Data.frame成为过滤/子集后的因子/向量 [英] Data.frame becomes factor/vector after filtering/subsetting

问题描述

我有一个数据框架，一列如下：

I have a data.frame with one column, like so:

>d = data.frame(animal=c("horse","dog","cat"))

然后我将其排除所有项目也存在于向量中。例如：

then I filter it by excluding all items also present in a vector. e.g.:

> res = d[!(d$animal %in% c("horse")),]
> res
[1] dog cat
Levels: cat dog horse
>class(res)
[1] "factor"

这里发生了什么？

推荐答案

欢迎你刚刚被 drop 烦恼所困：你需要明确告诉R不要放到一维：

Welcome to R. You've just been bitten by the drop annoyance: you need to explicitly tell R not to "drop to one-dimension":

res = d[!(d$animal %in% c("horse")), , drop = FALSE] 

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