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
Answer
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