r - Summarise does not accept counts()

Question

I am creating a summary data frame for multiple columns with dplyr, and it works well for desired functions excepts for count or n(). I want to show total number of observations.

 summ <- Test_data %>%            
   summarize(across(
     .cols = is.numeric,
     .fns = list(a=n(),mean=mean, stdev=sd, max=max,min=min), na.rm=TRUE,
     .names = "{col}_{fn}"
   ))

It gives error: Error: Problem with summarise() input ..1.x Can't convert an integer vector to function i Input ..1 is across(...).

Why just function for counts does not work, please advise.

question from:https://stackoverflow.com/questions/65924698/summarise-does-not-accept-counts

Answer 1

As @tjebo comments, n() is tricky inside of a summarize().

This is discussed the comment he linked - summarize_all with "n()" function - where @akrun explains "getting the n() for each column is not making much sense as it would be the same [for each summarized column]." The reason n() is giving you problems is because n() doesn't take any arguments, but the summarize is sending the value of the selected .cols as an argument.

Two solutions are to replace with the length function, or add a leading ~ to turn n() into ~ n(). (However, I don't know why ~ n() works, is it turning the code into a single-sided formulae? )

Also, use where(is.numeric) in the column selection.

iris %>% 
    summarize(across(.cols  = where(is.numeric), 
                     .fns   = list(n = ~ n(), mean = mean, sd = sd), 
                     .names = "{col}_{fn}"))

iris %>% 
    summarize(across(.cols  = where(is.numeric), 
                     .fns   = list(n = length, mean = mean, sd = sd), 
                     .names = "{col}_{fn}"))