[英]Can you use .SDcols to subset rows in data.table
Is there a way to use .SDcols in datatable to select rows.有没有办法在数据表中使用 .SDcols 来选择行。 For example, in mtcars, select rows where vs, am and carb all equal 1. I have a lot of columns and want to avoid a lot of typing.
例如,在 mtcars 中,选择 vs、am 和 carb 都等于 1 的行。我有很多列,希望避免大量输入。
I know this does not work我知道这行不通
mtcars[ lapply( .SD == 1), , .SDcols = c('vs, 'am', 'carb')]
We may need Reduce
to return a single logical vector and use that as index for subsetting我们可能需要
Reduce
返回单个逻辑向量并将其用作子集的索引
library(data.table)
mtcars[mtcars[, Reduce(`&`, lapply( .SD, `==`, 1)),
.SDcols = c('vs', 'am', 'carb')]]
-output -输出
mpg cyl disp hp drat wt qsec vs am gear carb
1: 22.8 4 108.0 93 3.85 2.320 18.61 1 1 4 1
2: 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
3: 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
4: 27.3 4 79.0 66 4.08 1.935 18.90 1 1 4 1
Or use rowSums
to create the logical vector或者使用
rowSums
创建逻辑向量
mtcars[mtcars[, .I[rowSums(.SD == 1) == 3], .SDcols = c('vs', 'am', 'carb') ]]
mtcars <- as.data.table(mtcars)
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