[R] Accessing specific data.frame columns within function

[Ulrik Stervbo]

Hi Clark,

In your function you are using the variable 'y' and not 'data'. If this
indeed is your intention, there is no need to pass 'data' to your function,
otherwise all 'y's in your function should be 'data'.

Does this work for you:

f <- function(data, oldnames, subset.val = 4){
  data <- data[ data[[ oldnames[2] ]] == subset.val, ]
  data$d <- data[[ oldnames[1] ]]^2 + data[[ oldnames[2] ]]
  return(data)
}

y <- data.frame(k=c(1,1,1),l=c(2,2,5),m=c(4,2,4))
f(data = y, oldnames = c("k","m"))

Its probably safer to pass the 'oldnames' as two arguments. Also, if you
want to subset your data.frame in the function, you should pass the value
or subset before you call the function, along the lines of

f <- function(data, oldname.1, oldname.2){
  data$d <- data[[ oldname.1 ]]^2 + data[[ oldname.2 ]]
  return(data)
}

y <- data.frame(k=c(1,1,1),l=c(2,2,5),m=c(4,2,4))
y <- subset(y, m == 4)
f(data = y, oldname.1 = "k", oldname.2 = "m")

Hope this helps,
Ulrik



On Fri, 5 Feb 2016 at 05:14 Clark Kogan <kogan.clark at gmail.com> wrote:

> Hello,
>
> I am trying to write a function that adds a few columns to a data.frame.
> The
> function uses the columns in a specific way. For instance, it might take
> a^2
> + c to produce a column d. Or it might do more complex manipulations that I
> don't think I need to discuss here. I want to keep x as a data.frame when I
> pass it into the function, as I want to use some data.frame functionality
> on
> x.
>
> Furthermore, I don't want the names in x to have to be specific. I want to
> be able to specify which columns the function should treat as "a" and "c".
>
> The way I am currently doing it, is that I pass the names of the columns
> that I want to treat as a and c.
>
> f <- function(data,oldnames) {
>   newnames <- c("a","c")
>   ix <- match(oldnames,names(y))
>   names(y)[ix] <- newnames
>   y <- subset(y,c==4)
>   y$d <- y$a^2 + y$c
>   ix <- match(newnames,names(y))
>   names(y)[ix] <- oldnames
>   y
> }
>
> y <- data.frame(k=c(1,1,1),l=c(2,2,5),m=c(4,2,4))
> f(y,c("k","m"))
>
> The way that I am doing it does not seem all that elegent or standard
> practice. My question is: are there potential problems programming with
> data.frames in this way, and are their standard practice methods of
> referencing data.frame names that deal with these problems?
>
> Thanks!
>
> Clark
>
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