How do I check if a dictionary has a key in it in Julia?

Question

Suppose I have a Dict object and a key value and I want to see if there's already an entry in the dictionary for that key? How do I do this?

Answer 1

There are a few ways to do this. Suppose this is your dictionary:

d = Dict(
    "aardvark" => 1,
    "bear"     => 2,
    "cat"      => 3,
    "dog"      => 4,
)

If you have a key you can check for its presence using the haskey function:

julia> haskey(d, "cat")
true

julia> haskey(d, "zebra")
false

A slightly fancier way to check this is to check if the key is in the set of keys returned by calling keys(d):

julia> ks = keys(d)
Base.KeySet for a Dict{String,Int64} with 4 entries. Keys:
  "aardvark"
  "bear"
  "cat"
  "dog"

julia> "cat" in ks
true

julia> "zebra" in ks
false

Finally, it's fairly common that you want to get the value associated with a key if it is present in the dictionary. You can do that as a separate step by doing d[k] after checking that k is present in keys(d) but that involves an additional dictionary lookup. Instead, if there is some sentinel value that you know cannot be a value in your dictionary, such as nothing, then you can use the get function to look up the key with a default:

v = get(d, k, nothing)
if v !== nothing
    # keys(d) contains k
end

If you know nothing about the kinds of values that d can map keys to, this is not a safe option since it could be the case that the pair k => nothing is present in d.