Using a parameter as a variable name inside a function in R

Question

I am trying to incorporate the name of the month in the name of the variable being stored.

import <- function(month) {
  dataobj <- letters
  assign("x", dataobj)
  save("x", file="data.rda")
}

works. But the following doesn't work -

import <- function(month) {
  dataobj <- letters
  assign(substr(month, 1, 3), dataobj)
  save(substr(month, 1, 3), file="data.rda")
}

It seems that save() will accept "x" but not substr(month, 1, 3).

Any ideas how to fix this?

McGarnagle · Accepted Answer · 2012-05-07 17:13:50Z

6

Use the list argument of save():

save(list=substr(month,1,3), file="data.rda")

edited May 7, 2012 at 17:13

McGarnagle

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answered May 7, 2012 at 9:09

baptiste

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Comments

Paul Hiemstra · Accepted Answer · 2012-05-07 08:14:15Z

In stead of creating objects in the environment with a specific, month dependent, name, I would use a list of objects where month is used as name.

dat = lapply(1:4, function(x) letters)
names(dat) = c("Jan","Feb","Mar","Apr")
> dat
$Jan
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" 
[20] "t" "u" "v" "w" "x" "y" "z"                                                 

$Feb                                                                             
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" 
[20] "t" "u" "v" "w" "x" "y" "z"                                                 

$Mar                                                                             
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" 
[20] "t" "u" "v" "w" "x" "y" "z"                                                 

$Apr                                                                             
 [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" 
[20] "t" "u" "v" "w" "x" "y" "z"

Saving this list can be done easily using save(dat). If you are keen on saving the months in separate objects:

lapply(names(dat), function(month) {
  save(dat[[month]], file = sprintf("%s.rda", month)
 })

or using the good old for loop:

for(month in names(dat)) {
  save(dat[[month]], file = sprintf("%s.rda", month)
}

This is a good solution. Let me try it out and see if anyone else comes up with a more direct solution.
This is a less flexible solution than baptiste's, because you've hard-coded the 3-letter month names. In general, code which allows arbitrary inputs to be processed, e.g. month <-'HitherePaulH';substr(month,8,4) :=)
I agree, but for the sake of illustration I thought this was fine.

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