Question

Suppose I have a variable that lasts for several periods.
Like the amount of years that I have an Ipod.
So I had the Ipod 1st generation from 2001 until 2004 and then in 2005 I’ve got Ipod 2 and so on. So my dataframe would look like:

  2001 Ipod1
  2002 Ipod1
  2003 Ipod1
  2004 Ipod1
  2005 Ipod2
  2006 Ipod2
  2007 Ipod2
  2008 Ipod2
  2009 Ipod3
  2010 Ipod3

What I want is to create a dummy for the period when a new variable arrives so I would get:

  Year  Var  Dummy
  2001 Ipod1  1
  2002 Ipod1  0
  2003 Ipod1  0
  2004 Ipod1  0
  2005 Ipod2  1
  2006 Ipod2  0
  2007 Ipod2  0
  2008 Ipod2  0
  2009 Ipod3  1
  2010 Ipod3  0

So far I have been able to do this:

df = structure(list(Year = 2001:2010, Var = structure(c(1L, 1L, 1L,
1L, 2L, 2L, 2L, 2L, 3L, 3L), .Label = c("Ipod1", "Ipod2", "Ipod3"
), class = "factor")), .Names = c("Year", "Var"), class = "data.frame", row.names = c(NA,
-10L))

df$number.in.group = unlist(lapply(table(df$Var),seq.int)) 
df$dummy = ifelse(df$number.in.group == 1,1,0)
df$dummy[1]=0

Actually I would like the first element of the dummy to be zero.

My question is: Is there any way of doing this in a better way?

Thanks

Answer 1

How about this:

df$Dummy <- as.numeric(!duplicated(df$Var))

# Or, if you want the first element to be 0,
df$Dummy <- c(0, as.numeric(!duplicated(df$Var))[-1])