lazy evaluation - 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