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Merge function #322

1danjordan opened this issue May 9, 2017 · 2 comments


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commented May 9, 2017

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.

@1danjordan 1danjordan closed this May 9, 2017

@1danjordan 1danjordan reopened this May 9, 2017


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commented May 9, 2017

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

@hadley hadley closed this in f355404 May 9, 2017


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commented May 9, 2017

Well that was spectacularly fast!

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