r - Alternative to expand.grid for data.frames

Question

I have a data.frame df and I want that every row in this df is duplicated lengthTime times and that a new column is added that counts from 1 to lengthTime for each row in df.

I know, it sounds pretty complicated, but what I basically want is to apply expand.grid to df. Here is an ugly workaround and I have the feeling that there most be an easier solution (maybe even a base-R function?):

df <- data.frame(ID   = rep(letters[1:3], each=3),
                 CatA = rep(1:3, times = 3),
                 CatB = letters[1:9])
lengthTime <- 3
nrRow <- nrow(df)
intDF <- df
for (i in 1:(lengthTime - 1)) {
  df <- rbind(df, intDF)
}
df$Time <- rep(1:lengthTime, each=nrRow)

I thought that I could just use expand.grid(df, 1:lengthTime), but that does not work. outer did not bring any luck either. So does anyone know a good solution?

Answer 1

It's been a while since this question was posted, but I recently came across it looking for just the thing in the title, namely, an expand.grid that works for data frames. The posted answers address the OP's more specific question, so in case anyone is looking for a more general solution for data frames, here's a slightly more general approach:

expand.grid.df <- function(...) Reduce(function(...) merge(..., by=NULL), list(...))

# For the example in the OP
expand.grid.df(df, data.frame(1:lengthTime))

# More generally
df1 <- data.frame(A=1:3, B=11:13)
df2 <- data.frame(C=51:52, D=c("Y", "N"))
df3 <- data.frame(E=c("+", "-"))
expand.grid.df(df1, df2, df3)