# Multi-dimensional type declaration within a mutable structure
**URL:** https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500
**Category:** New to Julia
**Created:** [June 7, 2018, 11:44am UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500 "2018-06-07T11:44:15Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@drarnau](https://discourse.julialang.org/u/drarnau)
#### Post date: [June 7, 2018, 11:44am UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500/1 "2018-06-07T11:44:15Z")
I’m new to Julia and I face the following issue. I try to declare a mutable structure like this:
“”"
mutable struct mystructname
X::Array{Float64}(4, 4)
end
“”"
But the compiler does not like it:
> TypeError: mystructname: in type definition, expected Type, got Array{Float64,2}
> include\_string(::String, ::String) at loading.jl:522
> include\_string(::String, ::String, ::Int64) at eval.jl:30
> include\_string(::Module, ::String, ::String, ::Int64, ::Vararg{Int64,N} where N) at eval.jl:34
> (::Atom.##102#107{String,Int64,String})() at eval.jl:82
> withpath(::Atom.##102#107{String,Int64,String}, ::String) at utils.jl:30
> withpath(::Function, ::String) at eval.jl:38
> hideprompt(::Atom.##101#106{String,Int64,String}) at repl.jl:67
> macro expansion at eval.jl:80 [inlined]
> (::Atom.##100#105{Dict{String,Any}})() at task.jl:80
Any ideas?
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [June 7, 2018, 11:47am UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500/2 "2018-06-07T11:47:02Z")
This should work:
```julia
mutable struct MyStructName
X::Array{Float64,2}
end
```
The difference is that `Array{Float64,2}` is a type whereas `Array{Float64}(4,4)` is an instance of that type. (Also, please quote your code with back-ticks `, triple for multiline code.)
Edit: also note that types are usually in CamelCase.
---
### Author:  [@drarnau](https://discourse.julialang.org/u/drarnau)
#### Post date: [June 7, 2018, 12:57pm UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500/3 "2018-06-07T12:57:09Z")
Thanks a lot!
I’ve tried that, but if I understand correctly `X::Array{Float64,2}` is for any 2-dimensional array, so I need to specify somewhere else in the structure that I only allow (5,5) elements, something like:
```julia
mutable struct mystructname
X::Array{Float64,2}
i,j = size(X)
if i != 5 || j != 5
error
end
end
```
Does that make sense?
---
### Author:  [@mauro3](https://discourse.julialang.org/u/mauro3)
#### Post date: [June 7, 2018, 1:07pm UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500/4 "2018-06-07T13:07:05Z")
Have a read through the constructors-section in the docs. In particular this section and its example: [https://docs.julialang.org/en/stable/manual/constructors/#Inner-Constructor-Methods-1](https://docs.julialang.org/en/stable/manual/constructors/#Inner-Constructor-Methods-1)
---
### Author:  [@Bernard\_GODARD](https://discourse.julialang.org/u/Bernard_GODARD)
#### Post date: [June 7, 2018, 1:10pm UTC](https://discourse.julialang.org/t/multi-dimensional-type-declaration-within-a-mutable-structure/11500/5 "2018-06-07T13:10:01Z")
If you know the size of your array you should use
> **[GitHub - JuliaArrays/StaticArrays.jl: Statically sized arrays for Julia](https://github.com/JuliaArrays/StaticArrays.jl)**
>
> Statically sized arrays for Julia. Contribute to JuliaArrays/StaticArrays.jl development by creating an account on GitHub.
If you still want to use array, you need to add your check in the constructor:
```julia
mutable struct mystructname
X::Array{Float64,2}
mystructname(x) = (size(x)!=(5,5)) ? error("invalid size") : new(x)
end
```