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Document how to turn missing values into normal values #378

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Dec 28, 2021
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Document how to turn missing values into normal values #378

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26 changes: 26 additions & 0 deletions docs/src/using.md
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Expand Up @@ -200,6 +200,32 @@ julia> levels!(y, ["Young", "Middle"]; allowmissing=true)

```

Conversely, all missing values can be turned into a "normal" value using `replace`
(or `recode`, whose syntax is identical for this operation):
```jldoctest using
julia> replace(y, missing => "missing value")
4-element CategoricalArray{String,1,UInt32}:
"missing value"
"Young"
"Middle"
"Young"
```
Note that the returned array no longer allows for missing values (which is usually what is expected).
This syntax works for array types other than `CategoricalArray`.

An in-place variant `replace!` (respectively `recode!`) is also provided. Note that `y` still allows
for missing values (since the type of an object cannot be changed).
```jldoctest using
julia> replace!(y, missing => "missing value");

julia> y
4-element CategoricalArray{Union{Missing, String},1,UInt32}:
"missing value"
"Young"
"Middle"
"Young"
```

## Combining levels

Some operations imply combining levels of two categorical arrays: this is the case when concatenating arrays (`vcat`, `hcat` and `cat`) and when assigning a `CategoricalValue` from another categorical array.
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