Optional argument programming with dplyr - r

Question

I'm new to programming with dplyr. Let's say I have a function like this :
example <- function(data, group, var){
group <- enquo(group)
var <- enquo(var)
data %>%
group_by(!! group) %>%
summarise(
Min = min(!! var),
Max = max(!! var)
)
}
> diamonds %>% example(cut, price)
# A tibble: 5 x 3
cut Min Max
<ord> <dbl> <dbl>
1 Fair 337 18574
2 Good 327 18788
3 Very Good 336 18818
4 Premium 326 18823
5 Ideal 326 18806
I'd like to add a new argument sort. If non specified, the function does nothing more, if specified the function does arrange(desc(sort)).
I tried things like this :
example <- function(data, group, var, sort = NULL){
sort <- enquo(sort)
group <- enquo(group)
var <- enquo(var)
data <-
data %>%
group_by(!! group) %>%
summarise(
Min = min(!! var),
Max = max(!! var)
)
if(is.null(sort)) data
else arrange(data, desc(!! sort))
}
It works when the sort argument is specified
> diamonds %>% example(cut, price, sort = Min)
# A tibble: 5 x 3
cut Min Max
<ord> <dbl> <dbl>
1 Fair 337 18574
2 Very Good 336 18818
3 Good 327 18788
4 Premium 326 18823
5 Ideal 326 18806
But if I'm leaving sort blank I get the error message :
Error: cannot arrange column of class 'NULL' at position 1
How can I fix this ? Thanks for help and sorry for bad english

Answer 1

Moving sort <- enquo(sort) to the else section gets you closer, but you will still have a problem because the if(is.null(sort)) line will return an error if you use sort = Min in the function because is.null will want to evaluate the Min object, but will not find it.
Instead of using is.null, you could use missing, which does not evaluate the Min object.
example <- function(data, group, var, sort = NULL){
group <- enquo(group)
var <- enquo(var)
data <-
data %>%
group_by(!! group) %>%
summarise(
Min = min(!! var),
Max = max(!! var)
)
if(missing(sort)) {
data
} else { sort <- enquo(sort)
arrange(data, desc(!! sort))
}
}
Now we get the following outputs:
diamonds %>% example(cut, price)
# A tibble: 5 x 3
cut Min Max
<ord> <dbl> <dbl>
1 Fair 337 18574
2 Good 327 18788
3 Very Good 336 18818
4 Premium 326 18823
5 Ideal 326 18806
diamonds %>% example(cut, price, sort = Min)
# A tibble: 5 x 3
cut Min Max
<ord> <dbl> <dbl>
1 Fair 337 18574
2 Very Good 336 18818
3 Good 327 18788
4 Premium 326 18823
5 Ideal 326 18806
See Testing a function that uses enquo() for a NULL parameter