set_intersect(), set_union(), set_diff(), and set_equal()

[DavisVaughan]

I really could have used these when doing some Date set comparisons, since base R support is not great.

intersect(as.Date("2019-01-01"), as.Date(c("2019-01-01", "2019-01-02")))
#> [1] 17897

^{Created on 2019-08-27 by the reprex package (v0.2.1)}

@lionel- mentioned that it may make sense to use this new namespaced naming scheme rather than vec_intersect() vec_union() vec_set_diff() and vec_set_equal()

[DavisVaughan]

First implementation pass. Open to improvement!

library(vctrs)

set_union <- function(x, y) {
  vec_unique(vec_c(x, y))
}

set_diff <- function(x, y) {
  x_in_y <- vec_in(x, y)
  vec_slice(x, !x_in_y)
}

set_intersect <- function(x, y) {
  loc_in_y <- vec_match(x, y)
  loc_in_y <- vec_slice(loc_in_y, !is.na(loc_in_y))
  vec_slice(y, loc_in_y)
}

set_equal <- function(x, y) {
  x_in_y <- vec_in(x, y)
  all_x_in_y <- all(x_in_y)
  
  if (all_x_in_y) {
    y_in_x <- vec_in(y, x)
    all_y_in_x <- all(y_in_x)
    all_y_in_x
  } else {
    FALSE
  }
}

set_union(1:5, 5:10)
#>  [1]  1  2  3  4  5  6  7  8  9 10
set_union(1:5, c(5, 5, 6))
#> [1] 1 2 3 4 5 6

set_diff(1:5, 1:5)
#> integer(0)
set_diff(1:5, 1:2)
#> [1] 3 4 5
set_diff(1, 2:3)
#> [1] 1
set_diff(1, 1:2)
#> numeric(0)

set_intersect(1:5, 1:6)
#> [1] 1 2 3 4 5
set_intersect(1, 2)
#> numeric(0)
set_intersect(1:3, 1:2)
#> [1] 1 2

set_equal(1:5, 1:5)
#> [1] TRUE
set_equal(1:6, 1:5)
#> [1] FALSE
set_equal(1:5, 1:6)
#> [1] FALSE

^{Created on 2019-08-27 by the reprex package (v0.2.1)}

[lionel-]

The implementation of set_intersect() would be simpler with a nomatch parameter (cf #568):

set_intersect <- function(x, y) {
  vctrs::vec_slice(x, match(y, x, 0))
}

Also if we keep the names we probably should be consistently slicing the LHS.

[lionel-]

Should not return duplicate elements, as in:

setdiff(c(1, 1), 0)
#> [1] 1

But then if we keep the names, which ones do we keep? Maybe that's why we shouldn't keep them. Alternatively, the set could be made of name-value pairs, but will that be surprising or error prone?

[lionel-]

set-intersect needs to sort the matched locations to preserve the order of the first input, see r-lib/tidyselect#186

[DavisVaughan]

Should these go in funs?

[lionel-]

I think so too.

[DavisVaughan]

Moved to tidyverse/funs#67