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Docs of construction undef arrays with missing #31091

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Oct 30, 2020
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Docs of construction undef arrays with missing #31091

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Docs of construction undef arrays with missing
bkamins Feb 16, 2019
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change the description
bkamins Feb 17, 2019
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Update doc/src/manual/missing.md
bkamins Oct 24, 2020
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Update doc/src/manual/missing.md
bkamins Oct 25, 2020
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17 changes: 17 additions & 0 deletions doc/src/manual/missing.md
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Expand Up @@ -254,6 +254,23 @@ julia> Array{Union{Missing, String}}(missing, 2, 3)
missing missing missing
```

For `T` that is bits type (`isbitstype(T)` returns `true`),
`Array{Union{Missing, T}}(undef, dims)` creates an array filled with
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Better to recommend Array{Union{Missing, T}}(undef, dims) (which should work even for non-isbits T)?

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This is the problem. If T is non isbitstype then the array is truly uninitialized (#undef). If I understand the reason correctly this is for performance reasons.

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You mean Array{Union{Missing, T}}(missing, dims) @fredrikekre?

I'm not sure whether it's a good idea to develop this here given that we want people to use Array{Union{Missing, T}}(missing, dims) rather than Array{Union{Missing, T}}(undef, dims), and that it makes things more complex with the introduction of the concept of isbits type. Maybe make this a shorter !!note without showing the REPL output? It would also be useful to mention for non-isbits type the entries will be #undef.

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You mean...

Yea...

missing values. Also calling `similar` to create an unitialized array that has
element type of the form `Union{Missing, T}` creates an array filled with `missing`
if `T` is bits type:
```jldoctest
julia> similar([1, missing])
2-element Array{Union{Missing, Int64},1}:
missing
missing

julia> similar([1, 2], Union{Missing, Float64})
2-element Array{Union{Missing, Float64},1}:
missing
missing
```

An array allowing for `missing` values but which does not contain any such value
can be converted back to an array which does not allow for missing values using
[`convert`](@ref). If the array contains `missing` values, a `MethodError` is thrown
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