How to convert a string in a function into an object?

Question

I would like to convert a string that I pass in a function into an object (or column name).

I know that this works:

df <- data.frame(A = 1:10, B =         


        

Answer 1

I just got an upvote which brought me back after 5 years to this question. I still think that the correct answer is [[ despite the OP's request not to use it, but here's a way to dress up [[ as a more functional "function".

df <-     structure(list(x = 1:3, y = 1:3), .Names = c("x", "y"), row.names = c(NA, 
-3L), class = "data.frame")

 test.function <- `[[`    # So simple, `test.function` now has all the features desired.
 df
 x y
 1 1
 2 2
 3 3
 test.function(df, "x")
#[1] 1 2 3

Or if it were desireable to hard code pulling an object named 'df' from the calling environment, a proposition that seems of dubious safety:

 test.df_txt <- function(var, dfn ='df' ){ get(dfn)[[var]] }
 test.df_txt("x")
#[1] 1 2 3

Original response (still not recommended):

You can sidestep around the limitations of "$" if you are willing to use eval(parse(text=...)) :

 test.function <- function(x)  {
   z <- eval(parse( text=paste("df$", x, sep = "")), env=.GlobalEnv)
   return(z)
   }
test.function("A")
# [1]  1  2  3  4  5  6  7  8  9 10

BUT ... it is much better to use "[[". (My initial efforts at eval(parse()-ing were stuck at not knowing enough to use the "text" argument to parse.)