Julia: append to an empty vector

Question

I would like to create an empty vector and append to it an array in Julia. How do I do that?

x = Vector{Float64}
append!(x, rand(10))

results in

`append!` has no method matching append!(::Type{Array{Float64,1}}, ::Array{Float64,1})

Thanks.

Answer 1

Your variable x does not contain an array but a type.

x = Vector{Float64}
typeof(x)  # DataType

You can create an array as Array(Float64, n) (but beware, it is uninitialized: it contains arbitrary values) or zeros(Float64, n), where n is the desired size.

Since Float64 is the default, we can leave it out. Your example becomes:

x = zeros(0)
append!( x, rand(10) )

Answer 2

I am somewhat new to Julia and came across this question after getting a similar error. To answer the original question for Julia version 1.2.0, all that is missing are ():

x = Vector{Float64}()
append!(x, rand(10))

This solution (unlike x=zeros(0)) works for other data types, too. For example, to create an empty vector to store dictionaries use:

d = Vector{Dict}()
push!(d, Dict("a"=>1, "b"=>2))

A note regarding use of push! and append!:

According to the Julia help, push! is used to add individual items to a collection, while append! adds an collection of items to a collection. So, the following pieces of code create the same array:

Push individual items:

a = Vector{Float64}()
push!(a, 1.0)
push!(a, 2.0)

Append items contained in an array:

a = Vector{Float64}()
append!(a, [1.0, 2.0])

Answer 3

You can initialize an empty Vector of any type by typing the type in front of []. Like:

Float64[] # Returns what you want
Array{Float64, 2}[] # Vector of Array{Float64,2}
Any[] # Can contain anything

Answer 4

New answer, for Julia 1. append! is deprecated, you now need to use push!(array, element) to add elements to an array

my_stuff = zeros()
push!(my_stuff, "new element")