我希望从一个 data.table 中仅获取按多列分组的第一行。
如果只有一列,这很容易实现,例如:
(dt <- data.table(x = c(1, 1, 1, 2),
y = c(1, 1, 2, 2),
z = c(1, 2, 1, 2)))
# x y z
# |1: 1 1 1
# |2: 1 1 2
# |3: 1 2 1
# |4: 2 2 2
dt[!duplicated(x)] # Remove rows 2-3
# x y z
# |1: 1 1 1
# |2: 2 2 2
但是当尝试基于两列进行删除时,这些方法都无法奏效;也就是说,在此情况下仅删除第二行:
dt[!duplicated(x, y)] # Keeps only original data set
# x y z
# |1: 1 1 1
# |2: 1 1 2
# |3: 1 2 1
# |4: 2 2 2
dt[!duplicated(list(x, y))] # Same as above
dt[!duplicated(c("x", "y"))] # Same as above
dt[!duplicated(list("x", "y"))] # Same as above
dt[!duplicated(c(x, y))] # Only removes duplicates from first column
# x y z
# |1: 1 1 1
# |2: 2 2 2
除此之外,在某些情况下才有效:
dt[!duplicated(paste0(x, y))]
# x y z
# |1: 1 1 1
# |2: 1 2 1
# |3: 2 2 2
key
排序,您可以指定按变量排序。 - mnel