Pau

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# Elixir Today: Combining a List of Map via Specific Field

references

### Intro

Recently, I've been working on a similar question about `How to combine a list of map via specific field`.

I looked at the elixir forum and gladly I found something. I saw that one of the answers there is using `Enum.group_by` + `Enum.map`.

Given my curiosity, `How can I come up with a solution using Enum.reduce`?

### Curiosity Kicks In

So I started working out using `Enum.reduce`.

``````
@list [
%{id: 1, list: [1]},
%{id: 1, list: [2]},
%{id: 2, list: [1]},
%{id: 3, list: [1]},
%{id: 1, list: [3, 4, 5]},
%{id: 2, list: [2, 3]},
%{id: 3, list: [2, 3, 4, 5]},
%{id: 4, list: [2, 3, 4, 5]},
%{id: 4, list: [1]}
]

@list
|> Enum.reverse()
|> Enum.reduce(%{}, fn %{id: id, list: _list} = f, acc ->
case acc do
%{^id => existing} -> Map.put(acc, id, [f | existing])
%{} -> Map.put(acc, id, [f])
end
end)
|> Enum.map(fn {id, list} -> %{id: id, list: Enum.flat_map(list, & &1.list)} end)

``````
1. In this way, I created a sample list with specific id and their list.
2. I reverse the order of the list
3. I used Enum.reduce to sort out and combine the list of the ids accordingly.
4. Finally, used `Enum.map` to transform the data.

### Benchmarking!

I started to compare the result between using `group_by` and `reduce`. In order to that, I used Benchee.

``````{:benchee, "~> 1.0", only: :dev}
``````

Given the setup

`````` @list [
%{id: 1, list: [1]},
%{id: 1, list: [2]},
%{id: 2, list: [1]},
%{id: 3, list: [1]},
%{id: 1, list: [3, 4, 5]},
%{id: 2, list: [2, 3]},
%{id: 3, list: [2, 3, 4, 5]},
%{id: 4, list: [2, 3, 4, 5]},
%{id: 4, list: [1]}
]

def groupby(list) do
Enum.group_by(list, & &1.id)
|> Enum.map(fn {id, list} -> %{id: id, list: Enum.flat_map(list, & &1.list)} end)
end

def reduce(list) do
list
|> Enum.reverse()
|> Enum.reduce(%{}, fn %{id: id, list: _list} = f, acc ->
case acc do
%{^id => existing} -> Map.put(acc, id, [f | existing])
%{} -> Map.put(acc, id, [f])
end
end)
|> Enum.map(fn {id, list} -> %{id: id, list: Enum.flat_map(list, & &1.list)} end)
end

``````

Then run

``````Benchee.run(
%{
"groupby_enum_map" => fn -> PracticeElixir.groupby(@list) end,
"reduce_enum_map" => fn -> PracticeElixir.reduce(@list) end
},
print: [fast_warning: false]
)

``````

### Benchmark Result

Given my dev environment, here is the result.

``````available_memory: "16 GB",
cpu_speed: "Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz",
elixir: "1.12.3",
erlang: "24.0.6",
num_cores: 16,
os: :macOS
``````
``````Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
reduction time: 0 ns
parallel: 1
inputs: none specified
Estimated total run time: 14 s

Benchmarking groupby_enum_map ...
Benchmarking reduce_enum_map ...

Name                       ips        average  deviation         median         99th %
reduce_enum_map         1.08 M        0.93 μs   ±208.21%        0.80 μs           5 μs
groupby_enum_map        0.97 M        1.03 μs   ±251.03%        0.90 μs           4 μs

Comparison:
reduce_enum_map         1.08 M
groupby_enum_map        0.97 M - 1.12x slower +0.109 μs

``````

You can view the full implementation here!

Happy Coding!

Alim Nastaev • Edited

You can do it even cleaner:

``````    @list
|> Enum.reverse()
|> Enum.reduce(%{}, fn %{id: id, list: list}, acc ->
case acc do
%{^id => existing} ->
Map.put(acc, id, list ++ existing)

# underscore here is much more explicit than just %{}
_ ->
Map.put(acc, id, list)
end
end)
|> Enum.map(fn {id, list} -> %{id: id, list: list} end)
``````

Usually, people religiously complain about `++` performance, but it's not always true:
The Seven Myths of Erlang Performance - 2.2 Myth: Operator "++" is Always Bad
erlang.org/doc/efficiency_guide/my...

P.S. Also, what's the purpose of `Enum.reverse/1` before the reduce function?

Pau

Thank you for this one will take a look at it. Reverse preserved the original ordering of the list.

Alim Nastaev

Hmmmm...?!
``````[