Hi:
Processing has a lenght() instruction that will help u find the lenght of the recovered array; but that will be too complicated...
If you just need the last value, then save the last value in another file, either txt or csv. Same way you save the values in the csv file you do it in another file; but you overwrite the previous value with the new one. In that way at the end you will have only the last value in that file and you just need to read one value. I don't know what you are using; but with processing use filename.print() instead of filename.println(). The lines will be overrwritten over and over and you will have your last value there. You need to create the file first of course with PrintWriter(); but if you did the csv file already I assume you know that.
Hope you can do it.