creating and assigning values to an array of structs in Julia

Question

I would like to define a struct to hold a vector of 3 elements

mutable struct Coords
    r::Array{Float64,3} # essentially holds x,y,z coords
end

I now want to make an array of these structures, and give random values to the vector inside each structure.

This is where I fade out. I have tried a few things which I will describe, but none of them worked.

---

trial 1:

x = 10                            # I want the array to be of length 10
arrayOfStructs::Array{Coords,x}
for i=1:x
    arrayOfStructs[i].r = rand(3)
end

The error message is

ERROR: LoadError: MethodError: Cannot `convert` an object of type Int64 to an object of type Array{C
oords,10}
Closest candidates are:
  convert(::Type{T<:Array}, ::AbstractArray) where T<:Array at array.jl:489
  convert(::Type{T<:AbstractArray}, ::T<:AbstractArray) where T<:AbstractArray at abstractarray.jl:1
4
  convert(::Type{T<:AbstractArray}, ::LinearAlgebra.Factorization) where T<:AbstractArray at C:\User
s\julia\AppData\Local\Julia-1.0.2\share\julia\stdlib\v1.0\LinearAlgebra\src\factorization.jl:46
  ...
Stacktrace:
 [1] setindex!(::Array{Array{Coords,10},1}, ::Int64, ::Int64) at .\array.jl:769
 [2] getindex(::Type{Array{Coords,10}}, ::Int64) at .\array.jl:366
 [3] top-level scope at C:\Users\Zarathustra\Documents\JuliaScripts\energyTest.jl:68 [inlined]
 [4] top-level scope at .\none:0
in expression starting at C:\Users\Zarathustra\Documents\JuliaScripts\energyTest.jl:67

I don't get why it thinks there are integers involved.

I have tried changing the inside of the for loop to be

arrayOfStructs[i] = Coords(rand(3))

to no avail.

I have also tried initializing arrayOfStructs = []

Answer 1

N in Array{T,N} defines the dimensionality of the array, that is, an N-dimensional array of type T.

You are not defining an array of size 3 to hold x, y, z coordinates in your struct definition, instead you are defining a 3D Array, which does not suit your purpose.

Yet again, you just declare the type of arrayOfStructs to be a 10-dimensional array without constructing it. You need to define and construct your array properly before using it.

Array types in Julia does not have static size information. Array is a dynamic structure and does not suit your case. For array types with static size information you might want to take a look at StaticArrays.jl.

Here is how I would go with your problem.

mutable struct Coords
    x::Float64
    y::Float64
    z::Float64
end

x = 10 # I want the array to be of length 10
# create an uninitialized 1D array of size `x`
arrayOfStructs = Array{Coords, 1}(undef, x) # or `Vector{Coords}(undef, x)`

# initialize the array elements using default constructor
# `Coords(x, y, z)`
for i = 1:x
    arrayOfStructs[i] = Coords(rand(), rand(), rand())
    # or you can use splatting if you already have values
    # arrayOfStructs[i] = Coords(rand(3)...)
end

You might instead create an empty outer constructor for your type that initializes the fields randomly.

# outer constructor that randomly initializes fields
Coords() = Coords(rand(), rand(), rand())

# initialize the array elements using new constructor
for i = 1:x
    arrayOfStructs[i] = Coords()
end

You may also use comprehensions to easily construct your array.

arrayOfStructs = [Coords() for i in 1:x]

If you still want to use an Array for your field, you may define r as a 1D-array and handle the construction of r in your constructors.

You might want to take a look at the relevant sections in the documentation for Arrays and Composite Types:

https://docs.julialang.org/en/v1/manual/arrays/

https://docs.julialang.org/en/v1/manual/types/#Composite-Types-1