Hello,

It might be a silly question, but I am struggling with it all day now. Hopefully someone with bright insight may be of any help.

I have this df which contains a column with ID names. Some ID names occur one time, other two, and some even 20 times. In other words some IDs are in 1 row, others in many rows.

What I would like to make is a new column which numbers the replicate IDs (from 1, 2, 3, etc.). I have tried ddply from plyr package, but that only gives me a new df with each unique ID followed by the number of replicates (that's not what I am looking for).

Any suggestions are welcome, thanks in advance!

Ben

With plyr you probably want to do something like:

library(plyr)
newdf <- ddply(df, .(ID), function(x) {
  transform(x, replicate=1:nrow(x))
})

plyr is sooo 2010, though ;-) Here are some alternatives:

Using dplyr

library(dplyr)
newdf <- df %>%
  group_by(ID) %>%
  mutate(replicate=seq(n()))

Using data.table:

library(data.table)
dt <- as.data.table(df)
dt[, replicate := 1:.N, by="ID"]

ave() will give the sequential numbers, so something like

with(mydf, paste0(ID, ave( ID, ID, FUN=seq_along )))

should do it.

Thanks for all the suggestions! Steve's first suggestion with 'old fashion' ;P plyr was exactly what I needed. A new column with sequential numbering of each replicate IDs. Many thanks!