在 R data.table 中保留多列的第一行 [英] Keep first row by multiple columns in an R data.table

问题描述

我只想从 data.table 中获取第一行，按多列分组.

I'd like to get the first row only from a data.table, grouped by multiple columns.

这很简单，只有一列，例如:

This is straightforward with a single column, e.g.:

(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，这些方法都不起作用；即在这种情况下仅删除第 2 行:

But none of these approaches work when trying to remove based on two columns; i.e. in this case removing only row 2:

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

除了这个，它只在某些情况下有效:

Except for this, which only works in certain cases:

dt[!duplicated(paste0(x, y))]
#     x y z
# |1: 1 1 1
# |2: 1 2 1
# |3: 2 2 2

推荐答案

data.table 为 unique、duplicated 和 duplicated 提供 S3 方法code>anyDuplicated

data.table provides S3 methods for unique, duplicated and anyDuplicated

unique(dt, by = c('x','y'))

会给你你想要的.

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