how to use tidyeval functions with loops?

Question

Consider this simple example

library(dplyr)

dataframe <- data_frame(id = c(1,2,3,4),
                        group = c('a','b','c','c'),
                        value = c(200,400,120,300))


> dataframe
# A tibble: 4 x 3
     id group value
  <dbl> <chr> <dbl>
1     1     a   200
2     2     b   400
3     3     c   120
4     4     c   300

and this tidyeval function that uses dplyr to aggregate my dataframe according to some input column.

func_tidy <- function(data, mygroup){
  quo_var <- enquo(mygroup)

  df_agg <- data %>% 
    group_by(!!quo_var) %>% 
    summarize(mean = mean(value, na.rm = TRUE),
              count = n()) %>% 
    ungroup()

  df_agg
}

now, this works

> func_tidy(dataframe, group)
# A tibble: 3 x 3
  group  mean count
  <chr> <dbl> <int>
1     a   200     1
2     b   400     1
3     c   210     2

but doing the same thing from within a loop FAILS

for(col in c(group)){
  func_tidy(dataframe, col)
}
 Error in grouped_df_impl(data, unname(vars), drop) : Column `col` is unknown 

What is the problem here? How can I use my tidyeval function in a loop?

Thanks!

Answer 1

For looping through column names you will need to use character strings.

for(col in "group")

When you pass this variable to your function, you will need to convert it from a character string to a symbol using rlang::sym. You use !! to unquote so the expression is evaluated.

So your loop would look like (I add a print to see the output):

for(col in "group"){
    print( func_tidy(dataframe, !! rlang::sym(col) ) )
}

# A tibble: 3 x 3
  group  mean count
  <chr> <dbl> <int>
1     a   200     1
2     b   400     1
3     c   210     2