# Custom indexing for an AbstractVector breaks display (but not show)
**URL:** https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813
**Category:** General Usage
**Created:** [May 19, 2017, 8:43pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813 "2017-05-19T20:43:04Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@Michael\_Eastwood](https://discourse.julialang.org/u/Michael_Eastwood)
#### Post date: [May 19, 2017, 8:43pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/1 "2017-05-19T20:43:04Z")
Consider the following example:
```julia
julia> struct Foo <: AbstractVector{Int}
bar :: Vector{Int}
end
Base.size(foo::Foo) = size(foo.bar)
Base.getindex(foo::Foo, idx) = foo.bar[idx]
Base.IndexStyle(::Foo) = Base.IndexLinear()
julia> Foo([1,2])
2-element Foo:
1
2
```
Life is wonderful and we have a functional `AbstractVector`. However, if we define the following additional method for `getindex`, things start to break:
```julia
julia> Base.getindex(foo::Foo, idx, jdx) = foo[idx+jdx]
julia> display(Foo([1,2])) # oops
2-element Foo:
2
#undef
julia> show(Foo([1,2])) # still works
[1, 2]
```
I encountered this while trying to define a type that represents a vector of spherical harmonic coefficients. This object is fundamentally a vector, so I made it a subtype of `AbstractVector`. However, it would also be convenient to index this object using the quantum numbers `l` and `m`, hence the second `getindex` function. However, this breaks the display at the REPL.
Is there a way to have my cake and eat it too? Are `AbstractVector`s allowed to define `getindex` with more than one index? Why does `display` break, but `show` still work?
```julia
julia> versioninfo()
Julia Version 0.6.0-pre.beta.unknown
Commit bd84fa1 (2017-03-31 12:58 UTC)
Platform Info:
OS: Linux (x86_64-linux-gnu)
CPU: Intel(R) Core(TM) i5-4210U CPU @ 1.70GHz
WORD_SIZE: 64
BLAS: libopenblas (USE64BITINT DYNAMIC_ARCH NO_AFFINITY Haswell)
LAPACK: libopenblas64_
LIBM: libopenlibm
LLVM: libLLVM-3.9.1 (ORCJIT, haswell)
```
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [May 19, 2017, 9:01pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/2 "2017-05-19T21:01:17Z")
Display treats vectors like matrices, and uses the same code path for both. This means it’ll index into your vector with trailing singleton dimensions.
---
### Author:  [@Michael\_Eastwood](https://discourse.julialang.org/u/Michael_Eastwood)
#### Post date: [May 19, 2017, 9:13pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/3 "2017-05-19T21:13:18Z")
Hm. That feels unintentional. Is there a workaround or do I need to use a different method name for the two element `getindex`?
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [May 19, 2017, 9:25pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/4 "2017-05-19T21:25:35Z")
Not sure if feature or bug. Both? [https://github.com/JuliaLang/julia/issues/14770](https://github.com/JuliaLang/julia/issues/14770)
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [May 19, 2017, 9:36pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/5 "2017-05-19T21:36:30Z")
To expand a bit: yes, it’s expected that N-dimensional arrays can be indexed with fewer or more than N indices. Depending on the index style you define, the fallback methods will convert to 1 or N indices with `sub2ind` or `ind2sub`. And, yes, some algorithms (beyond display) depend on this transformation.
---
### Author:  [@Michael\_Eastwood](https://discourse.julialang.org/u/Michael_Eastwood)
#### Post date: [May 19, 2017, 9:51pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/6 "2017-05-19T21:51:20Z")
Ok, I see. Reading through the #14770 it seems likely this behavior will change by 1.0. But for now I should just pick a different method name. Is that a fair assessment?
---
### Author:  [@mbauman](https://discourse.julialang.org/u/mbauman)
#### Post date: [May 19, 2017, 10:17pm UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/7 "2017-05-19T22:17:22Z")
I’m much less certain that things will change, but I’m also against it (and an optimist).
Either way an alternate name for this behavior is your best path forward right now.
---
### Author:  [@Michael\_Eastwood](https://discourse.julialang.org/u/Michael_Eastwood)
#### Post date: [May 20, 2017, 12:27am UTC](https://discourse.julialang.org/t/custom-indexing-for-an-abstractvector-breaks-display-but-not-show/3813/8 "2017-05-20T00:27:56Z")
Ok, thanks for the insight! (and the awesome `AbstractArray` interface).