Julia DefaultDict uses the same value (array) for each key

Question

I want to create a defaultdict of arrays. Problem is, it uses the same array for each key.

# using Pkg
# Pkg.add("DataStructures")
using DataStructures: DefaultDict
genome = DefaultDict{Tuple{String, String}, Array{Int64, 1}}(Int64[])
push!(genome["chr1", "+"], 5)
# 1-element Array{Int64,1}:
# 5

push!(genome["chrX", "-"], 10)
# 2-element Array{Int64,1}:
#  5
# 10

I have tried feeding it a lambda to create a new array x -> Int64, but that just gave a type error.

Answer 1

I don't know how to solve your problem using DefaultDict, but I think Julia's in-build dictionary structure offers a better solution. One can use

get!(collection, key, default)

to automatically give a default value where one hasn't been set already. The above code would be rewritten:

genome = Dict{Tuple{String, String}, Array{Int64, 1}}()
push!(get!(genome, ("chr1", "+"), Int64[]), 5)
# 1-element Array{Int64,1}:
# 5

push!(get!(genome, ("chrX", "-"), Int64[]), 10)
# 1-element Array{Int64,1}:
# 10