String to variable name in R

Question

I am cleaning a data set and I need to choose the variables depending on another variable. Let's say that if ID = 1, I need to introduce in the data frame the variable VAR01, if ID = 2, I need VAR02, and so on.

Thus, I'm doing a for loop where I paste the variable name 'VAR' with the ID number with the stringf function. The problem is that I need R to understand the string as a function name.

I've found in the forum this solution, which doesn't work for me:

> variable1 = c("monday", "tuesday", "wednesday")

> var_name = "variable1"

> eval(parse(text=var_name))
[1] "monday"    "tuesday"   "wednesday"

The problem is I cannot use it to refer to the variable:

> eval(parse(text=var_name)) = c(1,2,3)
Error in file(filename, "r") : cannot open the connection
In addition: Warning message:
In file(filename, "r") :
cannot open file 'variable1': No such file or directory

Has anyone got a solution?

Thanks!

Answer 1

You can use assign()

var_name <- 'test'
assign(var_name,1:3)
test

Note: assign as such will create the variable in the environment it is called. So, if you were to call assign within a function like this:

myfun <- function(var) { 
    assign(eval(substitute(var)), 5)
    print(get(var)) 
}

Calling the function assigns my_var a value of 5 within the function, whose environment is created only for the time the function is run and destroyed after.

> myfun("my_var")
# [1] 5

> my_var
# Error: object 'my_var' not found

So, if you want to retain the value after a function call, then, you'll have to specify an environment you'll have the variable thro' the time your task is run. For example, in the global environment:

myfun <- function(var, env = globalenv()) {
    assign(eval(substitute(var)), 5, envir = env)
    print(get(var))
}

> myfun("my_var")
# [1] 5
>  my_var
# [1] 5

Answer 2

You can use get:

result = get(var_name)

Although heavy use of get and set suggests you might want to start using list's of variables instead:

l = list(variable1 = c(1,2,3))
l$variable1 = c(4,5,6)