Pass arguments to custom function in R and filter

Question

I am trying to pass column and parameter into defined function. But I get empty dataset as result. Here is the example:

data = mtcars

param = 4

test_function = function(column, parameter){
data %>%
  filter(column == parameter) %>%
  mutate(mean_mpg = mean(mpg))
}

test_function("cyl", param) # I get empty table as result.

How to do it? What if I pass text not numeric variable - anything changes?

Thank you for your help! M

Answer 1

As you give column as a string, we need to unquote column which we can achieve with !!as.name(...)

test_function = function(column, parameter) {
    data %>%
        filter((!!as.name(column)) == parameter) %>%
        mutate(mean_mpg = mean(mpg))
}

test_function("cyl", param)
#    mpg cyl  disp  hp drat    wt  qsec vs am gear carb mean_mpg
#1  22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1 26.66364
#2  24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2 26.66364
#3  22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2 26.66364
#4  32.4   4  78.7  66 4.08 2.200 19.47  1  1    4    1 26.66364
#5  30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2 26.66364
#6  33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1 26.66364
#7  21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1 26.66364
#8  27.3   4  79.0  66 4.08 1.935 18.90  1  1    4    1 26.66364
#9  26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2 26.66364
#10 30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2 26.66364
#11 21.4   4 121.0 109 4.11 2.780 18.60  1  1    4    2 26.66364

Alternatively if you want to pass a symbol for column you could do

test_function2 = function(column, parameter) {
    column <- enquo(column)
    data %>%
        filter(!!column == parameter) %>%
        mutate(mean_mpg = mean(mpg))
}

test_function2(cyl, param)

Or – perhaps even nicer – you can write a function that accepts both a symbol and string for column

test_function3 = function(column, parameter) {
    column <- rlang::parse_expr(quo_name(enquo(column)))
    data %>%
        filter(!!column == parameter) %>%
        mutate(mean_mpg = mean(mpg))
}
test_function3(cyl, param)
test_function3("cyl", param)

All of the above methods give the same result.