Solving Vector Length Test Problem: EOF & Do-While Loops

[Vanadium 50]

the exit condition needs to be checked twice.

What is wrong with that? Logically, the condition for deciding to push the record is not necessarily the same as the condition to continue reading. They happen to be the same in this case, but they don't have to be.

 

[yungman]

I have to do this to make it work. I remember last time when I have the issue, the eof() does not become true upon reading the last element, it's one after the last element. But it will still contain the last element in the buffer. So the next push_back, it will repeat writing the last element again.

The following way can fix it:

We spent a lot of time on this, it's so important that I wrote down clearly in my notes.

Unless there is any reason against this way, I am going to put this in my program. This will definitely avoid reading empty file.

Thanks

 

[Mark44]

It has been stated at least three times by three different people that you should NOT be doing file I/O in the constructor, and same for the destructor.

 

[yungman]

It has been stated at least three times by three different people that you should NOT be doing file I/O in the constructor, and same for the destructor.

I know, give me time to change the program, want to resolve the double read first. I still need to change the names also.

Then I can delete the Constructor and Distructor, can't think of any reason to keep them.

I just created readFile() and writeFile() and copy the Constructor and Distructor into them resp. Then write code in main() to call readFile() at the beginning and call writeFile at the end of the program.

 

[yungman]

I fixed the program, file reading and writing are out of the constructor and distructor. In fact, I got rid of the constructor and distructor. I don't see any use of them. This is a running program.

I change the names to more readable. I don't need to have double read in line 30 after I move the while(!file.eof()) to the top instead of do-while().

This is the .h file

#ifndef VectorCall_H
#define VectorCall_H
#include <vector>
#include <fstream>
class DirectoryClass
{
  private:
    const static int nameLen = 25, phoneLen = 12;
  public:
    struct Directory { char name[nameLen];    char phone[phoneLen]; };
    std::vector<Directory>DirV;
    Directory Dir; bool newF, failOpen;
    std::fstream file;
 
    bool readFile()//
    {
        failOpen = false;
        file.open("Directory.dat", std::ios::in | std::ios::binary);
        if (file.fail())
        {//If file doesn't exist, create the file
            std::cout << " Fail to open file.\n";
            file.open("Directory.dat", std::ios::out | std::ios::binary | std::ios::app);
            file.close();
            failOpen = true;
        }
        else
        {//If the file exist, read entire file into vector DirV.
            while (!file.eof())
            {
                file.read(reinterpret_cast<char*>(&Dir), sizeof(Dir));
                if (file.eof()) {  break; }
                DirV.push_back(Dir);
            };
            file.close();         
        }
        return failOpen;
    }
    void createFile()
    { file.open("Directory.dat", std::ios::out | std::ios::binary | std::ios::app);
      file.close(); }
    void writeFile()
    {
        int ct2 = 0;
        file.open("Directory.dat", std::ios::out | std::ios::binary);
        do//write the updated data in vector DirV into the file before closing
        {
            file.write(reinterpret_cast<char*>(&DirV[ct2]), sizeof(DirV[ct2]));
            ct2++;
        } while (ct2 < DirV.size());
        file.close();
    }
    void push_backV() { DirV.push_back(Dir); }
    void print() { std::cout << "Name:  " << Dir.name <<
            "   Phone:  " << Dir.phone << "\n\n"; }
};
#endif

This is the source.cpp

#include <iostream>
#include <string>
#include <vector>
#include "VectorCall.h"
using namespace std;

int main()
{
  char more;  int index = 0; const int nameLen = 25, phoneLen = 12;
  DirectoryClass Information;
  char displayAll, tryAgain = false;
  Information.readFile();
  if (Information.failOpen == true)
  {cout << " Fail to open file, want to try again? "; cin >> tryAgain;}
  if (tryAgain == true) Information.readFile();
  if (Information.failOpen == true)
  {//First time set up the directory by entering all the names and information
    cout << " File does not exist, let's create the file. " <<
         "You need to enter informations:\n\n";
    Information.createFile();
    cin.ignore();
    do
    {   cout << " Enter name:  "; cin.getline(Information.Dir.name, nameLen);
        cout << " Enter phone:  "; cin.getline(Information.Dir.phone, phoneLen);
        //Info.print();
        Information.push_backV();//writing into vector DirV.
        cout << " Do you want to enter another name?  "; cin.get(more);
        cin.ignore();
    } while (tolower(more) == 'y');
  }
//Ask whether to display all the information in the file.
    cout << "  Press 'y' to see the whole list.  "; cin >> displayAll;
    cin.ignore(); cout << endl;
    if (displayAll == tolower('y'))
    {   do
        {  cout << "Name stored in DirV is:  " << Information.DirV[index].name <<
                  " phone:  " << Information.DirV[index].phone << "\n\n";
           index++;
        } while (index < Information.DirV.size());
    }
    else { cout << " Bye\n\n"; }
    Information.writeFile();
    return 0;
}

Hope this looks better. Let me know what else.I was looking at the next program in the book, I notice even with the Declaration and Implementation files, the book shows using a lot of internal functions within the main program body. I would think it's better to push as much to external as possible.

I understand Class is a blueprint to create object incidence. But I keep thinking it's more useful to put as much work ( functions) into the Specification and Implementation files as possible so the main() don't have to do a lot. It's like calling external functions to do the job. Isn't it the point that try to use external function( or whatever name you call) to do as much as possible so you can make the main() as simple as possible for fast turn around? It's like structure, define the structure and use it to make lives easier.

I can envision having a class to read and write file, using arguments like 1) c-string that contains the filename, path where to save or to open the file, 2) the kind of file and 3) the vector to write to the file like what I am doing. So this Class can read and write different files with different ADT datatype.

Then another class to define structure, cin information, store into a vector like what I am doing. Within that, I can put add element, delete element, sort elements.

With that, the main() just call them to do all the jobs. So far, things are still short, I use inline function. But I am sure when I add in the add, delete and sort, I have to have a separate Implementation file to avoid a very long declaration file.

Thanks

 

[jtbell]

What are the problems reading an empty file? If just reading garbage, then it's not too bad. But if it can do other damage, then it's not good.

If you try to read from an empty file, you reach end-of-file immediately. I tested this with the following program:

#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

int main ()
{
//  Open a file for output, but don't write anything to it.
//  Then close it.

    ofstream outputFile ("emptyfile.dat");
    cout << "Trying to open emptyfile.dat for output..." << endl;
    if (outputFile)
    {
        cout << "Successfully opened emptyfile.dat for output." << endl;
    }
    else
    {
        cout << "Failed to open emptyfile.dat for output." << endl;
    }
    outputFile.close();
    cout << "Closed emptyfile.dat." << endl;

//  Open that file for input, and try to read from it.

    cout << "Trying to open emptyfile.dat for input..." << endl;
    ifstream inputFile ("emptyfile.dat");
    if (inputFile)
    {
        cout << "Successfully opened emptyfile.dat for input." << endl;
        cout << "Trying to read from emptyfile.dat..." << endl;
        string line;
        if (getline (inputFile, line))
        {
            cout << "'" << line << "' was read from emptyfile.dat." << endl;
        }
        else
        {
            cout << "EOF on emptyfile.dat." << endl;
        }
        inputFile.close();
        cout << "Closed emptyfile.dat." << endl;
    }
    else
    {
        cout << "Can't open emptyfile.dat." << endl;
    }    

    return 0;
}

Output:

Trying to open emptyfile.dat for output...
Successfully opened emptyfile.dat for output.
Closed emptyfile.dat.
Trying to open emptyfile.dat for input...
Successfully opened emptyfile.dat for input.
Trying to read from emptyfile.dat...
EOF on emptyfile.dat.
Closed emptyfile.dat.

 

[yungman]

I am so glad I posted my program in post 65! I can't find my original program now that I am ready to get back to it! I have to copy both files from post 65 to recreate the program! That really save my day.

I finally finish the other part of the program in sorting, adding, deleting, now back to the Class and files.