How do I use a variable to name another variable in R? [duplicate]

Question

I have a function that uses readline to allow the user to enter the name they want to give for a variable I will be creating for them. Let's call this "USER.DEFINED.VARIABLE". It contains the name I want to use for another variable. Let's say that "USER.DEFINED.VARIABLE" gets set by readline to be "jimsfilename".

I know I can assign value to a variable named "jimsfilename" using:

assign(USER.DEFINED.VARIABLE,c(1,2,3,4,5))

"jimsfilename" will now have 1,2,3,4,5 in it. However, how do I now fuss with "jimsfilename", given that I don't (before readline assigns it to USER.DEFINED.VARIABLE) know what its name is?

In other words, lets say I now want to add 1 to every value in jimsfilename. I can't do:

USER.DEFINED.VARIABLE <- USER.DEFINED.VARIABLE + 1 # can't do this

because "USER.DEFINED.VARIABLE" is actually a text string name. I want instead to refer to jimsfilename, but all I have is USER.DEFINED.VARIABLE to indicate it. I'm sure this is something easy...

Answer 1

It depends bit on what you want to do, but here's an example of using get function:

x = 1
get("x") + 1

2

assign("name", get("x") + 1)
name

2

Answer 2

Why not just do all the manipulation of the variable (adding 1 or other changes) to a local copy of the variable with your own name, then at the end of the function/script/whatever do the assigning or other saving? That would be much simpler than creating the variable then having to use get to get a copy, change it, and assign it again.

Even better is to use your own variable name inside of a function, then just return the result and let the user decide what to name it at that point. This is the much more Rish way of doing things, it is best to not use the assign function at all. Most things that can be done using assign can be done much simpler by using a list and subscripting.

Functions should not change anything in the global environment, just return any values that the user might need and let the user make the assignment.

Answer 3

You can use eval and parse. The later interprets text as if it was an input in the console. The first evaluates the expression (generated by parse, for instance). Example:

> varname <- "user.defined.variable"
> varvalue <- 42
> eval(parse(text=paste(varname, varvalue, sep=" <- ")), envir=.GlobalEnv)
> ls()
[1] "user.defined.variable" "varname"               "varvalue"             
> user.defined.variable
[1] 42

Note that I've choosen the global environment as the destination for the new variable. You can make the appropriate changes if that is not the case.

To refer to the new variable later, you can use as.symbol. Just evaluate it under the environment where you assigned the new variable:

> eval(as.symbol(varname), envir=.GlobalEnv)
[1] 42

You can also use substitute to create expressions that eval can understand:

> eval(substitute(x+1, list(x=as.symbol(varname))), envir=.GlobalEnv)
[1] 43

To make changes to the new variable, just creat assignments expressions and evaluate them:

> eval(substitute(x <- x*10, list(x=as.symbol(varname))), envir=.GlobalEnv)
> eval(as.symbol(varname), envir=.GlobalEnv)
[1] 420