# Name of dataframe column created by function

**URL:** <https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058>\
**Category:** New to Julia\
**Tags:** dataframes\
**Created:** [December 1, 2020, 6:30pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058 "2020-12-01T18:30:48Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![jo-fleck](https://avatars.discourse-cdn.com/v4/letter/j/59ef9b/32.png) [@jo-fleck](https://discourse.julialang.org/u/jo-fleck)\
**Post date:** [December 1, 2020, 6:30pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/1 "2020-12-01T18:30:48Z")

</div>

I am computing weighted means of subgroups using the `groupby` and `transform` approach. See below for an illustration.

My understanding is that the new name is `sourcevar1_sourcevar2_function` because the weighted mean function does not return a single value or vector (explained [here](https://dataframes.juliadata.org/v0.21/man/split_apply_combine/)).

I need to get weighted means of several columns and so I am wondering if there is any way to set the column name within the transform command? Or does this have to be done in a separate step?

Thanks for helping with this!

```julia

using DataFrames
df = DataFrame(Region = ["state1", "state1", "state1", "state2", "state2", "state2"], Income = [10, 7, 12, 10, 7, 12], Weight = [51, 20, 86, 75, 125, 16])
gdf = groupby(df, :Region)

df_reg_mean_unweighted = transform(gdf, :Income => mean => :Region_mean_income_unweighted) # Income unweighted

│ Row │ Region │ Income │ Weight │ Region_mean_income_unweighted │
│ │ String │ Int64 │ Int64 │ Float64 │
├─────┼────────┼────────┼────────┼───────────────────────────────┤
│ 1 │ state1 │ 10 │ 51 │ 9.66667 │
│ 2 │ state1 │ 7 │ 20 │ 9.66667 │
│ 3 │ state1 │ 12 │ 86 │ 9.66667 │
│ 4 │ state2 │ 10 │ 75 │ 9.66667 │
│ 5 │ state2 │ 7 │ 125 │ 9.66667 │
│ 6 │ state2 │ 12 │ 16 │ 9.66667 │

df_reg_mean_weighted = transform(gdf, [:Income, :Weight] => (x, y) -> (mean(x, weights(y)))) # Income weighted

│ Row │ Region │ Income │ Weight │ Income_Weight_function │
│ │ String │ Int64 │ Int64 │ Float64 │
├─────┼────────┼────────┼────────┼────────────────────────┤
│ 1 │ state1 │ 10 │ 51 │ 10.7134 │
│ 2 │ state1 │ 7 │ 20 │ 10.7134 │
│ 3 │ state1 │ 12 │ 86 │ 10.7134 │
│ 4 │ state2 │ 10 │ 75 │ 8.41204 │
│ 5 │ state2 │ 7 │ 125 │ 8.41204 │
│ 6 │ state2 │ 12 │ 16 │ 8.41204 │

```

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<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [December 1, 2020, 8:10pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/2 "2020-12-01T20:10:27Z")

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I’m not sure I understand the question - you are showing in your example how you can specify the name of the created column, and you can do that for your weighted mean function as well?

```julia
julia> transform(gdf, [:Income, :Weight] => ((x, y) -> (mean(x, weights(y)))) => :Income_weighted) # Income weighted
6×4 DataFrame
 Row │ Region Income Weight Income_weighted 
     │ String Int64 Int64 Float64         
─────┼─────────────────────────────────────────
   1 │ state1 10 51 10.7134
   2 │ state1 7 20 10.7134
   3 │ state1 12 86 10.7134
   4 │ state2 10 75 8.41204
   5 │ state2 7 125 8.41204
   6 │ state2 12 16 8.41204

```

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<div class="post-metadata">

**Author:** ![jo-fleck](https://avatars.discourse-cdn.com/v4/letter/j/59ef9b/32.png) [@jo-fleck](https://discourse.julialang.org/u/jo-fleck)\
**Post date:** [December 1, 2020, 8:58pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/3 "2020-12-01T20:58:12Z")

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Sorry - I should have been clearer in the problem description.

I tried your suggested approach before asking but it didn’t work for me:

```julia
df_reg_mean_weighted2 = transform(gdf, [:Income, :Weight] => (x, y) -> (mean(x, weights(y))) => :Income_weighted)

6×4 DataFrame
│ Row │ Region │ Income │ Weight │ Income_Weight_function │
│ │ String │ Int64 │ Int64 │ Pair{Float64,Symbol} │
├─────┼────────┼────────┼────────┼───────────────────────────┤
│ 1 │ state1 │ 10 │ 51 │ 10.7134=>:Income_weighted │
│ 2 │ state1 │ 7 │ 20 │ 10.7134=>:Income_weighted │
│ 3 │ state1 │ 12 │ 86 │ 10.7134=>:Income_weighted │
│ 4 │ state2 │ 10 │ 75 │ 8.41204=>:Income_weighted │
│ 5 │ state2 │ 7 │ 125 │ 8.41204=>:Income_weighted │
│ 6 │ state2 │ 12 │ 16 │ 8.41204=>:Income_weighted │

```

However, your suggestion has an extra set of parentheses around the function (which I missed). They do the trick - many thanks!

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<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [December 1, 2020, 9:22pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/4 "2020-12-01T21:22:30Z")

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Ah yes this is an operator precedence issue, you need to enclose the anonymous function in brackets -glad it’s working now!

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<div class="post-metadata">

**Author:** ![jo-fleck](https://avatars.discourse-cdn.com/v4/letter/j/59ef9b/32.png) [@jo-fleck](https://discourse.julialang.org/u/jo-fleck)\
**Post date:** [December 1, 2020, 9:40pm UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/5 "2020-12-01T21:40:13Z")

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Is there a reason why this isn’t mentioned in the documentation? Seems like an easy way to name columns created by anonymous functions.

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<div class="post-metadata">

**Author:** ![nilshg](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/nilshg/32/2283_2.png) [@nilshg](https://discourse.julialang.org/u/nilshg)\
**Post date:** [December 2, 2020, 7:10am UTC](https://discourse.julialang.org/t/name-of-dataframe-column-created-by-function/51058/6 "2020-12-02T07:10:08Z")

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I think it’s just a case of this being the general way anonymous functions and operator precedence work, and not DataFrames specific:

```julia
julia> :a => sum => :b
:a => (sum => :b)

julia> :a => x -> sum(x) => :b
:a => var"#21#22"()

julia> :a => (x -> sum(x)) => :b
:a => (var"#23#24"() => :b)

```

I don’t think anyone would object to a small note in the docs on `transform` and friends that highlights this particular quirk around anonymous functions, it certainly comes up regularly as something that trips up new users.