Problems with attach() in R

问题

i have a list in R, which i attached.

> attach(myList)
> summary(myList)
            Length Class      Mode     
grData      3      data.frame list     
maxDate     1      POSIXct    numeric  
query       1      -none-     character
newData     3      data.frame list     
updateQuery 1      -none-     function

Okay, my list is really attached

> search()
 [1] ".GlobalEnv"          "myList"          "package:xlsx"        "package:xlsxjars"   
 [5] "package:rJava"       "package:quantmod"    "package:TTR"         "package:xts"        
 [9] "package:zoo"         "package:tidyr"       "package:stringr"     "package:RPostgreSQL"
[13] "package:DBI"         "package:dplyr"       "tools:rstudio"       "package:stats"      
[17] "package:graphics"    "package:grDevices"   "package:utils"       "package:datasets"   
[21] "package:methods"     "Autoloads"           "package:base"

grData data frame from my list has 11 rows:

> summary(myList$grData)
      date            application_id      value        
 Min.   :2016-10-01   Min.   : 7.0   Min.   :  5769  
 1st Qu.:2016-10-01   1st Qu.: 9.5   1st Qu.: 33113  
 Median :2016-10-01   Median :12.0   Median : 65821  
 Mean   :2016-10-01   Mean   :22.0   Mean   :106336  
 3rd Qu.:2016-10-01   3rd Qu.:37.5   3rd Qu.:108861  
 Max.   :2016-10-01   Max.   :49.0   Max.   :507376

But when i try to call grData without myList, i get data frame with 0 rows

> summary(grData)
      date    application_id      value     
 Min.   :NA   Min.   : NA    Min.   : NA  
 1st Qu.:NA   1st Qu.: NA    1st Qu.: NA  
 Median :NA   Median : NA    Median : NA  
 Mean   :NA   Mean   :NaN    Mean   :NaN  
 3rd Qu.:NA   3rd Qu.: NA    3rd Qu.: NA  
 Max.   :NA   Max.   : NA    Max.   : NA

UPD. There is no grData object in globalEnv

> ls()
[1] "checkDbLag"  "con"         "i"           "newDate"     "myList"  "updateQuery"
[7] "x"

Where could be problem?

回答1:

The use of attach() is actually not recommended by many R style guides (like http://google-styleguide.googlecode.com/svn/trunk/google-r-style.html#attach) because it can lead to wrong results.

As suggested by https://www.r-bloggers.com/to-attach-or-not-attach-that-is-the-question/ you have three better options:

Reference variables directly (e.g. lm(ds$x ~ ds$y))
Specify the dataframe for commands which support this (e.g. lm(y ~ x, data=ds))
Use the with() function, which returns the value of whatever expression is evaluated (e.g. with(ds,lm(y ~x))) (Also note the within() function, which is similar to with(), but returns a modified object.)

If you still want to use attach() even given all the alternatives, I suggest you provide a fully reproducible example. By reading your code it seems that you are providing an incomplete output (one time you reference your list as myList, other you reference it as RollingMau).

来源：https://stackoverflow.com/questions/40242904/problems-with-attach-in-r

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