将参数传递给 dplyr 切片

Question

I want to use slice to get the maximum value and the position (to get the date) over a large data.frame and I want to do it for several variables. So, I am writing a custom function to pass several variables as argument, but it's not working.

           mvar <- "WD"
          
           peak_max <- dataf%>%group_by(year=format(date,"%Y"))%>%
                   dplyr::slice(which.max(!!mvar))
          

I used summarise, but it also failed when passing the argument:

            peak_max <- dataf%>%group_by(year=format(date,"%Y"))%>%
                    dplyr::summarise(date=date[which.max(!!mvar)], wd=max(!!mvar))

I also tried with mvar <- enquo(mvar), but it doesn't work. What am I missing? Both worked when using WD directly, instead.!mvar.

Thanks in advance!

Answer 1

It looks like your using an older approach to programming with dplyr . Enquo() and !! aren't used anymore.

It's hard to be sure what you want to achieve without your data, but, if you pass the variable directly (as you do in dplyr pipes, you can embrace it {{ var }} in a function. If you're passing a string name for the variable, you need you use .data[[var]] :

library(tidyverse)

# pass variable directly
my_func_var <- function(df, var){
  df %>% slice_max({{ var }})
}

iris %>% my_func_var(Sepal.Length)
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#> 1          7.9         3.8          6.4           2 virginica

# pass variable as string
my_func_str <- function(df, var){
  df %>% slice_max(.data[[var]])
}

my_var <- "Sepal.Length"
iris %>% my_func_str(my_var)
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#> 1          7.9         3.8          6.4           2 virginica

^{Created on 2021-04-23 by the reprex package (v1.0.0)}

(I used slice_max() as it felt more natural, but which.max() seems to work too)