In Julia, how to merge a dictionary?

Question

What is the best way to merge a dictionary in Julia?

> dict1 = Dict("a" => 1, "b" => 2, "c" => 3)
> dict2 = Dict("d" => 4, "e" => 5, "f" => 6)
# merge both dicts
> dict3 = dict1 with dict2
> dict3
Dict{ASCIIString,Int64} with 6 entries:
  "f" => 6
  "c" => 3
  "e" => 5
  "b" => 2
  "a" => 1
  "d" => 4

Bureaucratize

Answer 1

https://docs.julialang.org/en/latest/base/collections/#Base.merge

merge(collection, others...)

Construct a merged collection from the given collections. If necessary, the types of the resulting collection will be promoted to accommodate the types of the merged collections. If the same key is present in another collection, the value for that key will be the value it has in the last collection listed.

julia> merge(dict1,dict2)
    Dict{ASCIIString,Int64} with 6 entries:
      "f" => 6
      "c" => 3
      "e" => 5
      "b" => 2
      "a" => 1
      "d" => 4

merge!(collection, others...)
Update collection with pairs from the other collections.

julia> merge!(dict1,dict2)
Dict{ASCIIString,Int64} with 6 entries:
  "f" => 6
  "c" => 3
  "e" => 5
  "b" => 2
  "a" => 1
  "d" => 4

julia> dict1
Dict{ASCIIString,Int64} with 6 entries:
  "f" => 6
  "c" => 3
  "e" => 5
  "b" => 2
  "a" => 1
  "d" => 4

Cushing

Just out if curiosity, how would thus handle conflicting key values? If I have values 'a =5' in 'dict1' and 'a =7' in 'dict2' what would the value of 'a' be in the resulting dictionary? – Epp

If you wanted to keep the conflicting values, you could use union(dict1, dict2); however, this would return an Array rather than a Dict. – Verve

Answer 2

You can use merge. If the Dicts have elements with the same key, the value for that key will be of the last Dict listed.

If you want to combine the elements of the Dicts that have the same key, you can use mergewith(combine, collection, others...). combine is a function that receive two values and return a value. See mergewith.

Example of the docs:

julia> a = Dict("foo" => 0.0, "bar" => 42.0)
Dict{String,Float64} with 2 entries:
  "bar" => 42.0
  "foo" => 0.0

julia> b = Dict("baz" => 17, "bar" => 4711)
Dict{String,Int64} with 2 entries:
  "bar" => 4711
  "baz" => 17

julia> mergewith(+, a, b)
Dict{String,Float64} with 3 entries:
  "bar" => 4753.0
  "baz" => 17.0
  "foo" => 0.0

Slackjawed

So if I would like to keep the second element, can I use merge((x, y) -> y, a, b)? – Trifolium

I think this would work, but you can use merge(a, b). – Slackjawed

Answer 3

You can use the ... operator, like this:

julia> dict1 = Dict("a" => 1, "b" => 2, "c" => 3);

julia> dict2 = Dict("d" => 4, "e" => 5, "f" => 6);

julia> Dict(dict1..., dict2...)
Dict{String, Int64} with 6 entries:
  "f" => 6
  "c" => 3
  "e" => 5
  "b" => 2
  "a" => 1
  "d" => 4

Bays

Neat! Works for Vectors and Sets too. – Trifolium