Add rows to dataframe based on existing rows

Question

I have this dataframe:

df <- data.frame(group=c("A", "A", "B", "B"), year=c(1980, 1986, 1990, 1992))
  group year
1     A 1980
2     A 1986
3     B 1990
4     B 1992

I'd like to modify it in the following way:

• add rows for each existing row with the two preceding years
• add a new column with a variable of the respective year
• delete the existing rows

This would be the outcome:

   group  year     pre
1      A  1978 pre1980
2      A  1979 pre1980
3      A  1984 pre1986
4      A  1985 pre1986
5      B  1988 pre1990
6      B  1989 pre1990
7      B  1990 pre1992
8      B  1991 pre1992

Adding the new column would be easy..

df$pre <- paste("pre", df$year, sep="")

But I am stuck on how to add the new rows with the respective years (of course creating a whole new data frame would be just as good). Any hints?

Answer 1

base R ftw:

data.frame(group = rep(df$group, each=2),
           year = df[rep(1:nrow(df), each=2),]$year-2:1,
           pre = paste0("pre",rep(df$year,each=2)))
#   group year     pre
# 1     A 1978 pre1980
# 2     A 1979 pre1980
# 3     A 1984 pre1986
# 4     A 1985 pre1986
# 5     B 1988 pre1990
# 6     B 1989 pre1990
# 7     B 1990 pre1992
# 8     B 1991 pre1992