C ++使用fstream对象读取和写入文本文件时遇到问题

Question

I'm a C++ rookie and am having a little trouble completing an assignment, and my teacher is not going to be available for the next week, so I'm stuck for help! I'm sure I'm just tired and am missing something small, but I'm having trouble getting the fstream object to create the file, and then read from it to print to the screen, everything else seems to work ok.

Here are the instructions for the assignment, pretty straightforward and basic:

1 - Write a program that will calculate the Area and Circumference of circles.

2 - From main input the radii of the circles via the keyboard and store in an array. This must be done via a Loop. Assume a maximum of 100 records.

3 - Call a function to calculate the circumference of each circle using the radii above and store in another array.

4 - Call another function to calculate the area of the circle and store in another array.

5 - From main Print to screen the radius, circumference and area of the circles. This information should be printed from the data in the 3 arrays. Before printing the actual data, print labels for "Radius", "Circumference" and "Area" and align the information under each label.

6 - In main create a fstream object for an output file called Lecture20Output.txt.

7 - Call a function to write the radius, circumference and area in the arrays above to Lab20Output.txt

8 - From main Print to screen the contents of Lab20Output.txt.

9 - Sample Run: Radius 5, 4, 7.

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

// prototypes
void getCircumf(const double, double, double &);
void getArea(const double, double, double &);
void writeOutFile(int, double[], double[], double[], fstream &);
void greeting();

int main()
{
    const int ARRAY_SIZE = 100;
    const double PI = 3.14;
    int i = 0, i2 = 0;
    double radii[ARRAY_SIZE],
           circumf[ARRAY_SIZE],
           area[ARRAY_SIZE];
    fstream myFile;
    string line;

    // use loop to prompt user for radii
    cout << "==============================================================" << endl;
    cout << "  Below, you may enter all of your radii (up to 100 entries)  " << endl;
    cout << "         *** Enter 0 (zero) when you are finished ***         " << endl;
    cout << "==============================================================" << endl;
    while (i<100)
    {
        cout << "\nEnter your radius: ";
        cin >> radii[i];
        if (radii[i] == 0)          // test if user has no more entries
            i = 100;
        else
        {
            getCircumf(PI, radii[i], circumf[i]);   // call function to calculate circumference
            getArea(PI,radii[i], area[i]);          // call function to calculate area
            i++;
            i2++;
        }
    }

    // print results table to screen
    cout << "\n======================================="
         << "\n| Radius | Circumference |    Area    |"
         << "\n=======================================" << endl;
    for (int i=0; i<i2; i++)
    {
        cout << fixed << setprecision(2);
        cout << "| "
             << setw(6) << radii[i]
             << " | "
             << setw(13) << circumf[i]
             << " | "
             << setw(10) << area[i]
             << " |" << endl;
        cout << "---------------------------------------" << endl;
    }

    // call function to print results table to output file
    myFile.open("Lab20Output.txt", ios::out | ios::in);
    if (!myFile)
    {
        cout << "FILE OPEN ERROR!" << endl;
        return 0;
    }
    cout << "\nWe are now writing this data to a file...";
    writeOutFile(i2,radii,circumf,area,myFile);
    cout << "done." << endl;

    // print to screen the contents of file "Lab20Output.txt"
    cout << "\nNow we will read back the data from the file..." << endl;
    while (getline(myFile, line))
    {
        cout << line << '\n';
    }

    myFile.close();
    greeting();
    return 0;
}

// function definitions

void getCircumf(const double PI, double radii, double &circumf)
{
    // caluculate the circumference of a circle
    circumf = 2 * PI * radii;
}

void getArea(const double PI, double radii, double &area)
{
    // caluculate the area of a circle
    area = PI * (radii * radii);
}

void writeOutFile(int i2, double radii[], double circumf[], double area[], fstream &myFile)
{
    // print results table to myFile
    myFile << "\n=======================================\n"
           << "| Radius | Circumference |    Area    |\n"
           << "=======================================" << endl;
    for (int i=0; i<i2; i++)
    {
        myFile << fixed << setprecision(2);
        myFile << "| "
               << setw(6) << radii[i]
               << " | "
               << setw(13) << circumf[i]
               << " | "
               << setw(10) << area[i]
               << " |" << endl;
        myFile << "---------------------------------------" << endl;
    }
}

void greeting()
{
    cout << "\n========================"
         << "\n    Have a nice day!    "
         << "\n========================" << endl;
}

Answer 1

Just rewind your stream before you try to print.

myFile.seekg(0, myFile.beg);  // <---

while (getline(myFile, line))
{
    cout << line << '\n';
}

Answer 2

I'm going to take a stab that this is what you're trying to do:

// call function to print results table to output file

// NOTE: open in out/trunc mode.
myFile.open("Lab20Output.txt", ios::out|ios::trunc);
if (!myFile)
{
    cout << "FILE OPEN ERROR!" << endl;
    return 0;
}
cout << "\nWe are now writing this data to a file...";
writeOutFile(i2,radii,circumf,area,myFile);
cout << "done." << endl;
myFile.close();

// print to screen the contents of file "Lab20Output.txt"
cout << "\nNow we will read back the data from the file..." << endl;

// NOTE: Open in read-mode
myFile.open("Lab20Output.txt", ios::in);
while (getline(myFile, line))
{
    cout << line << '\n';
}

myFile.close();
greeting();
return 0;

This always truncates the output file, writes, closes, then opens in read-mode. It is hard saying for sure this is what you're hoping for based on the question, but it seems to do what you appear to want.

Output

==============================================================
  Below, you may enter all of your radii (up to 100 entries)  
         *** Enter 0 (zero) when you are finished ***         
==============================================================

Enter your radius: 10

Enter your radius: 11

Enter your radius: 0

=======================================
| Radius | Circumference |    Area    |
=======================================
|  10.00 |         62.80 |     314.00 |
---------------------------------------
|  11.00 |         69.08 |     379.94 |
---------------------------------------

We are now writing this data to a file...done.

Now we will read back the data from the file...

=======================================
| Radius | Circumference |    Area    |
=======================================
|  10.00 |         62.80 |     314.00 |
---------------------------------------
|  11.00 |         69.08 |     379.94 |
---------------------------------------

========================
    Have a nice day!    
========================