Julia Complex Iterator Example

complex_iterator.jl

using Random

# The iterable instance
struct Iterable
    _ids::Vector{Int64}
    _other::Vector{String}
    Iterable() = new(rand(Int64, 10), [Random.randstring(3) for i in 1:10])
end

# Iterator, holds reference to instance
mutable struct ComplexIterator
    instance::Ref{Iterable}
    i::Int64
    id::Int64
    other::String
    ComplexIterator(i, instance::Iterable) = new(instance, i, instance._ids[i], instance._other[i])
end

# advance the iterator one step, updates its fields
function advance(iterator::ComplexIterator)

    if iterator.i != length(iterator.instance[]._ids)
        iterator.i += 1
        iterator.id = iterator.instance[]._ids[iterator.i]
        iterator.other = iterator.instance[]._other[iterator.i]
    end
    return iterator
end

# Iterable: start iteration by returning a ComplexIterator instead
function Base.iterate(instance::Iterable)
    return (ComplexIterator(1, instance), 1)
end

# Iterable: stop iteration 
function Base.iterate(instance::Iterable, i::Int64)
    if i == length(instance._ids)
        return nothing
    else
        return (ComplexIterator(i + 1, instance), i + 1)
    end
end

# iterator: start state
function Base.iterate(it::ComplexIterator)
    return (it, 1)
end

# iterator: succesive steps
function Base.iterate(it::ComplexIterator, i::Int64)
    if i == length(it.instance[]._ids)
        return nothing
    else
        advance(it) # advancing updates fields so they can be accessed
        return (it, i+1)
    end
end
   
# usage
instance = Iterable()
for it in instance
    println("Id: ", it.id)
    println("Other: ", it.other)
end

"""
Output:

Id: -5199682371322551963
Other: KxA
Id: 1963780643060477616
Other: cfz
Id: -8038754947851161495
Other: cRo
Id: -7086019444048395646
Other: lie
Id: -5416162732659090528
Other: w5V
Id: 6410769836315988433
Other: xFV
Id: 76395605441795193
Other: UN6
Id: 4689883532029073831
Other: O8W
Id: -5366039862844754873
Other: QUS
Id: -5366039862844754873
Other: QUS
"""