How to find object name passed to function-CodePudding

I have a function which takes a dataframe and its columns and processes it in various ways (left out for simplicity). We can put in column names as arguments or transform columns directly inside function arguments (like here). I need to find out what object(s) are passed in the function.

Reproducible example:

df <- data.frame(x= 1:10, y=1:10)

myfun <- function(data, col){
col_new <- eval(substitute(col), data)
# magic part
object_name <- ...
# magic part
plot(col_new, main= object_name)
}

For instance, the expected output for myfun(data= df, x*x) is the plot plot(df$x*df$x, main= "x"). So the title is x, not x*x. What I have got so far is this:

myfun <- function(data, col){
colname <- tryCatch({eval(substitute(col))}, error= function(e) {geterrmessage()})
colname <- gsub("' not found", "", gsub("object '", "", colname))
plot(eval(substitute(col), data), main= colname)
    }

This function gives the expected output but there must be some more elegant way to find out to which object the input refers to. The answer must be with base R.

CodePudding user response：

The rlang package can be very powerful when you get a hang of it. Does something like this do what you want?

library(rlang)

myfun <- function (data, col){
  .col <- enexpr(col)
  
  unname(sapply(call_args(.col), as_string))
}

This gives you back the "wt" column.

myfun(mtcars, as.factor(wt))    
# [1] "wt"

I am not sure your use case, but this would work for multiple inputs.

myfun(mtcars, sum(x, y))
# [1] "x" "y"

And finally, it is possible you might not even need to do this, but rather store the expression and operate directly on the data. The tidyeval framework can help with that as well.

CodePudding user response：

Use substitute to get the expression passed as col and then use eval and all.vars to get the values and name.

myfun <- function(data, col){
  s <- substitute(col)
  plot(eval(s, data), main = all.vars(s), type = "o", ylab = "")
}
myfun(df, x * x)

Anothehr possibility is to pass a one-sided formula.

myfun2 <- function(formula, data){
  plot(eval(formula[[2]], data), main = all.vars(formula), type = "o", ylab = "")
}
myfun2(~ x * x, df)