Hello, Read line by line from data2.txt, compare with data1.txt lines, if match , do nothing (NOT remove) , else append to a new file (result.txt) . mathces are exact string matches, both are text files. My code as follows. ------------------- #include <iostream> #include <fstream> #include <string> using namespace std; int main() { ifstream ifs1("data1.txt"); ifstream ifs2("data2.txt"); ofstream ofs("result.txt"); string line1; string line2; while (!ifs2.eof()) { getline(ifs2,line2); while (!ifs1.eof()) { getline(ifs1,line1); if (line1!=line2) { ofs << line1; ofs << "\n"; } else { } } } return 0; } ------------------------- data1.txt : This is line 1 This is line 2 This is line 3 This is line 4 This is line 5 data2.txt This is line 5 This is line 1 Result.txt should be: --------------------- This is line 2 This is line 3 This is line 4 But I don't get the required results. Appreciate your assistance. thanks.

If both sets of lines are not sorted order then you have to do multiple passes to find the matches. If they are sorted then you must do some sort of buffering to keep then in sync.