write function to replace variable with itself plus 1% of its median - r

Question

I'm new to writing functions in R, but want to write a function to add 1% of the median of a variable to itself, using dplyr, and replace the variable with this transformation.
x is a numeric variable.
add_median <- function(df, x) {
x <- enquo(x)
x <- quo_name(x)
mutate(x=x+.01*median(x, na.rm=T))
}
When I run newDF <- DF %>% add_median(variable_of_interest), I get the following error:
Error in 0.01 * median(x, na.rm = T) : non-numeric argument to binary operator
What am I doing wrong here?

Answer 1

We could change the function to evaluate with {{}} and then use assign (:=) instead of = in mutate
library(dplyr)
add_median <- function(df, x) {
df %>%
mutate({{x}} := {{x}} + .01 * median({{x}}, na.rm = TRUE))
}
If we need to change multiple columns, use mutate_at
add_median_multiple <- function(df, vec){
df %>%
mutate_at(vars(vec), ~ . + .01 * median(., na.rm = TRUE))
}
-testing
data(mtcars)
head(mtcars) %>%
add_median(mpg)
# mpg cyl disp hp drat wt qsec vs am gear carb
#Mazda RX4 21.21 6 160 110 3.90 2.620 16.46 0 1 4 4
#Mazda RX4 Wag 21.21 6 160 110 3.90 2.875 17.02 0 1 4 4
#Datsun 710 23.01 4 108 93 3.85 2.320 18.61 1 1 4 1
#Hornet 4 Drive 21.61 6 258 110 3.08 3.215 19.44 1 0 3 1
#Hornet Sportabout 18.91 8 360 175 3.15 3.440 17.02 0 0 3 2
#Valiant 18.31 6 225 105 2.76 3.460 20.22 1 0 3 1
comparison with original 'mpg' column
head(mtcars)
# mpg cyl disp hp drat wt qsec vs am gear carb
#Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
#Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
#Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
#Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
#Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
#Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
add_median_multiple(head(mtcars), c('mpg', 'wt'))
# mpg cyl disp hp drat wt qsec vs am gear carb
#Mazda RX4 21.21 6 160 110 3.90 2.65045 16.46 0 1 4 4
#Mazda RX4 Wag 21.21 6 160 110 3.90 2.90545 17.02 0 1 4 4
#Datsun 710 23.01 4 108 93 3.85 2.35045 18.61 1 1 4 1
#Hornet 4 Drive 21.61 6 258 110 3.08 3.24545 19.44 1 0 3 1
#Hornet Sportabout 18.91 8 360 175 3.15 3.47045 17.02 0 0 3 2
#Valiant 18.31 6 225 105 2.76 3.49045 20.22 1 0 3 1