填写data.table中的索引列 [英] Filling up index column in data.table

问题描述

此问题与以下相关：向索引组中出现次数的数据框添加列
我有以下data.table按前2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）），.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 
单位任何索引
 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 
  

索引列由每个单位填充1,2,3 ...。对于data.frame，我可以通过以下方式来实现：

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

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

This question is related to: Add a column to a data frame that index the number of occurrences in a group I have following data.table sorted by first 2 columns.

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

The index column is to be filled by 1,2,3... for each Unit. For a data.frame I can do it by :

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

But how to do it using data.table commands? Thanks for your help.

解决方案

Try

 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

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