标记R中的所有重复行，如在Stata中

从我的问题（这里）开始，我试图在R中复制Stata命令的功能 duplicateates tag ，它允许我根据给定的一组变量标记数据集中所有重复的行：

Following up from my question here, I am trying to replicate in R the functionality of the Stata command duplicates tag, which allows me to tag all the rows of a dataset that are duplicates in terms of a given set of variables:

clear *
set obs 16
g f1 = _n
expand 104
bys f1: g f2 = _n
expand 2
bys f1 f2: g f3 = _n
expand 41
bys f1 f2 f3: g f4 = _n
des  // describe the dataset in memory

preserve
sample 10  // draw a 10% random sample
tempfile sampledata
save `sampledata', replace
restore

// append the duplicate rows to the data
append using `sampledata'
sort f1-f4

duplicates tag f1-f4, generate(dupvar)
browse if dupvar == 1  // check that all duplicate rows have been tagged

编辑

这是Stata生成的（@ Arun的请求添加）：

Edit

Here is what Stata produces (added on @Arun's request):

f1   f2   f3   f4   dupvar  
 1    1    1    1        0  
 1    1    1    2        0  
 1    1    1    3        1  
 1    1    1    3        1  
 1    1    1    4        0  
 1    1    1    5        0  
 1    1    1    6        0  
 1    1    1    7        0  
 1    1    1    8        1  
 1    1    1    8        1

请注意，（f1，f2，f3，f4）= ，1，1，3）有两行，并且这两行都标记为 dupvar = 1 。类似地，对于（f1，f2，f3，f4）=（1,1,1,8）的重复的两行。

Note that for (f1, f2, f3, f4) = (1, 1, 1, 3) there are two rows, and both of those are marked dupvar = 1. Similarly, for the two rows that are duplicates for (f1, f2, f3, f4) =(1, 1, 1, 8).

基本函数重复仅标记第二个重复。所以，我写了一个函数来复制R中的Stata功能，使用 ddply 。

The base function duplicated tags only the second duplicate onwards. So, I wrote a function to replicate the Stata functionality in R, using ddply.

# Values of (f1, f2, f3, f4) uniquely identify observations
dfUnique = expand.grid(f1 = factor(1:16),
            f2 = factor(1:41),
            f3 = factor(1:2),
            f4 = factor(1:104))

# sample some extra rows and rbind them
dfDup = rbind(dfUnique, dfUnique[sample(1:nrow(dfUnique), 100), ])

# dummy data 
dfDup$data = rnorm(nrow(dfDup))

# function: use ddply to tag all duplicate rows in the data
fnDupTag = function(dfX, indexVars) {
  dfDupTag = ddply(dfX, .variables = indexVars, .fun = function(x) {
    if(nrow(x) > 1) x$dup = 1 else x$dup = 0
    return(x)
  })
  return(dfDupTag)
}

# test the function
indexVars = paste0('f', 1:4, sep = '')
dfTemp = fnDupTag(dfDup, indexVars)

在链接的问题，性能是一个巨大的问题。 另一个可能的解决方案是

But as in the linked question, performance is a huge issue. Another possible solution is

dfDup$dup = duplicated(dfDup[, indexVars]) | 
  duplicated(dfDup[, indexVars], fromLast = TRUE) 
dfDupSorted = with(dfDup, dfDup[order(eval(parse(text = indexVars))), ])

我有几个问题：

1.可以使 ddply 版本更快？

2.第二个版本是否使用重复的正确？对于多个副本的重复行？
3.我如何使用 data.table ？

I have a few questions:
1. Is it possible to make the ddply version faster?
2. Is the second version using duplicated correct? For more than two copies of the duplicated rows? 3. How would I do this using data.table? Would that be faster?