从数据表中,每组随机选择一行 [英] from data table, randomly select one row per group
问题描述
我正在寻找一种从数据表中选择行的有效方法,以便我为特定列中的每个唯一值都有一个代表行.
I'm looking for an efficient way to select rows from a data table such that I have one representative row for each unique value in a particular column.
让我举一个简单的例子:
Let me propose a simple example:
require(data.table)
y = c('a','b','c','d','e','f','g','h')
x = sample(2:10,8,replace = TRUE)
z = rep(y,x)
dt = as.data.table( z )
我的目标是通过对 z 列中的每个字母 a-h 采样一行来对数据表 dt 进行子集化.
my objective is to subset data table dt by sampling one row for each letter a-h in column z.
推荐答案
OP 在示例中仅提供了一个列.假设原始数据集中有多列,我们按'z'分组,sample
每组行序列中的1行,得到行索引(.I
),提取具有行索引 ($V1
) 的列,并使用它对 'dt' 的行进行子集化.
OP provided only a single column in the example. Assuming that there are multiple columns in the original dataset, we group by 'z', sample
1 row from the sequence of rows per group, get the row index (.I
), extract the column with the row index ($V1
) and use that to subset the rows of 'dt'.
dt[dt[ , .I[sample(.N,1)] , by = z]$V1]
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