R 将data.table中变量的唯一字符串与data.table的另一个变量的重复值连接起来

我有一个data.table，它有一列，我们称它为country，其中包含重复的值，另一列(survey) for each 观察值都有唯一的字符串.我想创建一个新变量，该变量包含单个国家/地区的所有survey个字符串的粘贴值.我已经使用INDEX和FOR循环做到了这一点，但我很好奇是否有更快的方法来使用一些data.table语法来完成这项工作.MWe:

library(data.table)

dt <- data.table(country = c("Belgium","Belgium","Bolivia","Brazil","Brazil","Brazil"),
                      survey = c("BE01, BE03, BE04","BE05, BE07, BE11",
                            "BO11, BO13", "BR01, BR02, BR03", "BR05","BR12, BR13"))
dt[, index := seq(1, .N), by = "country"]
for(cntry in unique(dt$country)){
  tmp_ind <- max(dt[country == eval(cntry)]$index)
  dt[country == eval(cntry) & index ==1, all_surveys := survey]
  if(tmp_ind > 1) {
    for(i in 2:tmp_ind){
      dt[country == eval(cntry) & index ==i,
         all_surveys := paste0(dt[country == eval(cntry) & index == i-1, all_surveys], survey)]
    }
  }
}

这就给了我们想要的

> dt
   country           survey index                      all_surveys
1: Belgium BE01, BE03, BE04     1                 BE01, BE03, BE04
2: Belgium BE05, BE07, BE11     2 BE01, BE03, BE04BE05, BE07, BE11
3: Bolivia       BO11, BO13     1                       BO11, BO13
4:  Brazil BR01, BR02, BR03     1                 BR01, BR02, BR03
5:  Brazil             BR05     2             BR01, BR02, BR03BR05
6:  Brazil       BR12, BR13     3   BR01, BR02, BR03BR05BR12, BR13