R code snippet that mimics Stata's "by & _n" behavior

by_n.R

## 
## See "Counting with by" for a Stata example
## http://www.ats.ucla.edu/stat/stata/notes/countn.htm

## Hadley's version (which I like most) using ave() and seq_along() 
mydf <- data.frame(id = c(1,1,1,2,2,2,2,3,3,3), v1 = 1)
mydf 
mydf$v2 <- ave(mydf$v1, mydf$id, FUN = seq_along)
mydf


## 1. Version with table()
mydf <- data.frame(id = c(1,1,1,2,2,2,2,3,3,3), v1 = 1)
mydf 
mydf <- data.frame(mydf, v2 = as.vector(unlist(lapply(table(mydf$id), seq_len))))
mydf

## > mydf <- data.frame(id = c(1,1,1,2,2,2,2,3,3,3), v1 = 1)
## > mydf
##    id v1
## 1   1  1
## 2   1  1
## 3   1  1
## 4   2  1
## 5   2  1
## 6   2  1
## 7   2  1
## 8   3  1
## 9   3  1
## 10  3  1
## > mydf <- data.frame(mydf, v2 = as.vector(unlist(lapply(table(mydf$id), seq_len))))
## > mydf
##    id v1 v2
## 1   1  1  1
## 2   1  1  2
## 3   1  1  3
## 4   2  1  1
## 5   2  1  2
## 6   2  1  3
## 7   2  1  4
## 8   3  1  1
## 9   3  1  2
## 10  3  1  3


## 2. Version with by()
mydf <- data.frame(id = c(1,1,1,2,2,2,2,3,3,3), v1 = 1)
underscore_n <- function(df){n <- dim(df)[1]; seq_len(n)}
underscore_n(mydf)
mydf <- data.frame(mydf, v2 = as.vector(unlist(by(mydf, mydf$id, underscore_n))))
mydf

Yes, much shorter... Thanks!