Enum.reduce(list1,list2,&[&1 | &2]) vs recursively merging a list

Questions & Help Questions
1

hey all, I am comparing 2 cases for merging 2 lists, the best way would be probably to use list1++list2, but lets forget about that for this question. So the 2 cases are-

# for merging 2 lists -> list1 & list2

#case1 use Enum.reduce
Enum.reduce(list1,list2,&[&1 | &2])

#case2 -> merge(list1,list2)
def merge([],list2) do
  list2
end

def merge(list1, list2) do
  [head | remaining_list1] = list1
  list2 = [head | list2]
  merge(remaining_list1, list2)
end

is there any difference between these 2 methods?
benchee claims that for 200 size lists and in case of merging 4 such lists these are the stats-

Benchmarking Enum.reduce ...
Benchmarking merge ...

Name                  ips        average  deviation         median         99th %
merge             30.72 K       32.55 μs    ±16.74%          32 μs          37 μs
Enum.reduce       14.41 K       69.39 μs    ±61.47%          68 μs          76 μs

Comparison: 
merge             30.72 K
Enum.reduce       14.41 K - 2.13x slower +36.84 μs

can someone please explain what is the difference in implementation of these 2 cases?

2

Enum.reduce(list1, list2, &[&1, &2]) does not do what you want it to do.

iex> l1 = [1, 2, 3]
iex> l2 = [4, 5, 6]
iex> Enum.reduce(l1, l2, &[&1, &2])
[3, [2, [1, [4, 5, 6]]]]

I think what you’re looking for is Enum.reduce(list1, list2, &[&1 | &2]). Not sure how that will benchmark against your merge function, my guess is they’ll be pretty similar

3

yeah sorry made a mistake in typing, will correct that.

hey @msimonborg I have corrected the code and updated it there still seems to be a difference in stats.

4

Thanks!

I benchmarked these as well, I got a slower run with reduce the first time around, on following passes they are pretty much the same.

First run:

Name             ips        average  deviation         median         99th %
merge       794.05 K        1.26 μs  ±4467.93%        0.85 μs        1.47 μs
reduce      406.70 K        2.46 μs  ±7602.35%        1.06 μs        3.25 μs

Comparison: 
merge       794.05 K
reduce      406.70 K - 1.95x slower +1.20 μs

Second run:

Name             ips        average  deviation         median         99th %
reduce      712.89 K        1.40 μs  ±3434.51%        0.89 μs        2.69 μs
merge       710.97 K        1.41 μs  ±2934.43%        0.97 μs        1.70 μs

Comparison: 
reduce      712.89 K
merge       710.97 K - 1.00x slower +0.00378 μs

Try configuring your benchmarks with variations in :time and :warmup options, and run the benchmarks multiple times back to back:

reduce = fn -> Enum.reduce(l1, l2, &[&1 | &2]) end
merge = fn -> Merge.merge(l1, l2) end

Benchee.run(
  %{
    "reduce" => reduce,
    "merge" => merge
  },
  warmup: 5,
  time: 10
)
5

try running it for merging 4 lists

list1 = Enum.to_list(1..300)
list2 = Enum.to_list(301..600)
list3 = Enum.to_list(601..900)
list4 = Enum.to_list(901..1200)


reduce = fn -> list1 |> Enum.reduce(list2,&[&1|&2]) |> Enum.reduce(list3,&[&1|&2]) |> Enum.reduce(list4,&[&1|&2]) end
merge = fn -> Draft.append(list1, list2) |> Draft.append(list3) |> Draft.append(list4) end

Benchee.run(
  %{
    "reduce" => reduce,
    "merge" => merge
  },
  warmup: 5,
  time: 10
)

the output-

❯ mix run benchmark.exs

Name                  ips        average  deviation         median         99th %
merge             30.64 K       32.64 μs    ±18.56%          32 μs          38 μs
Enum.reduce       14.50 K       68.95 μs     ±6.11%          68 μs          75 μs

Comparison: 
merge             30.64 K
Enum.reduce       14.50 K - 2.11x slower +36.31 μs

❯ mix run benchmark.exs

Name             ips        average  deviation         median         99th %
merge        30.84 K       32.43 μs    ±31.50%          32 μs          37 μs
reduce       14.52 K       68.88 μs     ±8.55%          68 μs          75 μs

Comparison: 
merge        30.84 K
reduce       14.52 K - 2.12x slower +36.45 μs

❯ mix run benchmark.exs

Name             ips        average  deviation         median         99th %
merge        30.59 K       32.69 μs    ±31.70%          32 μs          42 μs
reduce       14.48 K       69.07 μs     ±8.80%          69 μs          74 μs

Comparison: 
merge        30.59 K
reduce       14.48 K - 2.11x slower +36.38 μs

❯ mix run benchmark.exs

Name             ips        average  deviation         median         99th %
merge        30.70 K       32.58 μs    ±30.74%          32 μs          39 μs
reduce       14.23 K       70.26 μs    ±18.30%          68 μs          97 μs

Comparison: 
merge        30.70 K
reduce       14.23 K - 2.16x slower +37.68 μs
6

If you want optimization, this is what I would do:

def merge([head | remaining_list1], list2) do
  merge(remaining_list1, [head | list2])
end

def merge([],list2) do
  list2
end

Avoid unnecessary variable assignment, make use of function head pattern matching and put your base-case last since it will only be called once.

on the other hand, they benchmark the same because Enum.reduce/3 uses :lists.foldl/3 which is implemented in a similar way: otp/lib/stdlib/src/lists.erl at c578f61c17d836d7b4943767f6a08400c33cb3cf · erlang/otp · GitHub

7

I think your best option here it to use List.foldl/3 to avoid any indirection (actually I wonder why List.foldl/3 is not inlined by the compiler).

8

Nitpick: I don’t know what the compiler optimization passes do with them, but Enum.reduce on a list should naively be very very slightly faster because it has one term in its guard clause instead of two.

https://github.com/elixir-lang/elixir/blob/v1.13.4/lib/elixir/lib/list.ex#L247-L250

https://github.com/elixir-lang/elixir/blob/v1.13.4/lib/elixir/lib/enum.ex#L2395-L2397

9

You are right. I didn’t check the code. So i guess the most optimized way is to call :lists.foldl/3 directly.

10

Note that all these implementations are not maintaining the order of the list, neither are merging the lists recursively.