How does AbstractFloat[1.0f0] work?

New to Julia
1

In the blog post Tutorial on precompilation there is a line cabs = AbstractFloat[1.0f0] . While trying to understand the syntax I found one case in the documentation, in the section Conversion there is a = Any[1 2 3; 4 5 6].

What is this syntax? Does it have anything to do with abstract types?

2

In this case you are initializing a vector that can contain any type of Floats:

julia> x = AbstractFloat[1.f0]
1-element Array{AbstractFloat,1}:
 1.0f0

julia> push!(x,1.e0)
2-element Array{AbstractFloat,1}:
 1.0f0  # Float32
 1.0    # Float64

If you start a vector with a concrete type signature, like:

julia> y = Float32[1.f0]
1-element Array{Float32,1}:
 1.0

julia> push!(y,1.e0)
2-element Array{Float32,1}:
 1.0
 1.0

julia> typeof(y[1])
Float32

julia> typeof(y[2])
Float32

Only elements of that type will be possible. Depending on what you want the vector to have, you can even get an error:

julia> z = Int[1]
1-element Array{Int64,1}:
 1

julia> push!(z,sqrt(2))
ERROR: InexactError: Int64(1.4142135623730951)

AbstractFloat is an abstract type of which Float32 and Float64 are subtypes:

julia> subtypes(AbstractFloat)
4-element Array{Any,1}:
 BigFloat
 Float16
 Float32
 Float64

so an AbstractFloat[] vector can contain any of those types.

It is not a good a idea to have that kind of mixed type vector, that have performance issues, because each type of number has a different representation in the memory and has to be dispatched differently to the functions that will operate on them.

And Any[] can contain anything:

julia> x = Any[1]
1-element Array{Any,1}:
 1

julia> push!(x,"abc")
2-element Array{Any,1}:
 1
  "abc"
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3

Ok, thank you! Where is this documented?

4

There is a section in the julia documentation under performance tips named Avoid containers with abstract type parameters, which could be of help. In general, you should avoid using abstract types for arrays.

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5

Ok, thank you! This question is not about the performance of abstract types, it is about the syntax in the examples.

6

The syntax is explained here, under “Typed Array Literals”: Single- and multi-dimensional Arrays · The Julia Language

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