# Can I index a DataFrame using a String key?

**URL:** <https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251>\
**Category:** Data\
**Tags:** indexing, dictionary, dataframes, tables, hash\
**Created:** [November 14, 2022, 5:41pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251 "2022-11-14T17:41:13Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![bertulli](https://avatars.discourse-cdn.com/v4/letter/b/ecc23a/32.png) [@bertulli](https://discourse.julialang.org/u/bertulli)\
**Post date:** [November 14, 2022, 5:41pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/1 "2022-11-14T17:41:13Z")

</div>

Hi all!

So I’m writing some code where I need to access some properties of some machine register. I want to store both the value the register it’s initializated to and its binary weight. Using a `DataFrame` and the `count_ones` function, I could produce this:

```julia
8×3 DataFrame
 Row │ Register Value Weight 
     │ Any Any Any    
─────┼─────────────────────────────
   1 │ r0 68 2
   2 │ r1 0 0
   3 │ r2 136 2
   4 │ r3 51 4
   5 │ r4 170 4
   6 │ r5 0 0
   7 │ r7 -1 1
   8 │ r8 134217983 9

```

However, I’d like now to access the data I just populated, hopefully with something like

```julia
weight = df["r2", :Weight]

```

but IIUC, `DataFrames` don’t allow that. Is there a way I can do this? I know `Dict`s could be indexed using a String, but in that case I couldn’t be able to store multiple data (columns) for each register, right? Is there a way I can access a table indexing it using Strings?

Thanks!

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<div class="post-metadata">

**Author:** ![mrufsvold](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrufsvold/32/31600_2.png) [@mrufsvold](https://discourse.julialang.org/u/mrufsvold)\
**Post date:** [November 14, 2022, 6:32pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/2 "2022-11-14T18:32:23Z")

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```julia
#Get a boolean vector that's true where the string matches
r2_indices = df[:, :Register] .== "r2"
#use that to index the dataframe 
df[r2_indices, :Weight]

```

I’m sure there is a cleaner syntax for this with `select()`, but this should get the job done!

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<div class="post-metadata">

**Author:** ![mrufsvold](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrufsvold/32/31600_2.png) [@mrufsvold](https://discourse.julialang.org/u/mrufsvold)\
**Post date:** [November 14, 2022, 6:37pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/3 "2022-11-14T18:37:41Z")

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```julia
r2_df = groupby(df, :Register)["r2"]
select(r2_df, :Weight)

```

Got it!

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<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:** [November 14, 2022, 6:47pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/4 "2022-11-14T18:47:19Z")

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If you want a single lookup then Boolean mask it the standard approach to do it. If you want multiple lookups then use the `groupby` approach. A small comment is that you need to write:

```julia
groupby(df, :Register)[("r2",)]

```

---

<div class="post-metadata">

**Author:** ![mrufsvold](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mrufsvold/32/31600_2.png) [@mrufsvold](https://discourse.julialang.org/u/mrufsvold)\
**Post date:** [November 14, 2022, 7:00pm UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/5 "2022-11-14T19:00:36Z")

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Thank you for the catch! You’ve been cleaning up my messes all morning. 🤣

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<div class="post-metadata">

**Author:** ![bertulli](https://avatars.discourse-cdn.com/v4/letter/b/ecc23a/32.png) [@bertulli](https://discourse.julialang.org/u/bertulli)\
**Post date:** [November 15, 2022, 10:18am UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/6 "2022-11-15T10:18:23Z")

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Thanks 😃! But I don’t understand @bkamins , do you mean that using `groupby` has an initial heavier overhead, and then if I need only a single lookup then the boolean vector approach is faster? Because otherwise I don’t see why would the vector be “simpler”, if that’s what you meant

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<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:** [November 15, 2022, 10:21am UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/7 "2022-11-15T10:21:22Z")

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> [@bertulli](#):
>
> do you mean that using `groupby` has an initial heavier overhead, and then if I need only a single lookup then the boolean vector approach is faster?

This is what I meant:

1. if you use `groupby` more work is done initially, to allow for O(1) queries later (and there can be many).
2. if you use a bitmask the cost is O(n) but it is lower than doing `groupby`.

---

<div class="post-metadata">

**Author:** ![bertulli](https://avatars.discourse-cdn.com/v4/letter/b/ecc23a/32.png) [@bertulli](https://discourse.julialang.org/u/bertulli)\
**Post date:** [November 15, 2022, 10:39am UTC](https://discourse.julialang.org/t/can-i-index-a-dataframe-using-a-string-key/90251/8 "2022-11-15T10:39:59Z")

</div>

Thanks! As I thought then.

As a side note, I found out a `Dict` with a `NamedTuple` also works:

```julia
julia> reg_dict = Dict();

julia> reg_dict["r0"] = (value = 0x44, weight = count_ones(0x44))
(value = 0x44, weight = 2)

julia> reg_dict["r1"] = (value = 0, weight = count_ones(0))
(value = 0, weight = 0)

julia> reg_dict["r2"] = (value = 0x88, weight = count_ones(0x88))
(value = 0x88, weight = 2)

julia> reg_dict["r3"] = (value = 0x33, weight = count_ones(0x33))
(value = 0x33, weight = 4)

julia> reg_dict
Dict{Any, Any} with 4 entries:
  "r1" => (value = 0, weight = 0)
  "r2" => (value = 0x88, weight = 2)
  "r0" => (value = 0x44, weight = 2)
  "r3" => (value = 0x33, weight = 4)

julia> reg_dict["r3"].value
0x33

julia> reg_dict["r3"].weight
4

```

But it gets exported to CSV with the tuple converted to a String, and moreover I suspect the performance is close if not equal to the `groupby`ed `DataFrame`, thus making the latter preferable. Is this correct?