Struggling with CSV and multiple inputs

Okay now i have managed to let it work.
But, lets say 7.1 inject 2 times and 7.2 inject 2 times. Thats 4 msgs at the join node so it send it through to the function node which writes that 2 values but leave the other two blank. is there a way to code it so that the other two writes zero when it receives nothing or error in that row