r - rm(list = ls())不能完全清除工作区

这是一个非常小的问题，但是我想确切了解这里发生的事情。

说我执行以下操作:

library(RMySQL)
con <- dbConnect(MySQL(), host="some.server.us-east-1.rds.amazonaws.com",user="aUser", password="password", dbname="mydb")

values1 <- dbGetQuery(con,"select x,y from table1")
attach(values1)

至此，我可以

rm(list=ls())

values2 <- dbGetQuery("select x,y from table1")
attach(values2)

但是附件给了我关于屏蔽x和y的警告。我以为我已经破坏了这些。到底是怎么回事？如何完全清除工作区？

最佳答案

attach()不会在您的全局环境中复制x和y，而是将数据帧附加到搜索路径。

从?attach:

The database is not actually attached.  Rather, a new environment
 is created on the search path and the elements of a list
 (including columns of a data frame) or objects in a save file or
 an environment are _copied_ into the new environment.  If you use
 ‘<<-’ or ‘assign’ to assign to an attached database, you only
 alter the attached copy, not the original object.  (Normal
 assignment will place a modified version in the user's workspace:
 see the examples.)  For this reason ‘attach’ can lead to
 confusion.

例如:

> search()
[1] ".GlobalEnv"        "package:stats"     "package:graphics"
[4] "package:grDevices" "package:utils"     "package:datasets"
[7] "package:methods"   "Autoloads"         "package:base"
> a <- data.frame(stuff=rnorm(100))
> search()
[1] ".GlobalEnv"        "package:stats"     "package:graphics"
[4] "package:grDevices" "package:utils"     "package:datasets"
[7] "package:methods"   "Autoloads"         "package:base"
> attach(a)
> search()
 [1] ".GlobalEnv"        "a"                 "package:stats"
 [4] "package:graphics"  "package:grDevices" "package:utils"
 [7] "package:datasets"  "package:methods"   "Autoloads"
[10] "package:base"
> rm(list=ls())
> search()
 [1] ".GlobalEnv"        "a"                 "package:stats"
 [4] "package:graphics"  "package:grDevices" "package:utils"
 [7] "package:datasets"  "package:methods"   "Autoloads"
[10] "package:base"
> stuff
  [1] -0.91436377  0.67397624  0.62891651 -0.99669584  2.07692590 -0.62702302
  [...]
> detach(a)
> search()
[1] ".GlobalEnv"        "package:stats"     "package:graphics"
[4] "package:grDevices" "package:utils"     "package:datasets"
[7] "package:methods"   "Autoloads"         "package:base"

关于r - rm(list = ls())不能完全清除工作区，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/3536036/