# Mask on a 1D array

**URL:** <https://discourse.julialang.org/t/mask-on-a-1d-array/42740>\
**Category:** Performance\
**Created:** [July 8, 2020, 3:41pm UTC](https://discourse.julialang.org/t/mask-on-a-1d-array/42740 "2020-07-08T15:41:16Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![rdeits](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rdeits/32/286_2.png) [@rdeits](https://discourse.julialang.org/u/rdeits)\
**Post date:** [July 8, 2020, 3:55pm UTC](https://discourse.julialang.org/t/mask-on-a-1d-array/42740/2 "2020-07-08T15:55:23Z")

</div>

I’m not aware of any existing type that would do this, but you could create your own subtype of `AbstractArray` which holds onto the original array and the index you want to mask out:

```julia
julia> struct MaskedElementArray{T, N, A <: AbstractArray{T, N}} <: AbstractArray{T, N}
         parent::A
         masked_index::Int
       end

julia> function MaskedElementArray(parent::A, masked_index::Integer) where {T, N, A <: AbstractArray{T, N}}
         MaskedElementArray{T, N, A}(parent, masked_index)
       end
MaskedElementArray

julia> Base.size(a::MaskedElementArray) = size(a.parent)

julia> Base.eltype(a::MaskedElementArray) = eltype(a.parent)

julia> function Base.getindex(a::MaskedElementArray, i::Integer)
         if i == a.masked_index
           zero(eltype(a))
         else
           a.parent[i]
         end
       end

julia> x = [1, 2, 3, 4, 5];

julia> mask = 3
3

julia> M = MaskedElementArray(x, mask)
5-element MaskedElementArray{Int64,1,Array{Int64,1}}:
 1
 2
 0
 4
 5

```

The `MaskedElementArray` does not copy the contents of its parent array, so it should be as cheap as creating a `view`.

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