I want to lag the time variable itself, the format changes, simple example:
data<-data.frame(number=seq(1:5),
datetime=seq(as.POSIXct("2015/06/12"),as.POSIXct("2015/06/16"),by="1 day"))
number datetime
1 1 2015-06-12
2 2 2015-06-13
3 3 2015-06-14
4 4 2015-06-15
5 5 2015-06-16
What I want:
number datetime datetime.lag
1 1 2015-06-12 NA
2 2 2015-06-13 2015-06-12
3 3 2015-06-14 2015-06-13
4 4 2015-06-15 2015-06-14
5 5 2015-06-16 2015-06-15
data$datetime.lag<-c(NA, head(data$datetime, -1))
What I get:
number datetime datetime.lag
1 1 2015-06-12 NA
2 2 2015-06-13 1434092400
3 3 2015-06-14 1434178800
4 4 2015-06-15 1434265200
5 5 2015-06-16 1434351600
Why the format changes? any better suggestion?
You could use shift from the devel version of data.table i.e. v.1.9.5. The default type is lag and n is 1. Instructions to install the devel version are here
library(data.table)
setDT(data)[, lagdt:= shift(datetime)][]
# number datetime lagdt
#1: 1 2015-06-12 <NA>
#2: 2 2015-06-13 2015-06-12
#3: 3 2015-06-14 2015-06-13
#4: 4 2015-06-15 2015-06-14
#5: 5 2015-06-16 2015-06-15
We can also get multiple lags
setDT(data)[, paste0('lagDT', 1:2) :=shift(datetime, 1:2)][]
# number datetime lagDT1 lagDT2
#1: 1 2015-06-12 <NA> <NA>
#2: 2 2015-06-13 2015-06-12 <NA>
#3: 3 2015-06-14 2015-06-13 2015-06-12
#4: 4 2015-06-15 2015-06-14 2015-06-13
#5: 5 2015-06-16 2015-06-15 2015-06-14