paste formula in function (c constant?)

Question

I'm doing a function pasting a formula and then returning a feols result. But, I get a c at the beginning. How can I solve this?

library(dplyr)
library(fixest)

data(base_did)
base_did = base_did %>% mutate(D = 5*rnorm(1080),
                               x2 = 10*rnorm(1080),
                               rand_wei = abs(rnorm(1080)))

f <- function(data, arg=NULL){
  
  arg = enexpr(arg)
  
  if (length(arg) == 0) {
    formula = "D ~ 1"
  } 
  else {
    formula = paste(arg, collapse = " + ")
    formula = paste("D ~ ", formula, sep = "")
  }
  
  formula = paste(formula, " | id + period", sep = "")
  denom.lm <- feols(as.formula(formula), data = data, 
                    weights = abs(data$rand_wei))
  
  return(denom.lm)
}

f(base_did, arg = c(x1,x2))

#Error in feols(as.formula(formula), data = data, weights = abs(data$rand_wei)) : 
#  Evaluation of the right-hand-side of the formula raises an error: 
#  In NULL: Evaluation of .Primitive("c") returns an object of length 1
#while the data set has 1080 rows.

If I return(formula) at the end. I get [1] "D ~ c + x1 + x2 | id + period".

But I need only D ~ x1 + x2 | id + period.

Answer 1

Perhaps one option to make your function work would be to pass the arguments via ... so that c is not needed and which would prevent the c to be added to your formula. To make this work you also have switch to enexprs inside your function.

Note: I slightly adjusted your function for the reprex to return just the formula.

library(dplyr, warn = FALSE)
library(fixest)

data(base_did)

base_did = base_did %>% mutate(D = 5*rnorm(1080),
                               x2 = 10*rnorm(1080),
                               rand_wei = abs(rnorm(1080)))

f <- function(data, ...){
  arg = enexprs(...)
  
  if (length(arg) == 0) {
    formula = "D ~ 1"
  } 
  else {
    formula = paste(arg, collapse = " + ")
    formula = paste("D ~ ", formula, sep = "")
  }
  
  formula = paste(formula, " | id + period", sep = "")
  
  as.formula(formula)
}


f(base_did, x1, x2)
#> D ~ x1 + x2 | id + period
#> <environment: 0x7fe8f3567618>

f(base_did)
#> D ~ 1 | id + period
#> <environment: 0x7fe8f366f848>

UPDATE There is probably a better approach but after some research a possible option would be:

Note: When passing multiple arguments via c enexpr will return a call object which behaves like a list and where the first element contains the function name, i.e. c. That's why you get the c added to your formula.

f <- function(data, arg = NULL) {
  arg <- enexpr(arg)
  
  if (length(arg) == 0) {
    formula <- "D ~ 1"
  } else {
    if (length(arg) > 1) arg <- vapply(as.list(arg[-1]), rlang::as_string, FUN.VALUE = character(1))
    
    formula <- paste(arg, collapse = " + ")
    formula <- paste("D ~ ", formula, sep = "")
  }

  formula <- paste(formula, " | id + period", sep = "")

  as.formula(formula)
}


f(base_did, c(x1, x2))
#> D ~ x1 + x2 | id + period
#> <environment: 0x7fa763431388>

f(base_did, x1)
#> D ~ x1 | id + period
#> <environment: 0x7fa763538c40>

f(base_did)
#> D ~ 1 | id + period
#> <environment: 0x7fa765e22028>