Re-posted from: https://bkamins.github.io/julialang/2023/06/02/filter.html
Introduction
In DataFrames.jl a data frame can be grouped by columns.
In this way its view is created. The type of this view
is GroupedDataFrame and it allows for performing groupwise
operations on the data stored in the original data frame.
If you use SQL then you can think of GroupedDataFrame
as an object with is obtained by using GROUP BY command
but without performing an aggregation step. This is often
useful as after grouping data you might want to perform
several different aggregation operations on it without
having to group it each time.
In this post I want to discuss how filtering of
GroupedDataFrame works in DataFrames.jl.
In general there are two ways how you might want to filter it:
- by dropping whole groups;
- by dropping rows within group.
I will discuss both today.
The post was written under Julia 1.9.0 and DataFrames.jl 1.5.0.
Preparing the data
Start by creating a sample GroupedDataFrame:
julia> using DataFrames
julia> df = DataFrame(x = repeat(1:4, 2), id=1:8)
8×2 DataFrame
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 2 2
3 │ 3 3
4 │ 4 4
5 │ 1 5
6 │ 2 6
7 │ 3 7
8 │ 4 8
julia> gdf = groupby(df, :x, sort=true);
julia> show(gdf, allgroups=true)
GroupedDataFrame with 4 groups based on key: x
Group 1 (2 rows): x = 1
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 1 5
Group 2 (2 rows): x = 2
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 2 2
2 │ 2 6
Group 3 (2 rows): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
2 │ 3 7
Group 4 (2 rows): x = 4
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 4 4
2 │ 4 8
Dropping whole groups
The first type of operation is when you want to drop whole groups
from your grouped data frame.
First, assume that you want to drop
the first and last group from gdf. You can achieve this using indexing:
julia> gdf[[2, 3]]
GroupedDataFrame with 2 groups based on key: x
First Group (2 rows): x = 2
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 2 2
2 │ 2 6
⋮
Last Group (2 rows): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
2 │ 3 7
julia> gdf[Not(begin, end)]
GroupedDataFrame with 2 groups based on key: x
First Group (2 rows): x = 2
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 2 2
2 │ 2 6
⋮
Last Group (2 rows): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
2 │ 3 7
Now assume that you want to drop groups in which the value of the grouping
variable :x is even. You could still use indexing like this:
julia> gdf[isodd.(getproperty.(keys(gdf), :x))]
GroupedDataFrame with 2 groups based on key: x
First Group (2 rows): x = 1
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 1 5
⋮
Last Group (2 rows): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
2 │ 3 7
However, a more general approach is to use the filter function:
julia> filter(sdf -> isodd(first(sdf.x)), gdf)
GroupedDataFrame with 2 groups based on key: x
First Group (2 rows): x = 1
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 1 5
⋮
Last Group (2 rows): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
2 │ 3 7
If you wanted to drop grouping of the result you can pass ungroup=true in filter:
julia> filter(sdf -> isodd(first(sdf.x)), gdf, ungroup=true)
4×2 DataFrame
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 1 5
3 │ 3 3
4 │ 3 7
Dropping rows within group
Now that you know how to drop whole groups you might wonder how to drop individual rows
within group. Assume that we want to drop from each group all rows except
the row with minimal value of :id within group. You can achieve this using the
subset function:
julia> subset(gdf, :id => x -> x .== minimum(x))
4×2 DataFrame
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
2 │ 2 2
3 │ 3 3
4 │ 4 4
Note that by default subset ungroups data. If you want to keep it grouped
pass ungroup=false:
julia> show(subset(gdf, :id => x -> x .== minimum(x), ungroup=false),
allgroups=true)
GroupedDataFrame with 4 groups based on key: x
Group 1 (1 row): x = 1
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 1 1
Group 2 (1 row): x = 2
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 2 2
Group 3 (1 row): x = 3
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 3 3
Group 4 (1 row): x = 4
Row │ x id
│ Int64 Int64
─────┼──────────────
1 │ 4 4
Conclusions
I hope you found the examples using indexing, filter, and subset
useful and they improved your understanding of row filtering options
available in DataFrames.jl.
In particular remember that filter and subset have a different
default value of the ungoup keyword argument. This difference was
made to reflect the fact that when doing filter typically we do not
want to ungroup data, while when doing subset typically we want
to drop grouping.