R 为什么重新排序会将单列数据帧变成向量？

我只是好奇为什么对单列数据帧（或矩阵）重新排序会将其转换为向量。这有什么原因吗

k <- data.frame(a=c(2,10,3), b=c(8,3,9))
k <- k[order(k[,1]),]
class(k)
# [1] "data.frame"

k <- data.frame(a=c(2,10,3))
k <- k[order(k[,1]),]
class(k)
# [1] "numeric"

---

k查看
？'['
尤其是
drop
参数

drop: For matrices and arrays.  If ‘TRUE’ the result is coerced to
      the lowest possible dimension (see the examples).  This only
      works for extracting elements, not for the replacement.  See
      ‘drop’ for further details.

要回答您的问题，您需要

k[order(k[,1]), , drop=FALSE]

查看
？“[”
尤其是
drop
参数

drop: For matrices and arrays.  If ‘TRUE’ the result is coerced to
      the lowest possible dimension (see the examples).  This only
      works for extracting elements, not for the replacement.  See
      ‘drop’ for further details.

要回答您的问题，您需要

k[order(k[,1]), , drop=FALSE]

谢谢！这是我在帮助文档中的第二个问题和答案。我应该阅读它！谢谢！这是我在帮助文档中的第二个问题和答案。我应该阅读它！