how Julia determines index of dictionary keys?

January 29, 2022

I confronted strange behavior in Dictionary collection in Julia. a Dictionary can be defined in Julia like this:

dictionary = Dict(1 => 77, 2 => 66, 3 => 1)

and you can access keys using keys:

> keys(dictionary)

# [output] 
KeySet for a Dict{Int64, Int64} with 3 entries. Keys:
2
3
1

# now i want to make sure Julia consider above order. so i use collect and then i will call first element of it
> collect(keys(dictionary))[1]

# [output] 
2

as you can see the order of keys in keys(dictionary) output is so strange. seems Julia doesn’t consider the order of (key=>value) in input! even it doesn’t seem to be ordered ascending or descending. How Julia does indexing for keys(dictionary) output?

Expected Output:

> keys(dictionary)

# [output] 
KeySet for a Dict{Int64, Int64} with 3 entries. Keys:
1
2
3

> collect(keys(dictionary))[1]

# [output] 
1

I expect keys(dictionary) give me the keys in the order that I entered them in defining dictionary.

>Solution :

The key order in Dict is currently undefined (this might change in the future).

If you want order to be preserved use OrderedDict from DataStructures.jl:

julia> using DataStructures

julia> dictionary = OrderedDict(1 => 77, 2 => 66, 3 => 1)
OrderedDict{Int64, Int64} with 3 entries:
  1 => 77
  2 => 66
  3 => 1

julia> keys(dictionary)
KeySet for a OrderedDict{Int64, Int64} with 3 entries. Keys:
  1
  2
  3