When does dplyr return ties when using slice_min and slice_max? I'm seeing some inconsistencies and can't seem to find any clarification online or in their documentation.
Examples:
library(dplyr)
#there is a tie but only returns 5 rows, not the bottom 5 mpg's
mtcars %>% slice_min(mpg, n = 5, with_ties = TRUE)
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Cadillac Fleetwood 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4
#> Lincoln Continental 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4
#> Camaro Z28 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4
#> Duster 360 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4
#> Chrysler Imperial 14.7 8 440 230 3.23 5.345 17.42 0 0 3 4
#this will return the top two as a tie when above it did not
mtcars %>%
slice_min(mpg, n = 1, with_ties = TRUE)
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Cadillac Fleetwood 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4
#> Lincoln Continental 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4
#another example of it using ties to return more than 3 rows
starwars %>%
select(gender, mass) %>%
group_by(gender) %>%
slice_min(mass, n = 3, with_ties = TRUE)
# A tibble: 8 x 2
# Groups: gender [3]
# gender mass
#
#1 feminine 45
#2 feminine 49
#3 feminine 50
#4 feminine 50
#5 masculine 15
#6 masculine 17
#7 masculine 20
#8 NA 48
Am I missing something here?
CodePudding user response：
The "tie" refers to the borderline entry, not any ties at all. So if the last element included is tied with an element that would be excluded otherwise, "with_ties" pulls it into the output.
my_data <- data.frame(a = c(1, 1, 2, 2))
> slice_min(my_data, a, n = 1)
a
1 1
2 1
> slice_min(my_data, a, n = 2)
a
1 1
2 1
> slice_min(my_data, a, n = 3)
a
1 1
2 1
3 2
4 2
If you want the three lowest mpgs, you could start with a list of distinct mpgs, slice those, and join to original data:
mtcars %>%
distinct(mpg) %>%
slice_min(mpg, n = 3) %>%
left_join(mtcars)
Joining, by = "mpg"
mpg cyl disp hp drat wt qsec vs am gear carb
1 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4
2 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4
3 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4
4 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4
CodePudding user response：
From the documentation of slice_min/slice_max
It says that:
with_ties
Should ties be kept together? The default, TRUE, may return more rows than you request. Use FALSE to ignore ties, and return the first n rows.
This means that in cases the number of minimal values you ask for is smaller than the actual number of entries with this minimal value, you will get a larger output than you expected.
CodePudding user response：
There can be some issues with slice_min/slice_max when there is only a single value in the data. It also means that suppose the number of rows is 10000, it will return all the rows whether it is tied or not
dat <- tibble(a = rep(1, 5))
> slice_min(dat, a, n = 1)
# A tibble: 5 × 1
a
<dbl>
1 1
2 1
3 1
4 1
5 1
> slice_min(dat, a, n = 1, with_ties = TRUE)
# A tibble: 5 × 1
a
<dbl>
1 1
2 1
3 1
4 1
5 1
If there are duplicate values and option is to arrange and use slice
mtcars %>%
arrange(desc(mpg)) %>%
slice(1)
We may get the output in a single filter as well
mtcars %>% filter(mpg %in% tail(unique(sort(mpg)), 3))
mpg cyl disp hp drat wt qsec vs am gear carb
Fiat 128 32.4 4 78.7 66 4.08 2.200 19.47 1 1 4 1
Honda Civic 30.4 4 75.7 52 4.93 1.615 18.52 1 1 4 2
Toyota Corolla 33.9 4 71.1 65 4.22 1.835 19.90 1 1 4 1
Lotus Europa 30.4 4 95.1 113 3.77 1.513 16.90 1 1 5 2
> mtcars %>% filter(mpg %in% head(unique(sort(mpg)), 3))
mpg cyl disp hp drat wt qsec vs am gear carb
Duster 360 14.3 8 360 245 3.21 3.570 15.84 0 0 3 4
Cadillac Fleetwood 10.4 8 472 205 2.93 5.250 17.98 0 0 3 4
Lincoln Continental 10.4 8 460 215 3.00 5.424 17.82 0 0 3 4
Camaro Z28 13.3 8 350 245 3.73 3.840 15.41 0 0 3 4