What is Base.KeySet()?

When I use keys() the resulting data structure is a Base.KeySet(). Can someone elaborate on what that is? I searched the documentation, but I couldn’t find it.

character_inventory = Dict{String,Int32}("Sword" => 100, "Staff" => 50)
character_inventory_keys = keys(character_inventory)
println(typeof(character_inventory_keys))

Output:

Base.KeySet{String,Dict{String,Int32}}

The documentation for keys describe it a bit although it doesn’t mention KeySet explicitly:

help?> keys

    keys(a::AbstractDict)

Return an iterator over all keys in a dictionary. [...]

And to be more explicit, KeySet is one such iterator which yields the keys of the dictionary when iterated. It is simply a “lazy-view” of the keys (see https://github.com/JuliaLang/julia/blob/10b0a014275ad4836fad8d2a2590d457dc2286ad/base/abstractdict.jl#L39-L41) and it is implemented like this (presumably) for efficiency since you don’t have to materialize an array or similar with all the keys.

4 Likes

I think you meant 'use keys() without collect()', if you use collect you get an Array as expected.

Good catch, Fixed!