Assigning group ID with ddply

Question

Pretty basic performance question from an R newbie. I\'d like to assign a group ID to each row in a data frame by unique combinations of fields. Here\'s my current approach:

Answer 1

Try using the id function (also in plyr):

df$id <- id(df[c("st.num", "st.name")], drop = TRUE)

Update:

The id function is considered deprecated since dplyr version 0.5.0. The function group_indices provides the same functionality.

Answer 2

Is it necessary that the ID be a random 10 character string? If not, why not just paste together the columns of the data frame. If the IDs must be the same length in characters, convert factors to numeric, then paste them together:

df$ID <- paste(as.numeric(df$st.num), as.numeric(df$st.name), sep = "")

Then, if you really need to have 10 character IDs, I'd generate just the n number of IDs, and rename the levels of ID with them

df$ID <- as.factor(df$ID)
n <- nlevels(df$ID)

getID <- function(n, size=10){
  out <- {}
  for(i in 1:n){
    out <- c(paste(sample(c(0:9, LETTERS, letters), size, replace=TRUE), collapse=''))
  }
  return(out)
}

newLevels <- getID(n = n)

levels(df$ID) <- newLevels

Also, as an aside, you don't need to use function(x) with ddply that way with transform(). This code would work just the same:

ddply(df, c("st.num", "st.name"), transform, household=getString())