Lets make a data.table:
dt <- data.table(x.1=1:8, x.2=1:8, x.3=2:9, vessel=rep(letters[1:2], each=4), Year=rep(2012:2015, 2))
dt
x.1 x.2 x.3 vessel Year
1: 1 1 2 a 2012
2: 2 2 3 a 2013
3: 3 3 4 a 2014
4: 4 4 5 a 2015
5: 5 5 6 b 2012
6: 6 6 7 b 2013
7: 7 7 8 b 2014
8: 8 8 9 b 2015
I can aggregate it, using the functions length and sum, to get the sum of all x's in each year and the sum of unique vessels each year like this:
dt[,
list(
x.1=sum(x.1),
x.2=sum(x.2),
x.3=sum(x.3),
vessels=length(unique(vessel))),
by=list(Year=Year)]
Year x.1 x.2 x.3 vessels
1: 2012 6 6 8 2
2: 2013 8 8 10 2
3: 2014 10 10 12 2
4: 2015 12 12 14 2
This is what i want, but in my real data I have a lot of columns, so i would like to use grep or %like%, but i can not get it to work. I was thinking something in line with this:
dt[,grep("x",colnames(dt)),with = FALSE])
But how to merge that with the aggregate?