我有两个数据集，我试图找出每组的第一个观察结果 . 在以下示例中，您可以看到第一个数据集（“df1”）中的“id”分组按预期工作（case1） . 当我在第二个数据集（df2）（case2a）中按“id2”分组时，它也有效 . 但是，当我在第二个数据集（case2b）中按“id1”分组时，它不起作用（如预期的那样） . 令人惊讶的是，当我将“id1”转换为字符向量时，我得到了预期的输出 .
#case1
df1<- structure(list(id = c(1, 1, 1, 2, 2, 2, 3, 3, 3), stopId = structure(c(1L,
2L, 3L, 1L, 2L, 3L, 1L, 2L, 3L), .Label = c("a", "b", "c"), class = "factor"),
stopSequence = c(1, 2, 3, 3, 1, 4, 3, 1, 2)), .Names = c("id",
"stopId", "stopSequence"), row.names = c(NA, -9L), class = "data.frame")
# first observation of each id:
setDT(df)[,.SD[1,],by=.(id)] #worked
#df2
df2<-structure(list(id1 = c(201601072952201, 201601072952201, 201601072952201,
201601072952213, 201601072952213, 201601072952213, 201601072952212,
201601072952212, 201601072952212, 201601072952176), id2 = c("TXT",
"TXT", "TXT", "TXT", "TXT", "TXT", "PLP", "PLP", "PLP", "KYK"
), sb = c(32L, 32L, 32L, 32L, 32L, 32L, 58L, 58L, 58L, 6L), bb = c(7L,
7L, 7L, 56L, 56L, 56L, 28L, 28L, 28L, 47L), qt = c(21, 21, 21,
420, 420, 420, 1000, 1000, 1000, 13), amt = c(301, 301, 301,
306, 306, 306, 515, 515, 515, 368), rate = c(6321, 6321, 6321,
128520, 128520, 128520, 515000, 515000, 515000, 4784)), .Names = c("id1",
"id2", "sb", "bb", "qt", "amt", "rate"), class = "data.frame", row.names = c(NA,
-10L))
#case2a
setDT(df2)[,.SD[1,],by=.(id2)] #worked
id2 id1 sb bb qt amt rate
1: TXT 201601072952201 32 7 21 301 6321
2: PLP 201601072952212 58 28 1000 515 515000
3: KYK 201601072952176 6 47 13 368 4784
#case2b
setDT(df2)[,.SD[1,],by=.(id1)] #not worked as expected
id1 id2 sb bb qt amt rate
1: 201601072952201 TXT 32 7 21 301 6321
df2$id1<-as.character(df2$id1)
setDT(df2)[,.SD[1,],by=.(id1)] # worked
所以我的问题是为什么我需要在数据块2b中将数字变量转换为字符变量，而不是在case1中 .
1 回答
尝试使用
base中的标准函数 . 例如：输出：