Storing information result in a file without overwriting existing information

#include <iostream>
#include <stdlib.h>
#include <string>
#include <conio.h>
#include <fstream>

using namespace std;

class contest 
{

private:
    double index;
    float x;
    char name[50];
    char s[100],ans;

public:
    contest()
    { x=0; }
    void init();
    void define();
    void display();
    void clrscr();
   };
void  contest :: define()
{
    ifstream in;	   
    char a;
    int i=0;
   
    in.open("questions.txt"); //opening file
    while(in.eof()==0)
    {
        in.getline(s, 150); //number of lines to read
        cout<<endl<<s;
        if(i==5) //if number of lines is equal to five skip next line
        {
            in.get(a); //inputting answer
            cout<<"\nans:";
            cin>>ans;// input answer
            i=-1; //display question by questions
            if(ans==a)// if answer inputted is equal to value of a
            {
                x=x+5;//add plus  5 
                cout<<" *Correct.\n";
            }
            else
            {
                cout<<" #Wrong.\n";
            }
        }
        i++;
    }
    in.close();//close file
}
void contest :: init() //input students name and index number
{
    cout<<"Sur Name:";
    cin>>name;
    cout<<"Index Number:";
    cin>>index;
}
void contest :: display()
{
    cout<<"\n **Result:\n";  //display student's name and result
    cout<<"\t"<<name<<" you got total "<<x<<" marks, out of 50.\n";
}

int main()
{
    //clrscr();
    contest r;
    cout<<"      \t\t\t		QUIZ 	\t      Total Marks:50\n\n";
    r.init();
    cout<<" "<<endl;
    cout<<"\t 5 Marks Per Questions";
L:
    cout<<"\n\n\tSELECT YOUR CHOICE:\n"
        <<"\n\t*To start Quiz 		Press 1"
        <<"\n\t*To quit		Press 0	:";
    int c;
    cin>>c;
    switch(c)
    {
    case 1:
        r.define();
        r.display();
        break;
    case 0:
        exit(1);
    default:
        cout<<"invalid choice!";
        goto L;
    }
    getch();
}