error in reading and writing in files [closed] - c++

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This program works but prints garbage values at the beginning (ch=2) and prints the same output twice.
I am using this for my project.It contains more data and there i used a single object instead of array of objects.It didnt work.Each object of array stores a set of data.
#include<iostream>
#include<fstream>
using namespace std;
class rw //a class containing some data
{
public:
int n;
char a;
};
int main()
{
int i;
rw r[2]; //an array of objects
int ch;
fstream f1;
f1.open("file1.txt",ios::in|ios::out|ios::ate);
cout<<"1-read,2-write";
cin>>ch;
f1.seekp(0,ios::beg);
if(ch==1)//for saving
{
r[0].n=1;
r[0].a='a';
f1.write((char*) &r[0],sizeof(r[0]));
r[1].n=2;
r[1].a='b';
f1.write((char*)&r[1],sizeof(r[1]));
}
if(ch==2)//for reading
{
f1.seekg(0,ios::beg);
while(!f1.eof())
{
i=0;
cout<<r[i].n<<r[i].a;
cout<<"\n";
f1.read((char*)&r[i],sizeof(r[i]));
i++;
}
}
system("pause");
return 0;
}

Answer 1

Change following code
cout<<r[i].n<<r[i].a;
cout<<"\n";
f1.read((char*)&r[i],sizeof(r[i]));
to
f1.read((char*)&r[i],sizeof(r[i])); // Moved before printing
cout<<r[i].n<<r[i].a;
cout<<"\n";
You are outputting data then reading from the file, instead you should read and then print. Printing before reading is the reason for getting garbage in first run of loop. I think you moved the reading below to avoid double printing of last record. Continue reading to understand how to avoid double reading.
You should avoid using while (!f1.eof()), as it will print last data twice.
Better use while(f1.read((char*)&r[i],sizeof(r[i])));
Unrolling the eof version of loop for 2 inputs
while(!eof) // true
read first data
print first data
while(!eof) // true
read second data // read all, but was succesful and eof not set
print second data
while(!eof) // true
Try to read // Could not read, eof set
print data // Reprinting second data
while(!eof) // false

Answer 2

Don't use while (!f1.eof()), it will not work as you expect it to, because the eofbit flag isn't set until after you try to read from beyond the file. Instead do e.g. while (f1.read(...)).
Also be careful with a loop like this without bounds checking. If the file is not correct you might write out of bounds of the array r.