Passing a list of arguments to a function with quasiquotation

Question

I am trying to write a function in R that summarizes a data frame according to grouping variables. The grouping variables are given as a list and passed to group_by_at

Answer 1

You could make use of the ellipse .... Take the following example:

sum_fun <- function(df, sum_var, ...) {
  sum_var <- substitute(sum_var)
  grps    <- substitute(list(...))[-1L]
  return(
    df %>% 
      group_by_at(.vars = as.character(grps)) %>% 
      summarize(sum(!! sum_var))
  )
}

d %>% sum_fun(baz, foo, bar)

We take the additional arguments and create a list out of them. Afterwards we use non-standard evaluation (substitute) to get the variable names and prevent R from evaluating them. Since group_by_at expects an object of type character or numeric, we simply convert the vector of names into a vector of characters and the function gets evaluated as we would expect.

> d %>% sum_fun(baz, foo, bar)
# A tibble: 3 x 3
# Groups:   foo [?]
    foo   bar `sum(baz)`
         
1     1     2          3
2     1     3          5
3     4     5          7

If you do not want to supply grouping variables as any number of additional arguments, then you can of course use a named argument:

sum_fun <- function(df, sum_var, grps) {
  sum_var <- enquo(sum_var)
  grps    <- as.list(substitute(grps))[-1L]
  return(
    df %>% 
      group_by_at(.vars = as.character(grps)) %>% 
      summarize(sum(!! sum_var))
  )
}

sum_fun(mtcars, sum_var = hp, grps = c(cyl, gear))

The reason why I use substitute is that it makes it easy to split the expression list(cyl, gear) in its components. There might be a way to use rlang but I have not digged into that package so far.