[R] Hi , Is it possible select a different number of rows by each group with R????

Hi , Is it possible select a different number of rows by each group with R I must to select different number (specific quantity in field2:Table1) of rows in each group(field1:Table2). I have these 2 tables:Table1Table2field1field2field1field3field4field51310.375SpRm12610.416667SpRm23910.45833

[R] track on boats

Hi professors I'm finding the best package in R for manage a big database about vessels. I need to divided the travel of each boat into tracks. I have GPS points. I would join these points in a line on the map. I make that with the function "lines", but I can't divided by vessel or by track, I