How to assign function argument to variable name - r

Question

Say I have these data:
data <- data.frame(a = c(1,2,3))
And a simple function that creates a data frame. It also creates a new variable based on a simple function; this new variable takes the name that is passed in with varname. Here is my attempt (the assign line is wrong):
fun <- function(varname) {
data <- data.frame(a = c(1,2,3))
assign(paste0("data$", varname), sqrt(data$a))
data
}
fun("newvar")
Base R or tidyverse solutions are both great.

Answer 1

You were close! There are multiple ways of subsetting dataframes, including using the [[.]] notation (e.g., data[["var"]]. Simply assigning a value to a new column initializes the column.
fun <- function(varname) {
data <- data.frame(a = c(1,2,3))
data[[varname]] <- sqrt(data$a)
data
}
fun("newvar")

Answer 2

tidyverse
If you want to pass the variable name as a string then a tidyverse method would be:
library(dplyr)
fun <- function(varname) {
data.frame(a = c(1,2,3)) %>%
mutate(!! varname := sqrt(a))
}
fun("newvar")
Alternatively, you could use tidyeval so you don't have to quote the variable name:
library(dplyr)
fun <- function(varname) {
varname <- rlang::enquo(varname)
data.frame(a = c(1,2,3)) %>%
mutate(!! varname := sqrt(a))
}
fun(newvar)
base R
If you want to use base R I would recommend the solution posted by #Noah, but another base R option that is fairly obtuse would be:
fun <- function(varname) {
data.frame(a = c(1,2,3)) |>
within(eval(substitute(x <- sqrt(a), list(x = as.name(varname)))))
}
fun("newvar")