r - 如何使用 data.table 按组对变量的随机值进行子集化？

Question

我想在 data.table 中按组获取变量的最小样本和随机样本。

data.table(ggplot2::movies)[, list(min=min(rating), random=sample(rating, 1)), by=list(year, Action)]

不起作用:

Error in `[.data.table`(data.table(movies), , list(min(rating), sample(rating,  : 
Column 2 of result for group 88 is type 'integer' but expecting type 'double'. Column types must be consistent for each group.

如果我强制它为数字，我会得到这个惊人的结果:随机评分低于(？!!)同一类别的最小值的类别。

data.table(ggplot2::movies)[, list(min=min(rating), random=as.numeric(sample(rating, 1))), by=list(year, Action)][random<min]
   year Action min random
1: 1916      1 6.2      6
2: 1911      1 5.7      1
3: 1901      1 4.2      3
4: 1914      1 6.1      6
5: 1923      1 8.2      4
6: 1918      1 5.9      5
7: 1921      1 7.5      4

使用 .SD 不会改变任何东西:

data.table(ggplot2::movies)[, list(min=min(rating), random=as.numeric(sample(.SD$rating, 1))), by=list(year, Action)][random<min]
   year Action min random
1: 1916      1 6.2      2
2: 1911      1 5.7      4
3: 1893      0 7.0      2
4: 1901      1 4.2      4
5: 1914      1 6.1      5
6: 1923      1 8.2      8
7: 1918      1 5.9      4

更糟糕的是，当变量为整数时不会出现错误:

data.table(ggplot2::movies)[, list(min=min(votes), random=sample(votes, 1)), by=list(year, Action)][random<min]
   year Action min random
1: 1916      1 135     43
2: 1911      1  26      2
3: 1893      0  90     52
4: 1901      1  13     12
5: 1923      1 757    368
6: 1918      1  60     49
7: 1921      1  73     48

显然 sample 函数不想在子集上工作...

帮助!

Answer 1

您掉进了标准的 sample 陷阱。来自 ?sample:

If x has length 1, is numeric (in the sense of is.numeric) and x >= 1, sampling via sample takes place from 1:x. Note that this convenience feature may lead to undesired behaviour when x is of varying length in calls such as sample(x).

使用例如?sample 中的 resample 建议。