How do I push to an existing array?

[Eric Berry]

I'm VERY new to Elixir so forgive me for dumb questions. Here's a simple tool I am playing with to build keywords. I am doing something wrong, but the error message is not very clear.

Code:

defmodule Simple do

  def build_keywords do

    keywords = ["Car Insurance", "Insurance Car", "Car Ins", "Insurance Quote", "Insurance", "car Insurance Quote", "Insurance Car Quote", "I need Car Insurance"]

    state = "UT"

    cities = ["Alpine", "Altamont", "Alton", "Altonah", "American Fork", "Aneth", "Annabella", "Antimony", "Aurora", "Axtell", "Bear River City", "Beaver", "Beryl", "Bicknell", "Bingham Canyon", "Blanding", "Bluebell", "Bluff", "Bonanza", "Boulder", "Bountiful", "Brian Head", "Brigham City", "Bryce", "Bryce Canyon", "Cache Junction", "Cannonville", "Castle Dale", "Cedar City", "Cedar Valley", "Centerfield", "Centerville", "Central", "Chester", "Circleville", "Cisco", "Clarkston", "Clawson", "Clearfield", "Cleveland", "Coalville", "Collinston", "Corinne", "Cornish", "Croydon", "Dammeron Valley", "Delta", "Deweyville", "Draper", "Duchesne", "Duck Creek Village", "Dugway", "Dutch John", "Eagle Mountain", "East Carbon", "Echo", "Eden", "Elberta", "Elmo", "Elsinore", "Emery", "Enterprise", "Ephraim", "Escalante", "Eureka", "Fairview", "Farmington", "Fayette", "Ferron", "Fielding", "Fillmore", "Fort Duchesne", "Fountain Green", "Fruitland", "Garden City", "Garland", "Garrison", "Glendale", "Glenwood", "Goshen", "Grantsville", "Green River", "Greenville", "Greenwich", "Grouse Creek", "Gunlock", "Gunnison", "Hanksville", "Hanna", "Hatch", "Heber City", "Helper", "Henefer", "Henrieville", "Herriman", "Hildale", "Hill Afb", "Hinckley", "Holden", "Honeyville", "Hooper", "Howell", "Huntington", "Huntsville", "Hurricane", "Hyde Park", "Hyrum", "Ibapah", "Ivins", "Jensen", "Joseph", "Junction", "Kamas", "Kanab", "Kanarraville", "Kanosh", "Kaysville", "Kenilworth", "Kingston", "Koosharem", "La Sal", "La Verkin", "Lake Powell", "Laketown", "Lapoint", "Layton", "Leamington", "Leeds", "Lehi", "Levan", "Lewiston", "Lindon", "Loa", "Logan", "Lyman", "Lynndyl", "Magna", "Manila", "Manti", "Mantua", "Mapleton", "Marysvale", "Mayfield", "Meadow", "Mendon", "Mexican Hat", "Midvale", "Midway", "Milford", "Millville", "Minersville", "Moab", "Modena", "Mona", "Monroe", "Montezuma Creek", "Monticello", "Monument Valley", "Morgan", "Moroni", "Mount Carmel", "Mount Pleasant", "Mountain Home", "Myton", "Neola", "Nephi", "New Harmony", "Newcastle", "Newton", "North Salt Lake", "Oak City", "Oakley", "Ogden", "Orangeville", "Orderville", "Orem", "Panguitch", "Paradise", "Paragonah", "Park City", "Park Valley", "Parowan", "Payson", "Peoa", "Pine Valley", "Pleasant Grove", "Plymouth", "Portage", "Price", "Providence", "Provo", "Randlett", "Randolph", "Redmond", "Richfield", "Richmond", "Riverside", "Riverton", "Rockville", "Roosevelt", "Roy", "Rush Valley", "Saint George", "Salem", "Salina", "Salt Lake City", "Sandy", "Santa Clara", "Santaquin", "Saratoga Springs", "Scipio", "Sevier", "Sigurd", "Smithfield", "Snowville", "South Jordan", "Spanish Fork", "Spring City", "Springdale", "Springville", "Sterling", "Stockton", "Summit", "Sunnyside", "Syracuse", "Tabiona", "Talmage", "Teasdale", "Thompson", "Tooele", "Toquerville", "Torrey", "Tremonton", "Trenton", "Tridell", "Tropic", "Vernal", "Vernon", "Veyo", "Virgin", "Wales", "Wallsburg", "Washington", "Wellington", "Wellsville", "Wendover", "West Jordan", "Whiterocks", "Willard", "Woodruff", "Woods Cross"]

    auxs = ["in"]

    Enum.map(cities, fn(city) -> build_keywords_for_city(city, keywords, state, auxs) end)

  end

  def build_keywords_for_city(city, keywords, state, auxs) do

    results = []

    results = results ++ Enum.map(keywords, fn(keyword) -> map_keywords(keyword, city, state, auxs) end)

    Enum.map(results, fn(kw) -> IO.puts(kw) end)

  end

  def map_keywords(keyword, city, state, auxs) do

    results = []

    results = results ++ String.downcase(Enum.join([keyword, city], " "))

    results = results ++ String.downcase(Enum.join([keyword, city, state], " "))

    results

  end

end

Here's the results of my test:

  1) test build keywords (SimpleTest)

     test/simple_test.exs:8

     ** (ArgumentError) argument error

     stacktrace:

       :erlang.++("car insurance alpine", "car insurance alpine ut")

       (simple) lib/simple.ex:20: Simple.map_keywords/4

       (elixir) lib/enum.ex:977: anonymous fn/3 in Enum.map/2

       (elixir) lib/enum.ex:1261: Enum."-reduce/3-lists^foldl/2-0-"/3

       (elixir) lib/enum.ex:977: Enum.map/2

       (simple) lib/simple.ex:13: Simple.build_keywords_for_city/4

       (elixir) lib/enum.ex:977: anonymous fn/3 in Enum.map/2

       (elixir) lib/enum.ex:1261: Enum."-reduce/3-lists^foldl/2-0-"/3

Can somebody please let me know what I am doing wrong? I come from a Ruby background so I may be thinking too ruby-ish.

Thanks,

Eric

[José Valim]

According to your understanding of Elixir so far, what does the ++ operator do? What should be its arguments? What does the documentation say about it?

PS: Btw, you want to send those e-mails to elixir-lang-talk. This mailing list is for discussion of core language features. We can discuss this one here but please send the next ones there.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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[Eric Berry]

It was my understanding based on the docs that ++ joins arrays together (http://elixir-lang.org/getting-started/basic-types.html#linked-lists under linked lists)

"Two lists can be concatenated and subtracted using the ++/2 and --/2 operators:”

I was corrected by @aaronm on the slack channel with the following example:

https://gist.github.com/aaroncm/2eec12af864cbb06d36f

Thanks for your help. I will make sure I post to the correct place next time (likely slack overflow)

Eric

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[Stian Håklev]

Hi Eric,

it seems like you are trying to add a String to the List, but ++ only works on two lists. Also, you should generally avoid using ++ to add a single element to a list, because it has a poor performance. Instead, use the form [ head | tail ] (note that head must be a single element and tail must be a list). You can also use this form in pattern matching, for example

[ h | tl ] = [1, 2, 3]

after this, h will have the value of 1, and t of [2, 3]

But in fact, we don’t really need this in your code example, since we can just avoid all these intermediate variables anyway - which is idiomatic Elixir. We can directly construct the list to be returned, or use the pipe operator.

  def build_keywords_for_city(city, keywords, state, auxs) do

Enum.map(keywords, fn(keyword) -> map_keywords(keyword, city, state, auxs) end

)
    |> Enum.map(fn(kw) -> IO.puts(kw) end)
  

end

  def map_keywords(keyword, city, state, auxs) do

    [String.downcase(Enum.join([keyword, city], " ")), 
    String.downcase(Enum.join([keyword, city, state], " "))]
  end

I could probably rewrite your entire program quite a bit to make it more idiomatic, but since you were mainly concerned about the list addition (and I know how it is to begin learning a new language, and experiment with constructs etc), I made as few changes as possible.

Stian

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[José Valim]

Yes, the mistake in your code was that you were calling ++ with String.downcase on the right side, which won't return a list but a string.

The code in the gist could also be improved:

    for city <- cities,
        keyword <- keywords,
        entry <- ["#{keyword} #{city}", "#{keyword} #{city} #{state}"],
        do: entry

No need for nested "for" nor for calling List.flatten/1.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Director of R&D

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