r - Dplyr difference between select and group_by with respect to quoted variables? -


in current version of dplyr, select arguments can passed value:

variable <- "species" iris %>%      select(variable)  #       species #1       setosa #2       setosa #3       setosa #4       setosa #5       setosa #6       setosa #...

but group_by arguments cannot passed value:

iris %>%      group_by(variable) %>%      summarise(petal.length = mean(petal.length))  # error in grouped_df_impl(data, unname(vars), drop) :  # column `variable` unknown

the documented dplyr::select behaviour 

iris %>% select(species)

and documented documented dplyr::group_by behaviour 

iris %>%      group_by(species) %>%      summarise(petal.length = mean(petal.length))
  • why select , group_by different respect passing arguments value?
  • why first select call working , continue work in future?
  • why first group_by call not working? i'm trying figure out combination of quo(), enquo() , !! should use make work.

i need because create function takes grouping variable input parameter, if possible grouping variable should given character string, because 2 other function parameters given character strings.


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