Accessing an object using a string

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

--
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com