# Reapply groupby on a GroupedDataFrame

**URL:** <https://discourse.julialang.org/t/reapply-groupby-on-a-groupeddataframe/57046>\
**Category:** Data\
**Created:** [March 12, 2021, 9:58pm UTC](https://discourse.julialang.org/t/reapply-groupby-on-a-groupeddataframe/57046 "2021-03-12T21:58:57Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![jeremiedb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeremiedb/32/29150_2.png) [@jeremiedb](https://discourse.julialang.org/u/jeremiedb)\
**Post date:** [March 12, 2021, 9:58pm UTC](https://discourse.julialang.org/t/reapply-groupby-on-a-groupeddataframe/57046/1 "2021-03-12T21:58:57Z")

</div>

Is there a reason for `groupby` not supporting a `GroupedDataFrame` as an input?

In a use case, if I try to apply a transform base on different groups, it will hit an error:

```julia
dfg = groupby(df, "eid")
transform!(dfg, "amount" => ((x) -> cumsum(x)) => "amount_cumsum")
dfg = groupby(dfg, "date")

ERROR: MethodError: no method matching groupby(::GroupedDataFrame{DataFrame}, ::Array{String,1})
Closest candidates are:
  groupby(::AbstractDataFrame, ::Any; sort, skipmissing) at C:\Users\jerem\.julia\packages\DataFrames\oQ5c7\src\groupeddataframe\groupeddataframe.jl:187

```

A solution could be to use the non-mutating `transform` along `ungroup=true`, but on large data, it is a very costly operation (both time and RAM):

```julia
dfg = groupby(df, "eid")
df1 = transform(dfg, "amount" => ((x) -> cumsum(x)) => "amount_cumsum", ungroup=true)
dfg = groupby(df1, "date")

```

A way to circumvent this seems to be to call the second groupby on the `parent` of the `GroupedDataFrame`:

```julia
dfg = groupby(df, "eid")
transform!(dfg, "amount" => ((x) -> cumsum(x)) => "amount_cumsum")
dfg = groupby(dfg.parent, "date")

```

However, I have doubts whether this later approach might be exposed to some undesired side effects. If it’s legit, then wouldn’t it be desirable for `groupby` the handle a `GroupedDataFrame` as in input?

I might have some anchoring with regard to the behavior of R’s `data.table`, where a transformation sequentially be called on different grouping through `dt[, ..., by = "eid"]` and `dt[, ..., by = "date"]`. In `DataFrames.jl`, can it be assumed that to remove the _grouping key_, doing `df = dfg.parent` would be equivalent to _data.table_’s `setkey(dt, NULL)`?

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<div class="post-metadata">

**Author:** ![pdeffebach](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/pdeffebach/32/10320_2.png) [@pdeffebach](https://discourse.julialang.org/u/pdeffebach)\
**Post date:** [March 12, 2021, 10:11pm UTC](https://discourse.julialang.org/t/reapply-groupby-on-a-groupeddataframe/57046/2 "2021-03-12T22:11:37Z")

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Wait I don’t observe that behavior

```julia
julia> x = DataFrame(a =[1, 2], b = [100, 200])
2×2 DataFrame
 Row │ a b     
     │ Int64 Int64 
─────┼──────────────
   1 │ 1 100
   2 │ 2 200

julia> gd = groupby(x, :a)
GroupedDataFrame with 2 groups based on key: a
First Group (1 row): a = 1
 Row │ a b     
     │ Int64 Int64 
─────┼──────────────
   1 │ 1 100
⋮
Last Group (1 row): a = 2
 Row │ a b     
     │ Int64 Int64 
─────┼──────────────
   1 │ 2 200

julia> transform!(gd, :a => first => :c)
2×3 DataFrame
 Row │ a b c     
     │ Int64 Int64 Int64 
─────┼─────────────────────
   1 │ 1 100 1
   2 │ 2 200 2

```

`transform!` returns a `DataFrame`

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<div class="post-metadata">

**Author:** ![jeremiedb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jeremiedb/32/29150_2.png) [@jeremiedb](https://discourse.julialang.org/u/jeremiedb)\
**Post date:** [March 12, 2021, 10:39pm UTC](https://discourse.julialang.org/t/reapply-groupby-on-a-groupeddataframe/57046/3 "2021-03-12T22:39:30Z")

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You’re totally correct!  
I think I stuck to a narrow interpretation of the mutation concept in `transform!` and wrongly assumed it only modified the its input. By assigning a result to the transform operator `df = transform!(dfg...)` it effectively behave as desired.