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Generate a sequence within a group in R

I am trying to get consistency within a category.

My details:

A B 
1 1 
1 2
1 2
1 3
1 3
1 3
1 4
1 4

and I want to get the variable "c", for example, my data looks like this:

A B C
1 1 1
1 2 1
1 2 2
1 3 1
1 3 2
1 3 3
1 4 1
1 4 2
+1
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1 answer

Use avewith seq_along:

> mydf$C <- with(mydf, ave(A, A, B, FUN = seq_along))
> mydf
  A B C
1 1 1 1
2 1 2 1
3 1 2 2
4 1 3 1
5 1 3 2
6 1 3 3
7 1 4 1
8 1 4 2

If your data is already ordered (as in this case), you can also use sequencewith rle( mydf$C <- sequence(rle(do.call(paste, mydf))$lengths)), but you do not have this restriction with ave.

If you are a fan data.table, you can use .Nas follows:

library(data.table)
DT <- data.table(mydf)
DT[, C := sequence(.N), by = c("A", "B")]
DT
#    A B C
# 1: 1 1 1
# 2: 1 2 1
# 3: 1 2 2
# 4: 1 3 1
# 5: 1 3 2
# 6: 1 3 3
# 7: 1 4 1
# 8: 1 4 2
+8
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