此问题与:Add a column to a data frame that index the number of occurrences in a group 我有以下data.table按前2列排序。在data.table中填充索引列
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
对于每个单元,索引列将由1,2,3 ...填充。对于data.frame我可以做到这一点:
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
但如何使用data.table命令做到这一点?谢谢你的帮助。
使用'.N'的而不是'length'玩意儿 – eddi 2014-08-27 17:11:02
@ EDDI:能否请您进入这个作为一个答案。 – rnso 2014-08-27 17:11:46
@eddi谢谢你让我的脚趾。回答编辑:-) – Andrie 2014-08-27 17:12:38