# DataFrames invert index

**URL:** <https://discourse.julialang.org/t/dataframes-invert-index/84603>\
**Category:** New to Julia\
**Tags:** question, dataframes\
**Created:** [July 21, 2022, 8:22pm UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603 "2022-07-21T20:22:01Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![laut](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laut/32/15651_2.png) [@laut](https://discourse.julialang.org/u/laut)\
**Post date:** [July 21, 2022, 8:22pm UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/1 "2022-07-21T20:22:01Z")

</div>

@bkamins  
How do I get the inverted row index in DataFrames?

```julia
# make data frame
df = DataFrame(x=1:10, y=11:20)

# subset df (train set)
df2 = df[[1,3,9],:]

# invert subset (test set)
df3 = df[Not(df2,:]),:] # error
df3 = df[Not(rownumber(df2)),:] # error

```

---

<div class="post-metadata">

**Author:** ![bkamins](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bkamins/32/208538_2.png) [@bkamins](https://discourse.julialang.org/u/bkamins)\
**Post date:** [July 21, 2022, 10:00pm UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/2 "2022-07-21T22:00:02Z")

</div>

It is simpler than you think 🙂

```julia
julia> using DataFrames

julia> df = DataFrame(x=1:10, y=11:20)
10×2 DataFrame
 Row │ x y
     │ Int64 Int64
─────┼──────────────
   1 │ 1 11
   2 │ 2 12
   3 │ 3 13
   4 │ 4 14
   5 │ 5 15
   6 │ 6 16
   7 │ 7 17
   8 │ 8 18
   9 │ 9 19
  10 │ 10 20

julia> df2 = df[[1,3,9],:]
3×2 DataFrame
 Row │ x y
     │ Int64 Int64
─────┼──────────────
   1 │ 1 11
   2 │ 3 13
   3 │ 9 19

julia> df2 = df[Not([1,3,9]),:]
7×2 DataFrame
 Row │ x y
     │ Int64 Int64
─────┼──────────────
   1 │ 2 12
   2 │ 4 14
   3 │ 5 15
   4 │ 6 16
   5 │ 7 17
   6 │ 8 18
   7 │ 10 20

```

---

<div class="post-metadata">

**Author:** ![laut](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laut/32/15651_2.png) [@laut](https://discourse.julialang.org/u/laut)\
**Post date:** [July 22, 2022, 9:07am UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/3 "2022-07-22T09:07:19Z")

</div>

In my case I want to make df3 from df2 because I’m not saving the [1,3,9] used in df2 as an object (I’m using the ‘sample’ function to randomly select rows) and it is actually a much longer vector.

If there isn’t a way to do this, I can save the vector as an object and reference it in both data frames.

---

<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:** [July 22, 2022, 9:16am UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/4 "2022-07-22T09:16:38Z")

</div>

Yes that’s the simplest way of doing it, e.g. I tend to write something like:

```julia
using StatsBase
train_rows = sample(1:nrow(df), round(Int, nrow(df)*0.8); replace = false)
test_rows = Not(train_rows)

```

and then you can do `df[train_rows, :]` to get the training data and `df[test_rows, :]` to get the testing data.

---

<div class="post-metadata">

**Author:** ![bkamins](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bkamins/32/208538_2.png) [@bkamins](https://discourse.julialang.org/u/bkamins)\
**Post date:** [July 22, 2022, 9:17am UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/5 "2022-07-22T09:17:27Z")

</div>

But how do you want to relate `df2` to `df`? You can e.g. use `antijoin` if you can identify the key on which you can uniquely identify rows:

```julia
julia> antijoin(df, df2, on=names(df))
7×2 DataFrame
 Row │ x y
     │ Int64 Int64
─────┼──────────────
   1 │ 2 12
   2 │ 4 14
   3 │ 5 15
   4 │ 6 16
   5 │ 7 17
   6 │ 8 18
   7 │ 10 20

```

---

<div class="post-metadata">

**Author:** ![laut](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/laut/32/15651_2.png) [@laut](https://discourse.julialang.org/u/laut)\
**Post date:** [July 22, 2022, 9:21am UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/6 "2022-07-22T09:21:45Z")

</div>

With bigger data sets I’m guessing @nilshg , suggestion to just create the vector object will likely run faster then the anti join. Thanks for the suggestions.

---

<div class="post-metadata">

**Author:** ![bkamins](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bkamins/32/208538_2.png) [@bkamins](https://discourse.julialang.org/u/bkamins)\
**Post date:** [July 22, 2022, 9:37am UTC](https://discourse.julialang.org/t/dataframes-invert-index/84603/7 "2022-07-22T09:37:00Z")

</div>

Yes - it will be faster, my suggestion was for the case if you did not or could not save this vector for some reason.