Alphabet Iterator

type1fool

Hello, Elixir gang. I’m starting to get the hang of Elixir, which has been challenging since I’m not coming from Ruby. I have been struggling to find a solution for a particular problem. I need to generate a list of strings which increments over the alphabet. So["A", "B", "C"] is a very basic example.

When the end of the alphabet is reached, I want the result to look like this: [..."X", "Y", "Z", "AA", "BB"...]. The goal is to create a function like this: generate_list(first_letter, count).

I tried using the Alphabetify package, but it was pretty slow, and didn’t quite give me the results I needed. String.duplicate got me almost there, but I still wasn’t getting the right output.

Does anyone have an idea or package suggestion for this problem? I’ve hit a wall on this one.

Where I’m at

letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ" |> String.split("", trim: true)

def generate_list(first_letter, count)
when is_binary(first_letter)
when is_integer(count) do
    # letters
    # |> Stream.cycle
    # |> Stream.take(count)

    starting_index = letters |> Enum.find_index(fn x -> x == first_letter end)

    # 0..count
    # |> Enum.map(fn x ->  end)
end

13 comments

#lists #enumerables

0 4187 13

2018-08-11 15:42:07 UTC

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blatyo

Conduit Core Team

This should do it. The first_letter part isn’t super efficient, but the rest should be.

EDIT: Fix divisor

def generate_list(first_letter \\ "A", count) when is_binary(first_letter) and is_integer(count) do
  ?A..?Z 
  |> Enum.map(&to_string([&1])) 
  |> Stream.cycle() 
  |> Stream.with_index() 
  |> Stream.map(fn {string, index} ->
    String.duplicate(string, div(index, 26) + 1)
  end)
  |> Stream.drop_while(&(&1 != first_letter))
  |> Enum.take(count)
end

Post #1

type1fool

This is great, though it’s duplicating Z on the first pass.

iex(11)> generate_list("W", 10)
["W", "X", "Y", "ZZ", "AA", "BB", "CC", "DD", "EE", "FF"]

I changed div(index, 25) to div(index, 26) and the problem is solved. It correctly appends additional characters too:

iex(15)> generate_list("W", 10)
["W", "X", "Y", "Z", "AA", "BB", "CC", "DD", "EE", "FF"]
iex(20)> generate_list("ZZ", 60)
["ZZ", "AAA", "BBB", "CCC", "DDD", "EEE", "FFF", "GGG", "HHH", "III", "JJJ", "KKK", "LLL", "MMM", "NNN", "OOO", "PPP", "QQQ", "RRR", "SSS", "TTT", "UUU", "VVV", "WWW", "XXX", "YYY", "ZZZ", "AAAA", "BBBB", "CCCC", "DDDD", "EEEE", "FFFF", "GGGG", "HHHH", "IIII", "JJJJ", "KKKK", "LLLL", "MMMM", "NNNN", "OOOO", "PPPP", "QQQQ", "RRRR", "SSSS", "TTTT", "UUUU", "VVVV", "WWWW", ...]

Thank you @blatyo for this clean solution!

Post #2

kelvinst

That’s indeed a really good solution. But I got interested by the possibility of improving its performance and here are the results:

defmodule LettersAndNumbers do
  def list(from \\ 0, count) do
    from
    |> stream()
    |> Enum.take(count)
  end

  def stream(from \\ 0)

  def stream(from) when is_bitstring(from) or is_list(from) do
    from
    |> letters_to_number()
    |> stream()
  end

  def stream(from) do
    from
    |> Stream.iterate(&(&1 + 1))
    |> Stream.map(&number_to_letters/1)
  end

  def number_to_letters(number) do
    [rem(number, 26) + 65]
    |> to_string()
    |> String.duplicate(div(number, 26) + 1)
  end

  def letters_to_number(letters) when is_bitstring(letters) do
    letters
    |> String.to_charlist()
    |> letters_to_number()
  end

  def letters_to_number([char | _] = letters) do
    ((length(letters) - 1) * 26) + (char - 65)
  end
end

PS.: I also did some changes to the API, but basically, instead of cycling through the alphabet and navigating it with an index, I mathematically transform numbers into letters by adding 65 to them and duplicating them div(number, 26) + 1 times like you did.

Post #3

type1fool

@kelvinst Nice! I like this approach and I can’t wait to benchmark these two solutions.

Post #4

kelvinst

Yeah, it would be nice to benchmark them, since I’m not sure my solution will really be faster. I mean, of course it’s faster with a high “first_letter” because it will not iterate discarding everything, but I’m not sure if the calcs I’ve done didn’t add any extra complexity per iteration.

Post #5

type1fool

This is my first foray into benchmarking, so these results may be unreliable.

Operating System: macOS"
CPU Information: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
Number of Available Cores: 8
Available memory: 16 GB
Elixir 1.6.1
Erlang 20.2.4

Benchmark suite executing with the following configuration:
warmup: 3 s
time: 7 s
memory time: 0 μs
parallel: 1
inputs: 100K, 1K, 1M
Estimated total run time: 1 min


Benchmarking LettersAndNumbers.list/2 with input 100K...
Benchmarking LettersAndNumbers.list/2 with input 1K...
Benchmarking LettersAndNumbers.list/2 with input 1M...
Benchmarking generate_list/2 with input 100K...
Benchmarking generate_list/2 with input 1K...
Benchmarking generate_list/2 with input 1M...

##### With input 100K #####
Name                               ips        average  deviation         median         99th %
generate_list/2                   1.44      692.72 ms     ±4.56%      682.07 ms      747.01 ms
LettersAndNumbers.list/2          1.43      697.56 ms     ±4.66%      687.10 ms      787.46 ms

Comparison:
generate_list/2                   1.44
LettersAndNumbers.list/2          1.43 - 1.01x slower

##### With input 1K #####
Name                               ips        average  deviation         median         99th %
generate_list/2                 3.31 K      302.43 μs    ±23.01%         285 μs      695.32 μs
LettersAndNumbers.list/2        2.17 K      459.98 μs    ±14.67%         439 μs      750.16 μs

Comparison:
generate_list/2                 3.31 K
LettersAndNumbers.list/2        2.17 K - 1.52x slower

##### With input 1M #####
Name                               ips        average  deviation         median         99th %
generate_list/2                 0.0141       1.18 min     ±0.00%       1.18 min       1.18 min
LettersAndNumbers.list/2        0.0141       1.18 min     ±0.00%       1.18 min       1.18 min

Comparison:
generate_list/2                 0.0141
LettersAndNumbers.list/2        0.0141 - 1.00x slower

Benchmark Code:

defmodule TicketMarket.MutationBenchmarks do
  alias TicketMarket.LettersAndNumbers
  alias TicketMarket.MutateLayout

  inputs = %{
    "1K" => 1_000,
    "100K" => 100_000,
    "1M" => 1_000_000,
  }

  Benchee.run %{
    "LettersAndNumbers.list/2" => fn(count) -> LettersAndNumbers.list(0, count) end,
    "generate_list/2" => fn(count) -> MutateLayout.generate_list("A", count) end
  },
    formatters: [
      # Benchee.Formatters.HTML,
      Benchee.Formatters.Console
    ],
    time: 7,
    warmup: 3,
    inputs: inputs
end

It’s interesting that both approaches get exponentially slower as the count increases. I do like the simplicity of generate_list/2, so that may be the way I end up going. I don’t expect the count to be higher than 1000, so either function would be usable.

If you have any thoughts about the benchmark, let me know.
Thanks again!

Post #6

kelvinst

So, the thing is that you didn’t bench the case were mine is supposedly faster, which would be with a high starting letter.

Post #7

type1fool

This is true. I’ll run it again tonight with a higher starting letter.

Post #8

kelvinst

Also, be sure to get a high letter like “O” |> String.duplicate(40000) which is the number 1039988.

Post #9

type1fool

Indeed you are correct. Starting with a crazy high letter benefits the LettersAndNumbers approach. Thi scenario would not ever happen. The highest letter would be something like "ZZZZ". Fun experiment nonetheless.

inputs = %{
    # "10" => 10,
    # "100" => 100,
    "1K" => 1_000,
    "100K" => 100_000,
    "1M" => 1_000_000,
  }

  Benchee.run %{
    "LettersAndNumbers.list/2" => fn(count) -> LettersAndNumbers.list(1039988, count) end,
    "generate_list/2" => fn(count) -> MutateLayout.generate_list("O" |> String.duplicate(40_000), count) end
  },
    formatters: [
      # Benchee.Formatters.HTML,
      Benchee.Formatters.Console
    ],
    time: 7,
    warmup: 3,
    inputs: inputs

RESULTS:

Operating System: macOS"
CPU Information: Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
Number of Available Cores: 8
Available memory: 16 GB
Elixir 1.6.1
Erlang 20.2.4

Benchmark suite executing with the following configuration:
warmup: 3 s
time: 7 s
memory time: 0 μs
parallel: 1
inputs: 100K, 1K, 1M
Estimated total run time: 1 min


Benchmarking LettersAndNumbers.list/2 with input 100K...
Benchmarking LettersAndNumbers.list/2 with input 1K...
Benchmarking LettersAndNumbers.list/2 with input 1M...
Benchmarking generate_list/2 with input 100K...
Benchmarking generate_list/2 with input 1K...
Benchmarking generate_list/2 with input 1M...

##### With input 100K #####
Name                               ips        average  deviation         median         99th %
LettersAndNumbers.list/2        0.0712       0.23 min     ±0.00%       0.23 min       0.23 min
generate_list/2                 0.0132       1.27 min     ±0.00%       1.27 min       1.27 min

Comparison:
LettersAndNumbers.list/2        0.0712
generate_list/2                 0.0132 - 5.40x slower

##### With input 1K #####
Name                               ips        average  deviation         median         99th %
LettersAndNumbers.list/2          8.40    0.00199 min     ±2.23%    0.00198 min    0.00209 min
generate_list/2                 0.0162       1.03 min     ±0.00%       1.03 min       1.03 min

Comparison:
LettersAndNumbers.list/2          8.40
generate_list/2                 0.0162 - 518.33x slower

##### With input 1M #####
Name                               ips        average  deviation         median         99th %
LettersAndNumbers.list/2       0.00465       3.59 min     ±0.00%       3.59 min       3.59 min
generate_list/2                0.00365       4.57 min     ±0.00%       4.57 min       4.57 min

Comparison:
LettersAndNumbers.list/2       0.00465
generate_list/2                0.00365 - 1.27x slower

Post #10

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