I would like to create a new variable,litter, to indicate each sow or litter in different farrowing dates (fdate). Each litter is to be numbered from 1 to N with an increament of 1 as shown in the last column.
sow season piglet fdate litter
1M521 1 5702 14/09/2009 1
1M521 1 5703 14/09/2009 1
1M521 2 22920 17/02/2010 2
1M521 2 22920 17/02/2010 2
1M521 2 22920 17/02/2010 2
1M584 1 8516 28/09/2009 3
1M584 1 8516 28/09/2009 3
1M584 1 8516 28/09/2009 3
1N312 1 6192 16/09/2009 4
1N312 1 6193 16/09/2009 4
1N312 1 6194 16/09/2009 4
1N312 2 21818 11/02/2010 5
1N312 2 21819 11/02/2010 5
1N312 2 21820 11/02/2010 5
2A1526 1 9293 3/10/2009 6
2A1526 1 9294 3/10/2009 6
2A1526 1 9295 3/10/2009 6
2A1526 1 9296 3/10/2009 6
If I'm understanding correctly, you're just basing this on fdate? If your dataframe is piglets, try this:
piglets$litter <- as.numeric(factor(piglets$fdate))
来源：https://stackoverflow.com/questions/5798206/identifying-or-coding-unique-factors