C++ working with files, having trouble making a program

Question

I am trying to make a simple program that can continously add data to a .txt document. Take a look at my code:

#include <iostream>
#include <fstream>

using namespace std;

int main () {
    ofstream playersW("players.txt");
    ifstream playersR("players.txt");

    int id;
    string name, info = "";
    float money;

    while (playersR >> id >> name >> money) {
        if (playersR.is_open()) {
            info += to_string(id) + " " + name + " " + to_string(money) + "\n";
        }
    }

    playersW << info;

    playersR.close();

    cout << "Enter player ID, Name and Money (or press 'CTRL+Z' + 'Enter' to quit):" << endl;

    while (cin >> id >> name >> money) {
        if (playersW.is_open()) {
            playersW << id << " " << name << " " << money << endl;
        }
    }

    playersW.close();
}

What I want is the program to first read the data that is stored in players.txt and then write it again and also add the new additional data to players.txt.

EDIT: With the code I have now, my program only writes in the file players.txt the new information that the user enters.

why making such big string to store all the data? also include class string — Raindrop7, Dec 31 '16 at 21:24
Two streams for the same file? Not a great idea. Just use append mode? — Lightness Races in Orbit, Dec 31 '16 at 21:24
Can anyone post this as a answer please? Also, why should I include string? All my programs work just fine without including string. I think (from what I've read so far, that now string is included in iostream), so can you point out a reason or a simple program which is not going to work without including string? — S. A., Dec 31 '16 at 21:29
@S.A.: It "works" by pure chance. It may not tomorrow, or on a different OS, or a different version of your compiler, or a different compiler. Simply include the proper headers. Period! It only takes a very brief moment to fix. — Lightness Races in Orbit, Dec 31 '16 at 21:32
You can apply these suggestions to your code and write your own answer. I'm going to the pub. :) — Lightness Races in Orbit, Dec 31 '16 at 21:33
Can you write to a new file and, once complete, delete the old file and rename your new file? It is a bad idea to operate on the same file with two streams as you are. — D3C34C34D, Dec 31 '16 at 21:35
Okay, I will include string in every program since now! Also, the problem is that I don't know how to apply the suggestions, because I don't understand that. I guess I am just going to find some tutorials on working with files :), — S. A., Dec 31 '16 at 21:35

Raindrop7 · Accepted Answer · 2016-12-31T22:22:03.953

1

this is a simple program that can open the file players.txt in binary mode so it first reads the content and displays it then it asks the user to input new players until the user enters 0 or negative player id so the loop breaks and then it closes the file to save the new appended content:

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

int main ()
{

    fstream players("players.txt");

    int id;
    string name, info = "";
    float money;

    while (players >> id >> name >> money)
        cout << name << "   " << id << "   "  << money << endl;

    players.clear();
    players.seekp(0, ios::end);

    cout << "Enter player ID, Name and Money (or press 'CTRL+Z' + 'Enter' to quit):" << endl;

    while(1)
    {
        cout << "id: "; 
        cin >> id;
        cout << endl;

        if(!cin || id <= 0)
            break;

        cout << "name: ";
        cin >> name;
        if(!cin)
            break;
        cout << endl;

        cout << "money: ";
        cin >> money;
        if(!cin)
            break;
        cout << endl;

        players << id << "   " << name << "   " << money << endl;
    }

    players.close();

    return 0;
}

edited Dec 31 '16 at 22:22

answered Dec 31 '16 at 21:47

Raindrop7

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`if(id <= 0) // if so break` Insufficient. You don't ever check for success of the stream operations. – Lightness Races in Orbit Dec 31 '16 at 21:50
@LightnessRacesinOrbit: please explain – Raindrop7 Dec 31 '16 at 21:53
What happens when the end of the stream is reached? Or when "invalid" data is found in it? How will your loop be terminated? Are you [assuming C++11](http://stackoverflow.com/a/13379073/560648)? – Lightness Races in Orbit Dec 31 '16 at 21:55
@LightnessRacesinOrbit: yes for sure for example a string instead of float so do you recommend exception handling? – Raindrop7 Dec 31 '16 at 21:57
Eh, no... check the stream state. Like you do with _any_ stream operations. – Lightness Races in Orbit Dec 31 '16 at 21:57
@LightnessRacesinOrbit: like `if(players.good())` inside the loop? – Raindrop7 Dec 31 '16 at 21:59
No, as you read from `cin`, check `cin`. Which C++ book are you using? – Lightness Races in Orbit Dec 31 '16 at 22:13
1

@Raindrop7, your solution works just fine for me, Thank You! By the way, @ Lightness Races In Orbit, weren't you going to the pub?? :). Happy New Year to both of you! – S. A. Dec 31 '16 at 22:13
It doesn't work just fine. It has several critical bugs. – Lightness Races in Orbit Dec 31 '16 at 22:14
@LightnessRacesinOrbit: I am sorry I meant checking cin not fstream players object – Raindrop7 Dec 31 '16 at 22:15
@LightnessRacesinOrbit you mean `while(cin >> id >> name >> money)` then inside loop check for exit value? – Raindrop7 Dec 31 '16 at 22:16
No, you can't do that if you want your prompts. A few `if (!cin) break` would do, no? – Lightness Races in Orbit Dec 31 '16 at 22:19
@LightnessRacesinOrbit: now is it good or are there any other bugs? I tried it for a failing case and works ine breaking the loop – Raindrop7 Dec 31 '16 at 22:23
It's better now in the sense of error checking `cin`. I haven't checked for other bugs. BTW you will need more than just _one_ testcase to verify your new checks. – Lightness Races in Orbit Dec 31 '16 at 22:32
Okay, mister @LightnessRacesinOrbit, should I copy-paste my comment from above now? Or not yet? – S. A. Dec 31 '16 at 22:36
Wait how does his program run fine, when there are no {} on the first while ? – S. A. Dec 31 '16 at 22:53
@S.A. the first while has only one statement that is why I removed `{}` – Raindrop7 Dec 31 '16 at 22:56
Aha. So actually the only thing that I was missing was the moving of the pointer to the end of the document. However, I learned to use fstream isntead of both ofstream and ifstream. EDIT: and I guess i missed other small stuff... But however I now know how to do it :)! – S. A. Dec 31 '16 at 23:03
Now you're getting it – Lightness Races in Orbit Jan 01 '17 at 03:32

C++ working with files, having trouble making a program

1 Answers1