How do I combine make a Dict from two Vectors in Julia?

Question

If I have two vectors:

mykeys = ["a", "b", "c"]
myvals = [420, 69, 1337]

How can I make them into a Dict of corresponding pairs like:

Dict("a" => 420, "b" => 69, "c" => 1337)

Answer 1

Perfect use case for broadcasting:

julia> Dict(mykeys .=> myvals)
Dict{String, Int64} with 3 entries:
  "c" => 1337
  "b" => 69
  "a" => 420

There's also a Dict comprehension similar to e.g. Python:

julia> Dict((k, v) for (k, v) ∈ zip(mykeys, myvals))
Dict{String, Int64} with 3 entries:
  "c" => 1337
  "b" => 69
  "a" => 420

and as a general hint usually when you do ?SomeType you get a help entry which includes contructors:

help?> Dict
search: Dict IdDict WeakKeyDict AbstractDict redirect_stdin redirect_stdout redirect_stderr isdispatchtuple to_indices parentindices LinearIndices CartesianIndices deuteranopic optimize_datetime_ticks DimensionMismatch

  Dict([itr])

  Dict{K,V}() constructs a hash table with keys of type K and values of type V. Keys are compared with isequal and hashed with hash.

  Given a single iterable argument, constructs a Dict whose key-value pairs are taken from 2-tuples (key,value) generated by the argument.

  Examples
  ≡≡≡≡≡≡≡≡≡≡

  julia> Dict([("A", 1), ("B", 2)])
  Dict{String, Int64} with 2 entries:
    "B" => 2
    "A" => 1

  Alternatively, a sequence of pair arguments may be passed.

  julia> Dict("A"=>1, "B"=>2)
  Dict{String, Int64} with 2 entries:
    "B" => 2
    "A" => 1

Another trick is to just type your constructor with an opening bracket and hit \tab \tab to get the REPL to show you methods you can call:

julia> Dict(
Dict() in Base at dict.jl:118                               Dict(ps::Pair{K, V}...) where {K, V} in Base at dict.jl:124  Dict(kv) in Base at dict.jl:127
Dict(kv::Tuple{}) in Base at dict.jl:119                    Dict(ps::Pair...) in Base at dict.jl:125

Now as per your comment, I agree that it might not be immediately obvious why .=> works: what happens is that it creates a vector of Pairs:

julia> mykeys .=> myvals
3-element Vector{Pair{String, Int64}}:
 "a" => 420
 "b" => 69
 "c" => 1337

Which constructor method for Dict() does this call you ask?

julia> x = mykeys .=> myvals;

julia> @which Dict(x)
Dict(kv) in Base at dict.jl:127

(@which is one of the nicest things in Julia to figure out what's happening)