# Selecting arbitrary columns from DataFrameRow

**URL:** <https://discourse.julialang.org/t/selecting-arbitrary-columns-from-dataframerow/89950>\
**Category:** Data\
**Tags:** dataframes\
**Created:** [November 8, 2022, 6:18pm UTC](https://discourse.julialang.org/t/selecting-arbitrary-columns-from-dataframerow/89950 "2022-11-08T18:18:27Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![tom-plaa](https://avatars.discourse-cdn.com/v4/letter/t/d26b3c/32.png) [@tom-plaa](https://discourse.julialang.org/u/tom-plaa)\
**Post date:** [November 8, 2022, 6:18pm UTC](https://discourse.julialang.org/t/selecting-arbitrary-columns-from-dataframerow/89950/1 "2022-11-08T18:18:28Z")

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**EDIT: Disregard this, I don’t know why I wasn’t getting the results I expected before…**

I want to replicate finding duplicate composite primary keys in an sql database to some arbitrary dataframe.

Let’s set up some dummy dataframes (borrowed from some other topic here):

```julia

function grades_2020()
    name = ["Sally", "Bob", "Alice", "Hank"]
    grade_2020 = [1, 5, 8.5, 4]
    extra_col = [0, 0, 0, 0]
    DataFrame(; name, grade_2020, extra_col)
end

function grades_2020_alt()
    name = ["Sally", "Bob", "Alice", "Hank", "Extra1", "Extra2"]
    grade_2020 = [1, 5, 8.5, 4, 1000, 2000]
    extra_col = [0, 0, 0, 0, 0, 0]
    DataFrame(; name, grade_2020, extra_col)
end

test_df = grades_2020()
test_df_alt = grades_2020_alt()

```

So let’s imagine our composite primary key would be `["name", "grade_2020"]`. I want to filter out duplicates and get the full rows (including the extra column), but I do not want to explicitly write the column names in the filtering algorithm, I want a function that receives a vector of primary key names, such as `pkey_cols = ["name", "grades_2020"]` for this particular instance.

My attempt only allows me to retrieve the columns that make part of the primary key because I cannot select the relevant columns from a `DataFrameRow` object like `row["name", "grade_2020"]` or `row[["name", "grade_2020"]].`  
**EDIT:** `row[["name", "grade_2020"]]` **actually DOES work.**

```julia
function test_filter(dataindb, datatoinsert, pkey_cols)
    pkeys_indb = dataindb[!, pkey_cols]
    pkeys_indata = datatoinsert[!, pkey_cols]    
    return filter(x -> !in(x, eachrow(pkeys_indb)), eachrow(pkeys_indata)) |> DataFrame
end

```

Which results in:

```julia
julia> test_filter(test_df, test_df_alt, ["name", "grade_2020"])
2×2 DataFrame
 Row │ name grade_2020 
     │ String Float64    
─────┼────────────────────
   1 │ Extra1 1000.0
   2 │ Extra2 2000.0

```

I wish I could instead do something like:  
**(EDIT: this DOES actually work)**

```julia
function test_filter(dataindb, datatoinsert, pkey_cols)
    pkeys_indb = dataindb[!, pkey_cols]
    return filter(x -> !in(x[pkeys_cols], eachrow(pkeys_indb)), eachrow(datatoinsert)) |> DataFrame
end

```

**EDIT: and the output result (as intended) below**

```julia
julia> test_filter(test_df, test_df_alt, ["name", "grade_2020"])
2×3 DataFrame
 Row │ name grade_2020 extra_col 
     │ String Float64 Int64     
─────┼───────────────────────────────
   1 │ Extra1 1000.0 0
   2 │ Extra2 2000.0 0

```

I feel like this should have been something easy, but I’m surprisingly stuck.

---

<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:** [November 8, 2022, 7:01pm UTC](https://discourse.julialang.org/t/selecting-arbitrary-columns-from-dataframerow/89950/2 "2022-11-08T19:01:23Z")

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Wait so you have solved your problem? You are correct that you can use `pkeys` to select parts of a `DataFrameRow` programatically.

---

<div class="post-metadata">

**Author:** ![tom-plaa](https://avatars.discourse-cdn.com/v4/letter/t/d26b3c/32.png) [@tom-plaa](https://discourse.julialang.org/u/tom-plaa)\
**Post date:** [November 8, 2022, 7:02pm UTC](https://discourse.julialang.org/t/selecting-arbitrary-columns-from-dataframerow/89950/3 "2022-11-08T19:02:51Z")

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Apparently, I must have mistyped something when I was trying to index DataFrameRow on a vector of strings. This was embarassing… 🤣