[R] Accessing an object using a string

[Ista Zahn]

I'm surprised no one has given what I consider to be the standard
answer to this questions, namely ?get. Won't

  for (i in object_list) {
    print(paste0("Object '", i, "' in '", file_name, "' contains:"))
    str(get(i))
    print(names(get(i)))  # works
  }

do what you want?

Best,
Ista

On Tue, Aug 16, 2016 at 11:46 AM, Greg Snow <538280 at gmail.com> wrote:
> Interestingly I just gave a presentation on this last Friday.
>
> Using load it is a little more complicated since load reads the object
> name from the file as well (which can lead to overwriting objects if
> loading from more than one file).  So it is better to use another save
> format if possible.  One alternative is to use the saveRDS and readRDS
> functions.
>
> Here is the example that I presented:
>
> First, create some sample data files on the disk (here saving as .csv files)
>
> data('Oxboys', package='nlme')
> obl <- split(Oxboys, Oxboys$Subject)
> for(d in obl) {
>   write.csv(d, file=sprintf("OxBoys%02d.csv",
>                             d$Subject[1]),
>             row.names=FALSE)
> }
> rm(Oxboys,obl)
>
>
> Now read the files into R into a single list (and add names to the
> list, optional but can be nice):
>
> fnames <- list.files(pattern='^OxBoys')
> ob.new <- lapply(fnames, read.csv)
> nms <- paste0('OxBoys', sapply(ob.new,
>                   function(df) df$Subject[1]))
> names(ob.new) <- nms
> head( ob.new$OxBoys1 )
>
> The list.files function in the first line is one convenient way to
> create the vector of filenames, but other options would include
> paste0, sprintf, and others.
>
> Now perform the same analysis on each data frame in the list and
> nicely format the results (a simple regression in this case):
>
> slopes <- sapply(ob.new, function(df) {
>   fit <- lm(height~age, data=df)
>   coef(fit)
> })
> head(t(slopes))
>
>
>
>
> On Tue, Aug 16, 2016 at 12:43 AM,  <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
>>>
>>> ______________________________________________
>>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>>
>> --
>> Gregory (Greg) L. Snow Ph.D.
>> 538280 at gmail.com
>>
>>
>
>
>
> --
> Gregory (Greg) L. Snow Ph.D.
> 538280 at gmail.com
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.