Nested Dataframe loses format after mutating with ifelse-condition - r

Question

I'm trying to mutate a new variable in a nested dataframe with an ifelse-condition. But the problem is that after implementing the ifelse-condition the nested dataframe turns into a list.
I want to show this problem with the iris dataset:
Here you can see the original nested format:
iris %>% nest(data = -Species)
# A tibble: 3 x 2
Species data
<fct> <list>
1 setosa <tibble [50 x 4]>
2 versicolor <tibble [50 x 4]>
3 virginica <tibble [50 x 4]>
And now I want to mutate a new variable in the nested dataframes:
iris %>%
nest(data = -Species) %>%
mutate(data = map(data, function(x)
x %>% mutate(`Sepal.Length^2` = Sepal.Length^2)))
# A tibble: 3 x 2
Species data
<fct> <list>
1 setosa <tibble [50 x 5]>
2 versicolor <tibble [50 x 5]>
3 virginica <tibble [50 x 5]>
This code works. The data-column is as desired in the tibble-format.
But if I insert the ifelse-condition now, the tibble-format is lost:
iris %>%
nest(data = -Species) %>%
mutate(data = map(data, function(x)
ifelse(!is.na(x), x %>% mutate(`Sepal.Length^2` = Sepal.Length^2), NA)))
# A tibble: 3 x 2
Species data
<fct> <list>
1 setosa <list [200]>
2 versicolor <list [200]>
3 virginica <list [200]>
I want to keep the tibble-format even with the ifelse-condition.
Can anyone help me?

Answer 1

In the first step of the map() computation, i.e. data in setosa, the input x of your custom function is actually
x <- iris[1:50, 1:4]
Then you put x into ifelse()
ifelse(!is.na(x), # part 1
x %>% mutate(`Sepal.Length^2` = Sepal.Length^2), # part 2
NA) # part 3
The first part is !is.na(x), which returns 50x4=200 logical values. Hence, the second and third parts will be recycled to length 200. However, the second part, i.e.
x %>% mutate(`Sepal.Length^2` = Sepal.Length^2)
is a tibble with 5 variables, which is also a list with length 5, so each variable in this tibble will be recycled 40 times and subsequently a list with length 200 will be created. That is why you get 3 lists of length 200.
In your case, ifelse() may not be applicable. You can adjust it to
iris %>%
nest(data = -Species) %>%
add_row(Species = "example", data = NA) %>%
mutate(data = map(data, function(x) {
if(is.data.frame(x))
x %>% mutate(`Sepal.Length^2` = Sepal.Length^2)
else
NULL
}))
# # A tibble: 4 x 2
# Species data
# <chr> <list>
# 1 setosa <tibble [50 × 5]>
# 2 versicolor <tibble [50 × 5]>
# 3 virginica <tibble [50 × 5]>
# 4 example <NULL>
Make sure that the condition in if() must be a single logical value.
Grateful to #27ϕ9 for a neater version with map_if():
iris %>%
nest(data = -Species) %>%
add_row(Species = "example", data = NA) %>%
mutate(data = map_if(data, is_tibble,
~ mutate(.x, `Sepal.Length^2` = Sepal.Length^2),
.else = NULL))