Passing a variable name to a function in R

Question

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

Answer 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

Answer 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)]