File I/O merging and adding

Ok, I already have file1.txt and file2.txt which are created by me. file3.txt will add a line inputted by the user to the end of file1.txt. file4.txt will merge file3.txt and file2.txt. I have the following portoin which I thought would add a line and create file3.txt but it doesn't. It DOES create file3.txt but writes nothing to it. What I have here is just the first portion of my problem, I believe if I can figure this part out, it shouldnt be that hard to figure out the merging.

Code:

(original code delete new one posted)

>> O_hw7 << line << endl;
You output that line before you open the file.

Also, why do you specify ios::in for an output file?

Ok, i did what you said but no luck. Oh, and the bottom portion is something I'm supposed to have as well, so you can ignore it i guess. And you asked about ios::in, well like they say, if it aint broke, dont fix it. it works fine and i used a format similar to this for my last prog and it worked
anyways, my continuing problem:
I'm supposed to create file3.txt with the original contents of file1.txt in addition to the sentence(s) entered by the user.

Code:

string file= "HW7_1.txt";	
string line;
ifstream HW7_1txt;	
ofstream O_hw7;	
string sentence;
char choice;

O_hw7.open("HW7_3.txt", ios::in| ios ::app);		//Opens .txt file in input mode and append mode
HW7_1txt.open (file.c_str());
while (getline(HW7_1txt, line));
O_hw7 << line << endl;		
HW7_1txt.clear ();
HW7_1txt.close ();		

do
{
cout << "Enter a sentence" << endl;
getline (cin, sentence);	
			
std :: cout << "The sentence to be added is: " << sentence << endl;
O_hw7 << sentence<< endl;

cout << "would you like to enter another sentence?" << endl;
cin >> choice;
cin.ignore();
}
while (choice == 'y' || choice == 'Y');
O_hw7.close();

Code:

while (getline(HW7_1txt, line));         // remove the semicolon
     O_hw7 << line << endl;

Kurt

whoa, something that small..... thanks so much! you probably saved me a couple of hours(seriously....me and this programming) from troubleshooting it, thanks so much!!!!