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])