Passing a variable name to a function in R

I've noticed that quite a few packages allow you to pass symbol names that may not even be valid in the context where the function is called. I'm wondering how this works and how I can use it in my own code?

Here is an example with ggplot2:

a <- data.frame(x=1:10,y=1:10)
library(ggplot2)
qplot(data=a,x=x,y=y)

x and y don't exist in my namespace, but ggplot understands that they are part of the data frame and postpones their evaluation to a context in which they are valid. I've tried doing the same thing:

b <- function(data,name) { within(data,print(name)) }
b(a,x)

However, this fails miserably:

Error in print(name) : object 'x' not found

What am I doing wrong? How does this work?

Note: this is not a duplicate of Pass variable name to a function in r

Solution 1:

I've recently discovered what I think is a better approach to passing variable names.

a <- data.frame(x = 1:10, y = 1:10)

b <- function(df, name){
    eval(substitute(name), df)
}

b(a, x)
  [1]  1  2  3  4  5  6  7  8  9 10

Update The approach uses non standard evaluation. I began explaining but quickly realized that Hadley Wickham does it much better than I could. Read this http://adv-r.had.co.nz/Computing-on-the-language.html

Solution 2:

You can do this using match.call for example:

b <-  function(data,name) {

  ## match.call return a call containing the specified arguments 
  ## and the function name also 
  ## I convert it to a list , from which I remove the first element(-1)
  ## which is the function name

  pars <- as.list(match.call()[-1])
  data[,as.character(pars$name)]

}

 b(mtcars,cyl)
 [1] 6 6 4 6 8 6 8 4 4 6 6 8 8 8 8 8 8 4 4 4 4 8 8 8 8 4 4 4 8 6 8 4

explanation:

match.call returns a call in which all of the specified arguments are specified by their full names.

So here the output of match.call is 2 symbols:

b <-  function(data,name) {
  str(as.list(match.call()[-1]))  ## I am using str to get the type and name
}

b(mtcars,cyl)
List of 2
 $ data: symbol mtcars
 $ name: symbol cyl

So Then I use first symbol mtcars ansd convert the second to a string:

mtcars[,"cyl"]

or equivalent to :

eval(pars$data)[,as.character(pars$name)]

Solution 3:

Very old thread but you can also use the get command as well. It seems to work better for me.

a <- data.frame(x = 1:10, y = 11:20)

b <- function(df, name){

   get(name, df)

 }

b(a, "x")
 [1]  1  2  3  4  5  6  7  8  9 10