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(factor)]] : 

  object of type 'symbol' is not subsettable

As i don't have much experience programming i don't know exactly what's the purpose for the functions used for passing variables in functions:

substitute

,

deparse

,

eval

,

parse

. I've been reading the documentation and i'm not finding it very clear. So it would be interesting if someone could provide a clearer use for each function or perhaps point to resources to learn more about it.

Answer 1

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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