Create groups based on time period - r

Question

How can I create a new grouping variable for my data based on 5-year steps?
So from this:
group <- c(rep("A", 7), rep("B", 10))
year <- c(2008:2014, 2005:2014)
dat <- data.frame(group, year)
group year
1 A 2008
2 A 2009
3 A 2010
4 A 2011
5 A 2012
6 A 2013
7 A 2014
8 B 2005
9 B 2006
10 B 2007
11 B 2008
12 B 2009
13 B 2010
14 B 2011
15 B 2012
16 B 2013
17 B 2014
To this:
> dat
group year period
1 A 2008 2005_2009
2 A 2009 2005_2009
3 A 2010 2010_2014
4 A 2011 2010_2014
5 A 2012 2010_2014
6 A 2013 2010_2014
7 A 2014 2010_2014
8 B 2005 2005_2009
9 B 2006 2005_2009
10 B 2007 2005_2009
11 B 2008 2005_2009
12 B 2009 2005_2009
13 B 2010 2010_2014
14 B 2011 2010_2014
15 B 2012 2010_2014
16 B 2013 2010_2014
17 B 2014 2010_2014
I guess I could use cut(dat$year, breaks = ??) but I don't know how to set the breaks.

Answer 1

Here is one way of doing it:
dat$period <- paste(min <- floor(dat$year/5)*5, min+4,sep = "_")
I guess the trick here is to get the biggest whole number smaller than your year with the floor(year/x)*x function.
Here is a version that should work generally:
x <- 5
yearstart <- 2000
dat$period <- paste(min <- floor((dat$year-yearstart)/x)*x+yearstart,
min+x-1,sep = "_")
You can use yearstart to ensure e.g. year 2000 is the first in a group for when x is not a multiple of it.

Answer 2

cut should do the job if you create actual Date objects from your 'year' column.
## convert 'year' column to dates
yrs <- paste0(dat$year, "-01-01")
yrs <- as.Date(yrs)
## create cuts of 5 years and add them to data.frame
dat$period <- cut(yrs, "5 years")
## create desired factor levels
library(lubridate)
lvl <- as.Date(levels(dat$period))
lvl <- paste(year(lvl), year(lvl) + 4, sep = "_")
levels(dat$period) <- lvl
head(dat)
group year period
1 A 2008 2005_2009
2 A 2009 2005_2009
3 A 2010 2010_2014
4 A 2011 2010_2014
5 A 2012 2010_2014
6 A 2013 2010_2014