this is common thing that happened to me when i solve a problem and i want to submit it then i got WA because of i forgot to comment my testing file input... Now i saw that someone use this in main #ifdef NEREVAR_PROJECT freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout);

endif

but i don't understand that many people use many different thing at the position "NEREVAR_PROJECT" .. then i am trying to copy that and use but it's not work in my text editor . plz explain me what is it and what it means and what it does...and how it use... thanx in advance