# Buggy array of array

**URL:** <https://discourse.julialang.org/t/buggy-array-of-array/9867>\
**Category:** General Usage\
**Tags:** array\
**Created:** [March 21, 2018, 8:25pm UTC](https://discourse.julialang.org/t/buggy-array-of-array/9867 "2018-03-21T20:25:25Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![leoll2](https://avatars.discourse-cdn.com/v4/letter/l/22d042/32.png) [@leoll2](https://discourse.julialang.org/u/leoll2)\
**Post date:** [March 21, 2018, 8:25pm UTC](https://discourse.julialang.org/t/buggy-array-of-array/9867/1 "2018-03-21T20:25:25Z")

</div>

I’ve spent many hours trying to figure out why this code misbehaves. First, let’s explain this code (which is an excerpt of my real code, just enough to reproduce the issue).  
I’ve a custom type named MyScalar, which is basically an array with shifted indexes, and a type MyVector, meant to be a standard (i.e. no index shifting) array of MyScalar elements.  
However, when I try to run it (code below), the output becomes buggy and keeps printing “This is my type” after any other command I execute. Actually, twice per command (because 2 is the length of the MyScalar).  
I truly can’t understand what’s happening here!

> module MyModule
> 
> using Compat, OffsetArrays
> 
> export MyScalar, MyVector
> 
> struct MyScalar{T} \<: AbstractArray{T,1}  
> data::OffsetArray{T}  
> dim::Int  
> end
> 
> function my\_show(io::IO, MS::MyScalar)  
> println(“This is my type!\n”)  
> end
> 
> MyScalar{T}(::Type{T}, n::Int, off::Int = 0) = MyScalar(OffsetArray{T}(undef, off:(off+n-1)), n)  
> MyScalar(n::Int) = MyScalar(Float64, n)  
> MyScalar{T}(::Type{T}, oa::OffsetArray{T}) = MyScalar(oa, 1 + indices(oa)[1].stop - indices(oa)[1].start)  
> Base.size(MS::MyScalar) = (MS.dim,)  
> Base.IndexStyle(::Type{\<:MyScalar}) = IndexLinear()  
> Base.similar(MS::MyScalar, ::Type{T}, d::Dims) where {T} = MyScalar(T, d[1])  
> Base.getindex(MS::MyScalar, i::Int) = MS.data[i]  
> Base.setindex!(MS::MyScalar, v, i::Int) = (MS.data[i] = v)  
> Base.show(io::IO, MS::MyScalar) = my\_show(io, MS)  
> Base.indices(MS::MyScalar) = indices(MS.data)
> 
> struct MyVector \<: AbstractArray{MyScalar{Int32}, 1}  
> data::Array{MyScalar{Int32},1}  
> dim::Int  
> end
> 
> MyVector() = MyVector(Array{MyScalar{Int32},1}(3),3) #arbitrary fixed length  
> Base.size(MV::MyVector) = (MV.dim,)  
> Base.getindex(MV::MyVector, i::Int) = MV.data[i]  
> Base.setindex!(MV::MyVector, v, i::Int) = (MV.data[i] = v)  
> end

Example code:

> using MyModule  
> myv = MyVector()  
> myv[2] = MyScalar(Int32, 2)

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<div class="post-metadata">

**Author:** ![kristoffer.carlsson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/kristoffer.carlsson/32/22_2.png) [@kristoffer.carlsson](https://discourse.julialang.org/u/kristoffer.carlsson)\
**Post date:** [March 21, 2018, 8:38pm UTC](https://discourse.julialang.org/t/buggy-array-of-array/9867/2 "2018-03-21T20:38:03Z")

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```julia
function my_show(io::IO, MS::MyScalar)
    println(io, “This is my type!\n”) # note io
end

```

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<div class="post-metadata">

**Author:** ![leoll2](https://avatars.discourse-cdn.com/v4/letter/l/22d042/32.png) [@leoll2](https://discourse.julialang.org/u/leoll2)\
**Post date:** [March 21, 2018, 8:48pm UTC](https://discourse.julialang.org/t/buggy-array-of-array/9867/3 "2018-03-21T20:48:09Z")

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Wow! Can’t believe how much time I wasted looking elsewhere, when the issue was that simple!  
Now I’m just curious to understand what happened when I didn’t specify the “io”. The documentation of print explicitly says:

> Write to io (or to the default output stream STDOUT if io is not given)

So, why that odd behaviour?

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 21, 2018, 9:23pm UTC](https://discourse.julialang.org/t/buggy-array-of-array/9867/4 "2018-03-21T21:23:41Z")

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It’s easy (and common!) to swap out the default stream for something else like an `IOBuffer` to create a string. Functions like `sprint(show, MyScalar(...))` will do it for you! Of course, they’ll be buggy since they won’t receive any data written to their IO object, and instead you’ll get spurious output at stdout.