# Best method for creating many columns in dataframe for dummy variables

**URL:** <https://discourse.julialang.org/t/best-method-for-creating-many-columns-in-dataframe-for-dummy-variables/129940>\
**Category:** New to Julia\
**Tags:** dataframes\
**Created:** [June 16, 2025, 7:11pm UTC](https://discourse.julialang.org/t/best-method-for-creating-many-columns-in-dataframe-for-dummy-variables/129940 "2025-06-16T19:11:27Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Snowy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/snowy/32/36765_2.png) [@Snowy](https://discourse.julialang.org/u/Snowy)\
**Post date:** [June 16, 2025, 7:11pm UTC](https://discourse.julialang.org/t/best-method-for-creating-many-columns-in-dataframe-for-dummy-variables/129940/1 "2025-06-16T19:11:27Z")

</div>

Hi,

I’m working on a problem that requires creating a lot of dummy variables (30+) based on dates and I’m wondering if my approach is optimal.

Currently I am creating each variable as a set with the dates that correspond to the variable:

```julia
variable_1 = Set([

Date("2022-05-01"),

Date("2023-05-03"),

Date("2024-05-02")])

```

Then find the Boolean whether the date in a specific row is in the set:

```julia
df[:,:variable_1]= in.(df[!,:DATE],Ref(variable_1))

```

However, this approach seems fine for a few variables, but doesn’t really seem scalable. Also, it doesn’t seem like a great approach for code organization, but that’s a separate issue.

I thought about creating a set of sets or vector of sets and looping over to create new columns with the Boolean values, but this doesn’t seem to work as intended:

```julia
Variables = [variable_1,variable_2]

For x in Variables

Df[:,x] = in.(df[!,:DATE],Ref(Variables)

End

```

Also, the above approach (if it worked) I don’t think would give any type of appropriate name to the column. I.e. I can’t seem to extract the names in my vector, only the values (e.g. typing Variables[1] only gives me the values in variable\_1, but does not return the name “variable\_1”).

Any help appreciated.

_edit_  
I figured out the looping bit, this works:

```julia
i = 0
for x in Variables
    col_location = ncol(df)+1
    insertcols!(df,col_location,:auto => in.(df[!,:DATE],Ref(Variables[i+1])),makeunique=true)
    i = i+1
end

```

And would save a lot of time. However, still wondering , what is the best way to get appropriate column names here?

Thanks

---

<div class="post-metadata">

**Author:** ![bertschi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bertschi/32/33462_2.png) [@bertschi](https://discourse.julialang.org/u/bertschi)\
**Post date:** [June 16, 2025, 10:17pm UTC](https://discourse.julialang.org/t/best-method-for-creating-many-columns-in-dataframe-for-dummy-variables/129940/2 "2025-06-16T22:17:02Z")

</div>

In contrast to R you cannot get the name of variables that got handed to a function (well at least if you don’t want to write a macro).  
Instead, it’s better to provide names explicitly and use some data structure for that, e.g., a dictionary:

```julia
newcols = Dict(:variable_1 => Set([...]), :variable_2 => Set([...]))
for (col, vals) in pairs(newcols)
    df[:, col] = in.(df[!, :DATE], Ref(vals))
end

```