open " << name << "!" << endl;
                return EXIT_FAILURE;
            }
        writeHeaderLinesToOutputFile();    
        return 0;
    }
    void writeHeaderLinesToOutputFile()
    {
        ofstream outFile; 
        
              outFile << "Loan Amount: $1000" << '\n'; 
              outFile << left << setw(9) << "Interest" << left << setw(9) << "Duration" << left << setw(9)  << "Monthly"; 
              outFile << left << setw(9) << "Total" << '\n';
              outFile << left << setw(9)  << "rate" << left << setw(9) << "(years)" << left << setw(9) << "payment";
              outFile << left << setw(9) << "payment" << '\n';
              outFile.close();
    }
    Can someone tell me what I am going wrong? I am pretty sure the problem is occurring because the file is being opened in one function and written to in another, but I have no idea how to get around it.
    Last edited by kristentx; 09-06-2007 at 02:45 PM.

  2. #2
    C++ Witch laserlight's Avatar
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    I am pretty sure the problem is occurring because the file is being opened in one function and written to in another, but I have no idea how to get around it.
    You need to pass around the ofstream by reference. It could be something like:
    Code:
    int openOutputFile(char name[50])
    {
        ofstream outFile(name);
        if (outFile.fail())
        {
            cerr << "Could not open " << name << "!" << endl;
            return EXIT_FAILURE;
        }
        writeHeaderLinesToOutputFile(outFile);
        outFile.close();
        return 0;
    }
    void writeHeaderLinesToOutputFile(ofstream& outFile)
    {
        outFile << "Loan Amount: $1000" << '\n'; 
        outFile << left << setw(9) << "Interest" << left << setw(9) << "Duration" << left << setw(9)  << "Monthly"; 
        outFile << left << setw(9) << "Total" << '\n';
        outFile << left << setw(9)  << "rate" << left << setw(9) << "(years)" << left << setw(9) << "payment";
        outFile << left << setw(9) << "payment" << '\n';
    }
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  3. #3
    Registered User
    Join Date
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    Your outFile variables are local to the functions they are declared in. They are two different variables. Inside the openOutputFile function, you open the output file successfully, but when the function ends the outFile variable is destroyed. When an fstream is destroyed it closes the file.

    Later, in your writeHeaderLinesToOutputFile function, you create a new variable called outFile. Then you don't open a file there, you just try to write to it, which of course fails.

    The solution to this problem is to create the ofstream variable in main and pass it as an argument to the two functions, then have those functions work on it instead of a local variable. Since you will be modifying the ofstream, make sure to pass it by reference.

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