Call variable from custom function

Question

This would seem to be a straightforward problem but I can't find an answer for it...

How do I write a function where one of the calls refers to a specific variable name?

For example, if I have a data frame:

data=structure(list(x = 1:10, treatment = c(1L, 2L, 1L, 2L, 1L, 2L, 
1L, 2L, 1L, 2L)), .Names = c("x", "treatment"), row.names = c(NA, 
-10L), class = "data.frame")

I can write a trivial function that adds treatment to the other variable in the data frame but this only works if there is a variable called "treatment" in d.

ff=function(data,treatment){data+treatment)}
ff(data,data$treatment)

This works but I want to set it up so the user doesn't call data$Var in the function.

ff = function(data, var1, var2) {data[, var1] + data[, var2]} ? Is that what you're looking for? — Josh W.
– Josh W., Commented May 26, 2015 at 20:08
When I run that I get an error that the variables are not found — User7598
– User7598, Commented May 26, 2015 at 20:13
To use @JoshW.'s solution, var1 and var2 would need to be passed to the function as character strings, otherwise you'd need deparse(substitute(var1)). — Alex A.
– Alex A., Commented May 26, 2015 at 20:14
That's because quote() returns a symbol and you need a string. Try it with deparse(substitute()). — Alex A.
– Alex A., Commented May 26, 2015 at 20:17

Greg Snow · Accepted Answer · 2015-05-26 20:17:19Z

1

Is this what you want?

ff <- function(data, colname) {
   data + data[[colname]]
}
ff( data, "treatment" )

or

ff <- function(data, column) {
  colname <- deparse(substitute(column))
  data + data[[colname]]
}
ff( data, treatment )

(the later can lead to hard to find bugs if someone tries something like ff(data, 1:10))

answered May 26, 2015 at 20:17

Greg Snow

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