[SOLVED] Mutate across multiple columns based on condition (length of unique values)

Issue

I’m trying to create a function inside mutate() + across() that changes into factor any variable which has five or less unique values (or any arbitrary number) wit the idea of using later that factors to do some grouping. I think the logic of the function is correct but I’m getting some incorrect dimensions error (error in Spanish). For the sake of simplicity, I’m using the mtcars database.

mtcars %>%
    mutate(across(1:ncol(.), 
        function(x) {
                if_else(length(unique(x[,i]))<=5, 
            as.factor(x), 
          x)}
    ))

Error: Problem with `mutate()` input `..1`.
i `..1 = across(...)`.
x número incorreto de dimensiones
Run `rlang::last_error()` to see where the error occurred.

Any help or advice will be much appreciated.

Solution

Here we need if/else as ifelse/if_else requires all arguments to be of equal length. The length(unique expression returns a logical value of length 1 and this may break the condition. Also, with dplyr, we can use select-helpers i.e. everything() to select all the columns

library(dplyr)
out <- mtcars %>%
    mutate(across(everything(), 
        function(x) {
                if(length(unique(x))<=5) 
            as.factor(x) else  
          x}
    ))

-output

> str(out)
'data.frame':   32 obs. of  11 variables:
 $ mpg : num  21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
 $ cyl : Factor w/ 3 levels "4","6","8": 2 2 1 2 3 2 3 1 1 2 ...
 $ disp: num  160 160 108 258 360 ...
 $ hp  : num  110 110 93 110 175 105 245 62 95 123 ...
 $ drat: num  3.9 3.9 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 ...
 $ wt  : num  2.62 2.88 2.32 3.21 3.44 ...
 $ qsec: num  16.5 17 18.6 19.4 17 ...
 $ vs  : Factor w/ 2 levels "0","1": 1 1 2 2 1 2 1 2 2 2 ...
 $ am  : Factor w/ 2 levels "0","1": 2 2 2 1 1 1 1 1 1 1 ...
 $ gear: Factor w/ 3 levels "3","4","5": 2 2 2 1 1 1 1 2 2 2 ...
 $ carb: num  4 4 1 1 2 1 4 2 2 4 ...

---

In addition, the lambda function can be concise with ~ and make use of n_distinct

mtcars %>% 
   mutate(across(everything(), 
        ~ if(n_distinct(.x) <=5) as.factor(.x) else .x))

Answered By – akrun

Answer Checked By – Gilberto Lyons (BugsFixing Admin)