R help - Aug 2009 - Symbolic references - passing variable names into functions

Hello All,

I am trying to write a function which would operate on columns of a
dataframe specified in parameters passed to that function.

    f = function(dataf, col1 = "column1", col2 = "column2")
{
        dataf$col1 = dataf$col2 # just as an example
    }

The above, of course, does not work as intended.  In some languages one
can force evaluation of a variable, and then use that evaluation as the
variable name.  Thus,

    > a = "myvar"
    > (operator)a = 1
    > myvar
    [1] 1

Is there some operator which allows this symbolic referencing in R?

Thanks,
Allie

I think ONE answer to what you actually want to do might be

f <- function(dataf, col1 = "column1", col2 = "column2")
{
        dataf[[col1]] <- dataf[[col2]] # just as an example
	  dataf
    }

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Alexander Shenkin
Sent: Wednesday, August 12, 2009 9:27 AM
To: r-help at r-project.org
Subject: [R] Symbolic references - passing variable names into functions

Hello All,

I am trying to write a function which would operate on columns of a
dataframe specified in parameters passed to that function.

    f = function(dataf, col1 = "column1", col2 = "column2")
{
        dataf$col1 = dataf$col2 # just as an example
    }

The above, of course, does not work as intended.  In some languages one
can force evaluation of a variable, and then use that evaluation as the
variable name.  Thus,

    > a = "myvar"
    > (operator)a = 1
    > myvar
    [1] 1

Is there some operator which allows this symbolic referencing in R?

Thanks,
Allie

______________________________________________
R-help at r-project.org mailing list
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.

Returning the changed value as in Erik's answer is probably the
most common and R-like solution but here are two others.  The
assign/get approach is perhaps the closest to what you are asking
for.

# replacement function approach

# replacement function - rhs is formula whose response is assigned to
"dataf<-" <- function(data, value) {
	v <- all.vars(value)
	data[[v[1]]] <- data[[v[2]]]
	data
}
BOD <- datasets::BOD
BOD
dataf(BOD) <- Time ~ demand
BOD

# assign/get approach

dataf <- function(data, col1, col2, env = parent.frame()) {
	data.name <- deparse(substitute(data))
	data <- get(data.name, env)
	data[col1] <- data[col2]
	assign(data.name, data, env)
}
BOD <- datasets::BOD
BOD
dataf(BOD, "Time", "demand")
BOD

On Wed, Aug 12, 2009 at 10:27 AM, Alexander Shenkin<ashenkin at ufl.edu>
wrote:
> Hello All,
>
> I am trying to write a function which would operate on columns of a
> dataframe specified in parameters passed to that function.
>
> ? ?f = function(dataf, col1 = "column1", col2 =
"column2") {
> ? ? ? ?dataf$col1 = dataf$col2 # just as an example
> ? ?}
>
> The above, of course, does not work as intended. ?In some languages one
> can force evaluation of a variable, and then use that evaluation as the
> variable name. ?Thus,
>
> ? ?> a = "myvar"
> ? ?> (operator)a = 1
> ? ?> myvar
> ? ?[1] 1
>
> Is there some operator which allows this symbolic referencing in R?
>
> Thanks,
> Allie
>
> ______________________________________________
> R-help at r-project.org mailing list
> 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.
>