Julia: Cannot append tuple to array

Question

I would like to have an array of tuples. However it seems I cannot append a tuple to it. Here is a minimal code example that raises the error.

julia> a = [(1,1),(2,2)]
2-element Array{Tuple{Int64,Int64},1}:
 (1, 1)
 (2, 2)

julia> append!(a, (3,3) )
ERROR: MethodError: Cannot `convert` an object of type Int64 to an object of type Tuple{Int64,Int64}
This may have arisen from a call to the constructor Tuple{Int64,Int64}(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] _append!(::Array{Tuple{Int64,Int64},1}, ::Base.HasLength, ::Tuple{Int64,Int64}) at ./array.jl:644
 [2] append!(::Array{Tuple{Int64,Int64},1}, ::Tuple{Int64,Int64}) at ./array.jl:637

Is something wrong with my syntax? I don't get why it complains that it has to convert a number to a tuple. What gives?

Answer 1

append! adds all of the individual elements of another collection to the existing object. Julia raises the error here because (3, 3) is a collection of two integers and it cannot reconcile an individual integer of type Int64 with the array's Tuple{Int64,Int64} type.

The method you need is push!, which will add one or more individual items to an existing collection:

julia> push!(a, (3, 3))
3-element Array{Tuple{Int64,Int64},1}:
 (1, 1)
 (2, 2)
 (3, 3)

The individual item, the tuple (3, 3), was successfully pushed onto the array a.

To accomplish the same task with append!, the tuple needs to be contained in a collection of some sort itself, such as an array:

julia> append!(a, [(4, 4)])
4-element Array{Tuple{Int64,Int64},1}:
 (1, 1)
 (2, 2)
 (3, 3)
 (4, 4)

This is documented on the collections page here.