# How to add new column to Dataframe

**URL:** <https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794>\
**Category:** Data\
**Tags:** question, dataframes\
**Created:** [April 7, 2021, 10:27pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794 "2021-04-07T22:27:41Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![Shazman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shazman/32/23093_2.png) [@Shazman](https://discourse.julialang.org/u/Shazman)\
**Post date:** [April 7, 2021, 10:27pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/1 "2021-04-07T22:27:41Z")

</div>

Hi  
I want to add dataframe A as a first column of dataframe of B.  
so the first column of new dataframe will be `IID` from A  
how can I do that ? thank you in advance!

dataframe A

```julia
  Row │ IID       
      │ Int64     
──────┼───────────
    1 │ 408610274
    2 │ 409588054
    3 │ 409894206
    4 │ 410902281
    5 │ 411257293

```

dataframe B

```julia
 Row │ x1 x2 x3 x4 x5      
     │ Float64 Float64 Float64 Float64 Float64 
─────┼─────────────────────────────────────────────
   1 │ 1.0 1.0 2.0 1.0 0.0
   2 │ 2.0 2.0 2.0 2.0 1.0
   3 │ 1.0 2.0 1.0 1.0 0.0
   4 │ 2.0 0.0 2.0 2.0 1.0
   5 │ 2.0 2.0 1.0 2.0 1.0

```

---

<div class="post-metadata">

**Author:** ![TheCedarPrince](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/thecedarprince/32/17323_2.png) [@TheCedarPrince](https://discourse.julialang.org/u/TheCedarPrince)\
**Post date:** [April 7, 2021, 11:02pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/2 "2021-04-07T23:02:11Z")

</div>

You could do `hcat(A, B)` if I am understanding you correctly.

```julia
using DataFrames
A = DataFrame([1:5], [:IID])
B = rand(5,5) .* 10 |> DataFrame

hcat(A, B)

5×6 DataFrame
 Row │ IID x1 x2 x3 x4 x5
     │ Int64 Float64 Float64 Float64 Float64 Float64
─────┼───────────────────────────────────────────────────────
   1 │ 1 9.28669 8.78076 1.19252 5.05751 1.59102
   2 │ 2 3.0458 6.38638 2.4602 7.59882 2.78558
   3 │ 3 4.7442 4.91209 9.38128 5.55845 0.0363324
   4 │ 4 4.13458 4.19869 5.46127 2.87679 2.81345
   5 │ 5 9.74654 8.61283 1.99002 0.444639 6.98284

```

---

<div class="post-metadata">

**Author:** ![Shazman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shazman/32/23093_2.png) [@Shazman](https://discourse.julialang.org/u/Shazman)\
**Post date:** [April 7, 2021, 11:32pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/3 "2021-04-07T23:32:34Z")

</div>

thank you,

I have another problem,  
my dataframe look like this,  
the columns names `x1`,`x2`…`x8` are gene’s names.  
and `0`,`1`,`2`, stands for genotypes.  
and the column `IID` is for individuals names and they grouped in number `10004`,  
now I want to have avarage genotype for group `10004`,  
how can I do that?

```julia
 Row │ Herd.when.genotyped IID x1 x2 x3 x4 x5 x6 x7 x8       
     │ Int64 Int64 Float64? Float64? Float64? Float64? Float64? Float64? Float64? Float64? 
─────┼────────────────────────────────────────────────────────────────────────────────────────────────────────────────
   1 │ 10004 410240170 2.0 1.0 2.0 2.0 1.0 2.0 1.0 1.0
   2 │ 10004 412396339 2.0 0.0 0.0 2.0 2.0 1.0 1.0 0.0
   3 │ 10004 412442014 2.0 2.0 1.0 2.0 1.0 1.0 2.0 0.0
   4 │ 10004 412409256 2.0 1.0 0.0 2.0 2.0 0.0 2.0 0.0
   5 │ 10004 412419664 2.0 2.0 0.0 2.0 1.0 1.0 2.0 1.0
   6 │ 10004 412442177 1.0 2.0 2.0 1.0 1.0 2.0 2.0 2.0
   7 │ 10004 412556442 2.0 1.0 2.0 2.0 2.0 2.0 2.0 1.0
   8 │ 10004 412502732 2.0 1.0 0.0 2.0 2.0 1.0 2.0 0.0
   9 │ 10004 412490450 2.0 1.0 1.0 2.0 2.0 2.0 2.0 2.0
  10 │ 10004 412788155 2.0 1.0 2.0 2.0 1.0 2.0 2.0 1.0

```

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<div class="post-metadata">

**Author:** ![TheCedarPrince](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/thecedarprince/32/17323_2.png) [@TheCedarPrince](https://discourse.julialang.org/u/TheCedarPrince)\
**Post date:** [April 7, 2021, 11:40pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/4 "2021-04-07T23:40:28Z")

</div>

Hi @Shazman no problem. As your new line of questioning is not related to your topic, I suggest you open up a new Discourse question with additional questions you have.

I will answer your latest question here however. I am not sure I understand you entirely so you may need to rephrase your question but I would do this (using the dataframes I posted from my last comment):

```julia
using Statistics
C = hcat(A, B)
C.average = [mean(row[2:6]) for row in eachrow(C)]

 Row │ IID x1 x2 x3 x4 x5 average
     │ Int64 Float64 Float64 Float64 Float64 Float64 Float64
─────┼────────────────────────────────────────────────────────────────
   1 │ 1 9.28669 8.78076 1.19252 5.05751 1.59102 5.1817
   2 │ 2 3.0458 6.38638 2.4602 7.59882 2.78558 4.45535
   3 │ 3 4.7442 4.91209 9.38128 5.55845 0.0363324 4.92647
   4 │ 4 4.13458 4.19869 5.46127 2.87679 2.81345 3.89696
   5 │ 5 9.74654 8.61283 1.99002 0.444639 6.98284 5.55537

```

---

<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:** [April 8, 2021, 12:12am UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/5 "2021-04-08T00:12:46Z")

</div>

You can insert a new column into the _first_ position with `insertcols`.

```julia
insertcols!(B, 1, :a => A.IID)

```

You can also use a `leftjoin` (or other `join` types) if the orders don’t match exactly.

For the `collapse`-ing operation

```julia
@chain df begin 
    groupby("Herd.when.genotyped")
    combine(:x1 => mean)
end

```

To get the mean of all the columns, do

```julia
@chain df begin 
    groupby("Herd.when.genotyped")
    combine(names(df, Between(:x1, :x5)) .=> mean)
end

```

---

<div class="post-metadata">

**Author:** ![Shazman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/shazman/32/23093_2.png) [@Shazman](https://discourse.julialang.org/u/Shazman)\
**Post date:** [April 8, 2021, 12:31am UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/6 "2021-04-08T00:31:15Z")

</div>

thank you !  
it works perfectly. 🤩

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<div class="post-metadata">

**Author:** ![jonjilla](https://avatars.discourse-cdn.com/v4/letter/j/7ba0ec/32.png) [@jonjilla](https://discourse.julialang.org/u/jonjilla)\
**Post date:** [April 8, 2021, 3:59am UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/7 "2021-04-08T03:59:31Z")

</div>

If we aren’t concerned with the position, that the column is inserted, could we simply do this?

```julia
B.a = A.IID

```

Actually, I’m not clear on the difference between this and

```julia
B[:,:a] = A.IID

```

Are they effectively the same?

---

<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:** [April 8, 2021, 2:12pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/8 "2021-04-08T14:12:48Z")

</div>

They are effectively the same. The columns `:a` and `:IID` will be the exact same in memory.

The first one does _not_ copy anywhere. The second one creates a copy so the the two columns are stored in different places.

---

<div class="post-metadata">

**Author:** ![jonjilla](https://avatars.discourse-cdn.com/v4/letter/j/7ba0ec/32.png) [@jonjilla](https://discourse.julialang.org/u/jonjilla)\
**Post date:** [April 8, 2021, 11:31pm UTC](https://discourse.julialang.org/t/how-to-add-new-column-to-dataframe/58794/9 "2021-04-08T23:31:03Z")

</div>

Thanks for the info. What is the implication that one makes a copy and the other doesn’t?

I’ve noticed that in some cases, I cannot use the first form, and only the second form works, but I haven’t quite figure out when that is, so I’ve started to use the second form only, which seems to work, but I figure there must be a reason why those two forms exist.

When you should use one vs the other?