R: How to write a function that gets the levels of column in a data.table

Question

Some data: ``` require(data.table) set.seed(123) DT <- data.table(factor = c("a", "b", "c"), num = rpois(6, 30)) DT[["factor"]] <- factor(DT[["factor"]]) levels(DT[["factor"]]) # [1] "a" "b" "c" ``` I'm trying to write a function that gets the levels of the the DT. Here's what I've attempted so far: ``` get_levels <- function(data, factor){ data = substitute(data) factor = substitute(factor) factor_levels = levels(data[["factor"]]) print(factor_levels) } get_levels(DT, factor) get_levels2 <- function(data, factor){ data = substitute(data) factor = substitute(factor) factor_levels = levels(data[[factor]]) print(factor_levels) } get_levels2(DT, factor) get_levels3 <- function(data, factor){ data = substitute(data) factor = substitute(factor) factor_levels = levels(eval(data[[deparse(factor)]])) print(factor_levels) } get_levels3(DT, factor) ``` I'm getting this error: ``` Error in data[["factor"]] : object of type 'symbol' is not subsettable ``` and this one: ``` Error in data[[deparse(…

Accepted Answer

If you're not experienced with R (or programming in general) avoid `substitute`, `deparse`, `eval`, etc. They are rarely necessary.

```
DT <- data.table(f = c("a", "b", "c"), num = rpois(6, 30))
DT[["f"]] <- factor(DT[["f"]])

get_levels <- function(data,fac){
  levels(data[[fac]])
}

get_levels(DT,'f')
```

And don't call your column `factor`. That's a function, and it's just confusing.

R: How to write a function that gets the levels of column in a data.table

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