my aim is: when i reach the last line of the file.. i will add some text to that line and finish the program.

Oh "XY", then: you really want to ask X, but you think you need Y to do that, and ask about Y.

This sounds like: "I want to append to the end of file". If so, then just either open in >> mode if you only want to append, or +> if you also want to read it before writing something in it.