# A gotcha with OffsetArrays

**URL:** <https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858>\
**Category:** General Usage\
**Tags:** offsetarrays\
**Created:** [July 18, 2025, 7:18pm UTC](https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858 "2025-07-18T19:18:45Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Joris\_Pinkse](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joris_pinkse/32/216398_2.png) [@Joris\_Pinkse](https://discourse.julialang.org/u/Joris_Pinkse)\
**Post date:** [July 18, 2025, 7:18pm UTC](https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858/1 "2025-07-18T19:18:45Z")

</div>

With

```julia
using OffsetArrays

import Base: length, size, lastindex, firstindex, eachindex, getindex, setindex!, show, display

struct Polly{T} <: AbstractVector{ T }
    d :: Int
    a :: OffsetArray{ T, 1 }

    function Polly( d :: Int, 𝒯::Type{T} = BigInt ) where T
        d ≥ 0 || error( "Need d ≥ 0, but d = $d" )
        return new{T}( d, OffsetArray( vcat( ones( T, 1 ), zeros( T, d ) ), 0:d ) )
    end
end

length( p :: Polly ) = length( p.a )
size( p :: Polly ) = size( p.a )
size( p :: Polly, j ) = size( p.a, j )
lastindex( p :: Polly ) = lastindex( p.a )
firstindex( p :: Polly ) = firstindex( p.a )
eachindex( p :: Polly ) = firstindex( p ) : lastindex( p )
getindex( p :: Polly, j :: Int ) = getindex( p.a, j )

function setindex!( p :: Polly{T}, x :: T, j :: Int ) where T
    j ∈ eachindex( p ) && return setindex!( p.a, x, j )
    j < 0 && error( "Cannot deal with negative indices" )
    append!( p.a, zeros( T, j - length( p)))
    setindex!( p.a, x, j )
    p.d = j + 1
    return x
end

x = Polly( 4, Rational{BigInt} )

```

I get

```julia
  0
   0
   0
   0
 #undef

```

whereas I had expected the first element to be one given that `eachindex` is defined. I can solve the problem by overloading `show` and `display` but is there a cleaner solution?

Thanks!

---

<div class="post-metadata">

**Author:** ![Joris\_Pinkse](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joris_pinkse/32/216398_2.png) [@Joris\_Pinkse](https://discourse.julialang.org/u/Joris_Pinkse)\
**Post date:** [July 18, 2025, 7:26pm UTC](https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858/2 "2025-07-18T19:26:13Z")

</div>

It appears that OffsetArrays uses `Base.replace_in_print_matrix` to address the same problem; this is a method that I’m not familiar with.

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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:** [July 18, 2025, 7:31pm UTC](https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858/3 "2025-07-18T19:31:55Z")

</div>

No, I think you’re just missing a `Base.axes(p::Polly) = axes(p.a)`. And then you don’t need to define firstindex/lastindex/eachindex.

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<div class="post-metadata">

**Author:** ![Joris\_Pinkse](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joris_pinkse/32/216398_2.png) [@Joris\_Pinkse](https://discourse.julialang.org/u/Joris_Pinkse)\
**Post date:** [July 18, 2025, 7:45pm UTC](https://discourse.julialang.org/t/a-gotcha-with-offsetarrays/130858/4 "2025-07-18T19:45:37Z")

</div>

Wow: thanks!