Running count based on field in R

Question

I have a data set of this format

User       
1 
2
3
2
3
1  
1      

Now I want to add a column saying count which counts the occurrence of the

Answer 1

You can use getanID from my "splitstackshape" package:

library(splitstackshape)
getanID(mydf, "User")
##    User .id
## 1:    1   1
## 2:    2   1
## 3:    3   1
## 4:    2   2
## 5:    3   2
## 6:    1   2
## 7:    1   3

This is essentially an approach with "data.table" that looks something like the following:

as.data.table(mydf)[, count := seq(.N), by = "User"][]