Julia: Append an element to an array of custom typ

Question

Appending an element to an array in Julia works like this:

v = Array{Int32, 1}(0)
append!(v, 1)
append!(v, 2)
println(v)  # prints: Int32[1,2]

When I try this with a custom type

type Node
    label::String
    value::Int32
end
nodes = Array{Node, 1}(0)
append!(nodes, Node("a", 42))

I get the following error:

ERROR: LoadError: MethodError: no method matching length(::Node)

I assume I have to 'implement' the length method but do not know how.

Answer 1

Try this

Base.append!(x::Array{Node,1}, val::Node) = push!(x, val)

then you get

append!(nodes, Node("a", 42))
1-element Array{Node,1}:
 Node("a",42)

you've got to explicitly create a function for this particular type as append! or any of the Base functions sometimes (or perhaps always I havent checked) wont accept Any

Answer 2

The append! command doesn't do what you think it does. You're thinking of the push! command.

The append! command appends two arrays together. Both arguments need to be arrays:

julia> append!(nodes, [Node("a", 42)])
1-element Array{Node,1}:
 Node("a",42)

No length implementing necessary
_{(that error was just telling you it tried to read the length of your array for the second argument and found something that was not an array.)}