r - Create counter with multiple variables

Question

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Answer 1

Use ave. Assuming your data.frame is called "mydf":

mydf$counter <- with(mydf, ave(CustomerID, CustomerID, FUN = seq_along))
mydf
#   CustomerID TripDate counter
# 1          1 1/3/2013       1
# 2          1 1/4/2013       2
# 3          1 1/9/2013       3
# 4          2 2/1/2013       1
# 5          2 2/4/2013       2
# 6          3 1/2/2013       1

For what it's worth, I also implemented a version of this approach in a function included in my "splitstackshape" package. The function is called getanID:

mydf <- data.frame(IDA = c("a", "a", "a", "b", "b", "b", "b"),
                   IDB = c(1, 2, 1, 1, 2, 2, 2), values = 1:7)
mydf
# install.packages("splitstackshape")
library(splitstackshape)
# getanID(mydf, id.vars = c("IDA", "IDB"))
getanID(mydf, id.vars = 1:2)
#   IDA IDB values .id
# 1   a   1      1   1
# 2   a   2      2   1
# 3   a   1      3   2
# 4   b   1      4   1
# 5   b   2      5   1
# 6   b   2      6   2
# 7   b   2      7   3

As you can see from the example above, I've written the function in such a way that you can specify one or more columns that should be treated as ID columns. It checks to see if any of the id.vars are duplicated, and if they are, then it generates a new ID variable for you.