在data.table中将日期拆分为年和月

Question

Given,

 set.seed(1234)
 library(data.table)
 dt <- data.table(date=c(201405, 201406, 201501, 201503), x = rnorm(4, 0, 1))

return,

     date          x
1: 201405 -1.2070657
2: 201406  0.2774292
3: 201501  1.0844412
4: 201503 -2.3456977

I want to split date into year and month as follows:

   year month          x
1: 2014     5 -1.2070657
2: 2014     6  0.2774292
3: 2015     1  1.0844412
4: 2015     3 -2.3456977

How can I do this?

Answer 1

Converting comment to an answer, you could simply use substr combined with the assignment by reference semantics. You can wrap it up into as.integer if you prefer it not to return a string

dt[, `:=`(year = substr(date, 1L, 4L), month = substr(date, 5L, 6L))]

Answer 2

You can use this->

 dt$month=as.integer(substr(dt$date,5,6))  
 dt$year=substr(dt$date,1,4)    
 dt  
 date               x month year  
 1: 201405 -1.2070657     5 2014  
 2: 201406  0.2774292     6 2014  
 3: 201501  1.0844412     1 2015  
 4: 201503 -2.3456977     3 2015