C++ reading data from a file

Disclaimer: this question is directly related to my programming homework.

My C++ assignment consists of opening a .txt file, performing a bunch of operations on it, and then saving the .t开发者_如何学Cxt file. Problem is, I'm having a hard time just grasping the basic concepts of reading and writing files.

My code:

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

int main () {
    ifstream inData;
    ofstream outData;

    // is it necessary to open datalist.txt for both the in and out streams?
    inData.open ("datalist.txt");
    outData.open("datalist.txt");

    if (inData.is_open()) { 
        cout << "yay, i opened it\n"; // this outputs as expected

        char fileData[100]; // have to use char arrays as per instructor. no strings
        inData >> fileData; // store text from datalist.txt in fileData char array
        cout << fileData; // nothing happens here... why?

        outData << "changing file text cause I can"; // this works just fine.
    }
    else {
        cout << "boo, i couldn't open it";
    }

    inData.close();
    outData.close();

    return 0;
}

The main issue I'm encountering is that I don't understand how to read the data in a file at even a basic level, let alone parse the file into relevant information (the purpose of the program is to read, write, and manipulate information in a semi-colon delimited list).

In addition to this question, I also am a little confused on two other things. First, is it necessary to open datalist.txt for both the in and out streams, for some reason it just feels weird that I have to open the same file twice. Second, my instructor doesn't want us to use the string class, and instead use char arrays. I don't understand the logic behind this and was hoping someone could elaborate why (or perhaps give a counter argument to why) strings are bad.

You don't open a file for reading and writing at the same time. Well, not with two different objects that don't know about each other at any rate. You either use a std::fstream (which can do simultaneous reading and writing), or you read first, close the file, process the data, then write it.

Also:

//have to use char arrays as per instructor. no strings

I think you may want to get a better instructor. The use of a naked, stack-based char* array is not something that any C++ teacher worth their salt should endorse.

This is where buffer overruns come from.

Opening the same file for reading and writing via two different file objects is generally a poor idea. In your case, it has also led to (part of) your problem. By default, opening an ofstream truncates it. So when you read from inData you get nothing. That is why nothing happens here:

cout << fileData; // nothing happens here... why?

At the end, your file contains:

changing file text cause I can

And nothing else.

So to read the file, you must not open it for writing as you have. If you want to change the file text to just your string, you can simply do two separate operations. Open inData, read it, and close it. Now open outData, write our string, and close it.

On the other hand, if what you wanted was to append your string to the end of the existing file, you should open a single stream for reading and writing. Read until end of file, then with the file pointer still at the end, write your string.

That's the basic idea. Any more and I'd be doing your homework for you. :)