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Fix empty unstack on empty data frame #2842

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Aug 24, 2021
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Fix empty unstack on empty data frame #2842

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a82da24
Fix empty unstack on empty data frame
bkamins Aug 23, 2021
c9338aa
Update NEWS.md
bkamins Aug 23, 2021
b466895
Update reshape.jl
bkamins Aug 23, 2021
ca1dc67
Update src/abstractdataframe/reshape.jl
bkamins Aug 24, 2021
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5 changes: 5 additions & 0 deletions NEWS.md
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Expand Up @@ -19,6 +19,11 @@
integers in a small range.
([#2812](https://github.com/JuliaData/DataFrames.jl/pull/2812))

## Bug fixes

* fix a problem with `unstack` on empty data frame
([#2842](https://github.com/JuliaData/DataFrames.jl/issues/2842))

# DataFrames.jl v1.2.2 Patch Release Notes

## Bug fixes
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5 changes: 3 additions & 2 deletions src/abstractdataframe/reshape.jl
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Expand Up @@ -418,10 +418,11 @@ function _unstack(df::AbstractDataFrame, rowkeys::AbstractVector{Int},
mask_filled[row_id, col_id] = true
end

# note that Symbol.(renamecols.(colref_map)) must produce unique column names
# note that Symbol(renamecols(colref_map)) must produce unique column names
# and names between df1 and df2 must be unique
df1 = df[row_group_row_idxs, g_rowkey.cols]
df2 = DataFrame(unstacked_val, Symbol.(renamecols.(colref_map)), copycols=false)
df2 = DataFrame(unstacked_val, Symbol[Symbol(renamecols(x)) for x in colref_map],
copycols=false)

@assert length(col_group_row_idxs) == ncol(df2)
# avoid reordering when col_group_row_idxs was already ordered
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8 changes: 8 additions & 0 deletions test/reshape.jl
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Expand Up @@ -654,4 +654,12 @@ end
@test IndexStyle(DataFrames.StackedVector) == IndexLinear()
end

@testset "empty unstack" begin
df = DataFrame(a = [], b = [], c = [])
dfu = unstack(df, :b, :c)
@test isempty(dfu)
@test names(dfu) == ["a"]
@test dfu.a isa Vector{Any}
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Any particular reason to check this type?

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For current and future safety I guess. I was surprised that:

julia> Symbol.(Any[])
Any[]

because I knew that:

julia> map(Symbol, Any[])
Symbol[]

so I thought it does not hurt to be extra careful with empty collections and add a test.

end

end # module