[R] Antwort: Re: Accessing an object using a string

[Jeff Newmiller]

My example shows that using a dedicated environment allows you to segregate the items loaded from the save file from the objects you are working with. It has the advantage that any kind of object or collection of objects can be loaded that way. It has the disadvantage that you have to keep track of what kinds of objects they are (as in not data frames).

For example, suppose you want to make a data table that summarizes information about the contents of several save files. If you name the data frame you are building up "infodf" and one of the save files has a data frame by that name in it, then loading it into the global environment will wipe out the information so far accumulated and replace it with something else.

Greg's option of saving individual objects into rds files is another solution. It has the advantage of letting you control the name of the imported object, but you still need to independently insure that the object is actually a data frame somehow before digging into its columns.

Personally, I prefer to avoid keeping lots of rds or RData files around... I favor restarting from original input files to insure reproducibility. Any output values that I want to emit from my analysis I write into CSV or other portable format. I only use rds/RData files for temporary storage (caching) for performance acceleration. 
-- 
Sent from my phone. Please excuse my brevity.

On August 15, 2016 11:43:14 PM PDT, G.Maubach at weinwolf.de wrote:
>Hi Greg 
>and all others who replied to my question,
>
>many thanks for all your answers and help. Currently I store all my 
>objects in .GlobalEnv = Workspace. I am not yet familiar working with 
>different environments nor did I see that this would be necessary for
>my 
>analysis.
>
>Could you explain why working with different environments would be 
>helpful?
>
>You suggested to read variables into lists rather than storing them in 
>global variables. This sounds interesting. Could you provide an example
>of 
>how to define and use this?
>
>Kind regards
>
>Georg
>
>
>
>Von:    Greg Snow <538280 at gmail.com>
>An:     G.Maubach at weinwolf.de, 
>Kopie:  r-help <r-help at r-project.org>
>Datum:  15.08.2016 20:33
>Betreff:        Re: [R] Accessing an object using a string
>
>
>
>The names function is a primitive, which means that if it does not
>already do what you want, it is generally not going to be easy to
>coerce it to do it.
>
>However, the names of an object are generally stored as an attribute
>of that object, which can be accessed using the attr or attributes
>functions.  If you change your code to not use the names function and
>instead use attr or attributes to access the names then it should work
>for you.
>
>
>You may also want to consider changing your workflow to have your data
>objects read into a list rather than global variables, then process
>using lapply/sapply (this would require a change in how your data is
>saved from your example, but if you can change that then everything
>after can be cleaner/simpler/easier/more fool proof/etc.)
>
>
>On Mon, Aug 15, 2016 at 2:49 AM,  <G.Maubach at weinwolf.de> wrote:
>> Hi All,
>>
>> I would like to access an object using a sting.
>>
>> # Create example dataset
>> var1 <- c(1, 2, 3)
>> var2 <- c(4, 5, 6)
>> data1 <- data.frame(var1, var2)
>>
>> var3 <- c(7, 8, 9)
>> var4 <- c(10, 11, 12)
>> data2 <- data.frame(var3, var4)
>>
>> save(file = "c:/temp/test.RData", list = c("data1", "data2"))
>>
>> # Define function
>> t_load_dataset <- function(file_path,
>>                            file_name) {
>>   file_location <- file.path(file_path, file_name)
>>
>>   print(paste0('Loading ', file_location, " ..."))
>>   cat("\n")
>>
>>   object_list <- load(file = file_location,
>>                       envir = .GlobalEnv)
>>
>>   print(paste(length(object_list), "dataset(s) loaded from",
>> file_location))
>>   cat("\n")
>>
>>   print("The following objects were loaded:")
>>   print(object_list)
>>   cat("\n")
>>
>>   for (i in object_list) {
>>     print(paste0("Object '", i, "' in '", file_name, "' contains:"))
>>     str(i)
>>     names(i)  # does not work
>>   }
>> }
>>
>> I have only the character vector object_list containing the names of
>the
>> objects as strings. I would like to access the objects in object_list
>to
>> be able to print the names of the variables within the object
>(usuallly 
>a
>> data frame).
>>
>> Is it possible to do this? How is it done?
>>
>> Kind regards
>>
>> Georg
>>
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