Merge function

[1danjordan]

Saw this on HN today. The post is about the various methods of merging Maps (Dicts/Tables/...) in different languages. I especially liked the Clojure merge-with implementation.

In R, with identically sized lists A and B, Map(c, A, B) is a simple solution, or in purrr pmap(list(A, B), c).

Once the lists become more complicated, or the type of merge logic changes, this becomes much more complicated. For example, we might want to merge by name (key), position or just pack all elements together individually:

  A <- list(a = 1, b = "abc")
  B <- list(a = 2, b = "def", c = 1:5)
  C <- list(c = 6:10)

merge(list(A, B), .by = "name")
# => list(a = c(1, 2), b = c("abc", "def"), c = 1:5)

merge(list(A, C), .by = "position")
# => list(c(1, 6:10))
#          ^ strip names

merge(list(A, B, C))
# => list(a = 1, b = "abc", a = 2, b = "def", c = 1:5, c =  6:10)
# this is the same as flatten(list(A, B, C))

I figured there might be a place in purrr for merge.

[hadley]

It seems like matching the list_modify() semantics for name vs. position would be most useful here.

[1danjordan]

Well that was spectacularly fast!