Question

此问题与：Add a column to a data frame that index the number of occurrences in a group有关我有以下data.table按前两列排序。

ddt = structure(list(Unit = structure(c(1L, 1L, 2L, 2L, 3L, 3L), .Label = c("A", 
"A1", "B"), class = "factor"), Anything = c(3.4, 6.9, 1.1, 2.2, 
2, 3), index = c(0, 0, 0, 0, 0, 0)), .Names = c("Unit", "Anything", 
"index"), row.names = c(NA, -6L), class = c("data.table", "data.frame"
), .internal.selfref = <pointer: 0x8948f68>, sorted = c("Unit", 
"Anything"))

ddt
   Unit Anything index
1:    A      3.4     0
2:    A      6.9     0
3:   A1      1.1     0
4:   A1      2.2     0
5:    B      2.0     0
6:    B      3.0     0

每个单位的索引列将由1,2,3 ...填充。对于data.frame，我可以通过以下方式完成：

for(U in unique(ddt$Unit)){
    ddt[ddt$Unit==U,]$index = 1:length(ddt[ddt$Unit==U,]$Unit)
}

ddt
  Unit Anything index
1    A      3.4     1
3    A      6.9     2
4   A1      1.1     1
2   A1      2.2     2
5    B      2.0     1
6    B      3.0     2

但是如何使用data.table命令呢？谢谢你的帮助。

Answer 1

尝试

 ddt[, indx:=1:.N, by=Unit]
 #     Unit Anything indx
 #1:    A      3.4    1
 #2:    A      6.9    2
 #3:   A1      1.1    1
 #4:   A1      2.2    2
 #5:    B      2.0    1
 #6:    B      3.0    2

Answer 2

试试这个：

ddt[, index := as.numeric(seq_len(.N)), by="Unit"]
ddt

   Unit Anything index
1:    A      3.4     1
2:    A      6.9     2
3:   A1      1.1     1
4:   A1      2.2     2
5:    B      2.0     1
6:    B      3.0     2

Answer 3

一个问题是您无法使用:=更改列的类（因为索引是类型double，理想情况下您需要整数）。我建议删除index并使用:=重新创建：

ddt$index = NULL
ddt[,index:= 1:nrow(.SD), by=Unit]
> ddt
   Unit Anything index
1:    A      3.4     1
2:    A      6.9     2
3:   A1      1.1     1
4:   A1      2.2     2
5:    B      2.0     1
6:    B      3.0     2

填写data.table中的索引列

3 个答案: