Giúp bài tập C++ về ghi và đọc File

#include <fstream> 
ifstream Docn ("tên file");
Docn>>n;

#include<iostream>
#include<fstream> 
using namespace std;
int checkb=0;
int checkc=0;
int snt(int n)
{
	if(n < 2)
		return 0;
	if(n ==2 || n ==3 )
		return 1;
	if(n %2 ==0 || n %3 ==0)
		return 0;
	for(int i = 5 ; i < n ; i +=2)
		if(n % i ==0)
			return 0;
	return 1;
}
int docn(ifstream &filein)
{
	int n;
	filein>>n;
	return n;
}
long tongchan(ifstream &filein, int a[], int n)
{
	long s=0;
	for(int i = 0 ; i < n ; i ++)
	{
		filein>>a[i];
		if(a[i] % 2 ==0)
			s+=a[i];
	}
	return s;
}
void chiak(int a[], int n, int k, int b[])
{
	for(int i = 0 ; i < n ; i ++)
		if(a[i] % k == 0)
			b[checkb++] = a[i];
}
void ktsnt(int a[], int n, int c[])
{
	for(int i = 0 ; i < n ; i ++)
		if(snt(a[i])== 1)
			c[checkc++] = a[i];
}
void ghifile(long tongsochan, int k,int b[], int c[], int n)
{
	ofstream fileout ("number2.out");
	if(! fileout.is_open())
 	{
		cout<<"Loi mo file"<<endl;
		return; 	
 	}
   	fileout<<"so n la : " << n << endl;
   	fileout<<"tong so chan la: "<<tongsochan<<endl;
   	if(checkb != 0)
	{
		fileout<<"so chia het cho " << k <<" la: ";
		for(int i = 0 ; i < checkb ; i ++)
			fileout<<" "<< b[i]<<" ";
		fileout<<endl;
	}
	else
		fileout<<"khong tim thay so chia het cho "<<k<<endl;
	if(checkc != 0)
	{
		fileout<<"so nguyen to la: ";
		for(int i = 0 ; i < checkc ; i ++)
			fileout<<" "<< c[i]<<" ";
		fileout<<endl;
	}
	else
		fileout<<"khong tim thay so nguyen to "<<k<<endl;
	fileout.close();
	
   	
}
int main()
{
	ifstream filein("number2.in");
	if(! filein.is_open())
    {
        return NULL;
    }
	int n;
	n=docn(filein);
	int a[n];
	int b[n];
	int c[n];
	long tongsochan=tongchan(filein,a,n);
	filein.close();
	cout<<"nhap so nguyen k: ";
	int k;
	cin>>k;
	chiak(a,n,k,b);
	ktsnt(a,n,c);
	ghifile(tongsochan,k,b,c,n);
}

[COLOR=#141823][FONT=helvetica]Cho tập tin văn bản number2.in[/FONT][/COLOR]

#include <cstdlib>#include <iostream>
#include <fstream>


using namespace std;
void Nhap(int n)
{   
    cout<<"Nhap n: ";
    cin>>n;
    ofstream number2 ("number2.in");
    number2 << " n="<<n<<"\n";
    for(int i=0;i<n;i++)
    {
            number2 << i;
            number2 << "\n";
    }
    number2.close();


}


    
int main(int argc, char *argv[])
{
    int n;
    Nhap(n);
    system("PAUSE");
    return EXIT_SUCCESS;
}