strain
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parent
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{
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"authors": [
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"petehuang"
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],
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"contributors": [
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"angelikatyborska",
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"Cohen-Carlisle",
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"devonestes",
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"DoggettCK",
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"fxn",
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"kytrinyx",
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"neenjaw",
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"sotojuan"
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],
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"files": {
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"solution": [
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"lib/strain.ex"
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],
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"test": [
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"test/strain_test.exs"
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],
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"example": [
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".meta/example.ex"
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]
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},
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"blurb": "Implement the `keep` and `discard` operation on collections.",
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"source": "Conversation with James Edward Gray II",
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"source_url": "http://graysoftinc.com/"
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}
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{"track":"elixir","exercise":"strain","id":"b1072621432641a18b1ba8d05e41d7c8","url":"https://exercism.org/tracks/elixir/exercises/strain","handle":"negrienko","is_requester":true,"auto_approve":false}
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# Used by "mix format"
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[
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inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
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]
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# The directory Mix will write compiled artifacts to.
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/_build/
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# If you run "mix test --cover", coverage assets end up here.
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/cover/
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# The directory Mix downloads your dependencies sources to.
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/deps/
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# Where third-party dependencies like ExDoc output generated docs.
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/doc/
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# Ignore .fetch files in case you like to edit your project deps locally.
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/.fetch
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# If the VM crashes, it generates a dump, let's ignore it too.
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erl_crash.dump
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# Also ignore archive artifacts (built via "mix archive.build").
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*.ez
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# Ignore package tarball (built via "mix hex.build").
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strain-*.tar
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# Help
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## Running the tests
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From the terminal, change to the base directory of the exercise then execute the tests with:
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```bash
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$ mix test
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```
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This will execute the test file found in the `test` subfolder -- a file ending in `_test.exs`
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Documentation:
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* [`mix test` - Elixir's test execution tool](https://hexdocs.pm/mix/Mix.Tasks.Test.html)
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* [`ExUnit` - Elixir's unit test library](https://hexdocs.pm/ex_unit/ExUnit.html)
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## Pending tests
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In test suites of practice exercises, all but the first test have been tagged to be skipped.
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Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.
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For example:
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```elixir
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# @tag :pending
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test "shouting" do
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assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
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end
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```
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If you wish to run all tests at once, you can include all skipped test by using the `--include` flag on the `mix test` command:
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```bash
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$ mix test --include pending
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```
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Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the file `test/test_helper.exs`.
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```elixir
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# ExUnit.configure(exclude: :pending, trace: true)
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```
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## Useful `mix test` options
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* `test/<FILE>.exs:LINENUM` - runs only a single test, the test from `<FILE>.exs` whose definition is on line `LINENUM`
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* `--failed` - runs only tests that failed the last time they ran
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* `--max-failures` - the suite stops evaluating tests when this number of test failures
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is reached
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* `--seed 0` - disables randomization so the tests in a single file will always be ran
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in the same order they were defined in
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## Submitting your solution
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You can submit your solution using the `exercism submit lib/strain.ex` command.
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This command will upload your solution to the Exercism website and print the solution page's URL.
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It's possible to submit an incomplete solution which allows you to:
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- See how others have completed the exercise
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- Request help from a mentor
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## Need to get help?
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If you'd like help solving the exercise, check the following pages:
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- The [Elixir track's documentation](https://exercism.org/docs/tracks/elixir)
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- The [Elixir track's programming category on the forum](https://forum.exercism.org/c/programming/elixir)
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- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
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- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
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Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
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If you're stuck on something, it may help to look at some of the [available resources](https://exercism.org/docs/tracks/elixir/resources) out there where answers might be found.
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# Hints
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## General
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- `apply` will let you pass arguments to a function, as will `fun.(args)`.
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# Strain
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Welcome to Strain on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
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## Instructions
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Implement the `keep` and `discard` operation on collections.
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Given a collection and a predicate on the collection's elements, `keep` returns a new collection containing those elements where the predicate is true, while `discard` returns a new collection containing those elements where the predicate is false.
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For example, given the collection of numbers:
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- 1, 2, 3, 4, 5
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And the predicate:
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- is the number even?
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Then your keep operation should produce:
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- 2, 4
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While your discard operation should produce:
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- 1, 3, 5
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Note that the union of keep and discard is all the elements.
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The functions may be called `keep` and `discard`, or they may need different names in order to not clash with existing functions or concepts in your language.
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## Restrictions
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Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
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Solve this one yourself using other basic tools instead.
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## Source
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### Created by
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- @petehuang
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### Contributed to by
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- @angelikatyborska
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- @Cohen-Carlisle
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- @devonestes
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- @DoggettCK
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- @fxn
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- @kytrinyx
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- @neenjaw
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- @sotojuan
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### Based on
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Conversation with James Edward Gray II - http://graysoftinc.com/
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defmodule Strain do
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@doc """
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Given a `list` of items and a function `fun`, return the list of items where
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`fun` returns true.
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Do not use `Enum.filter`.
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"""
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@spec keep(list :: list(any), fun :: (any -> boolean)) :: list(any)
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def keep([], _fun), do: []
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def keep([head | tail], fun) do
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if fun.(head), do: [head | keep(tail, fun)], else: keep(tail, fun)
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end
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@doc """
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Given a `list` of items and a function `fun`, return the list of items where
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`fun` returns false.
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Do not use `Enum.reject`.
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"""
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@spec discard(list :: list(any), fun :: (any -> boolean)) :: list(any)
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def discard(list, fun), do: keep(list, &!fun.(&1))
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end
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defmodule Strain.MixProject do
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use Mix.Project
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def project do
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[
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app: :strain,
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version: "0.1.0",
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# elixir: "~> 1.8",
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start_permanent: Mix.env() == :prod,
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deps: deps()
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]
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end
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# Run "mix help compile.app" to learn about applications.
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def application do
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[
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extra_applications: [:logger]
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]
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end
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# Run "mix help deps" to learn about dependencies.
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defp deps do
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[
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# {:dep_from_hexpm, "~> 0.3.0"},
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# {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
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]
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end
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end
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defmodule StrainTest do
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use ExUnit.Case
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defp is_odd?(n), do: rem(n, 2) == 1
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defp is_even?(n), do: rem(n, 2) == 0
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describe "keep" do
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# @tag :pending
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test "on empty list returns empty list" do
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assert Strain.keep([], fn _ -> true end) == []
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end
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# @tag :pending
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test "keep everything" do
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assert Strain.keep([1, 3, 5], fn _ -> true end) == [1, 3, 5]
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end
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# @tag :pending
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test "keep nothing" do
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assert Strain.keep([1, 3, 5], fn _ -> false end) == []
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end
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# @tag :pending
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test "keep first and last" do
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assert Strain.keep([1, 2, 3], &is_odd?/1) == [1, 3]
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end
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# @tag :pending
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test "keep neither first nor last" do
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assert Strain.keep([1, 2, 3], &is_even?/1) == [2]
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end
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# @tag :pending
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test "keep strings" do
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words = ~w(apple zebra banana zombies cherimoya zelot)
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assert Strain.keep(words, &String.starts_with?(&1, "z")) == ~w(zebra zombies zelot)
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end
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# @tag :pending
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test "keep lists" do
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rows = [
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[1, 2, 3],
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[5, 5, 5],
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[5, 1, 2],
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[2, 1, 2],
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[1, 5, 2],
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[2, 2, 1],
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[1, 2, 5]
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]
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assert Strain.keep(rows, fn row -> 5 in row end) == [
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[5, 5, 5],
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[5, 1, 2],
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[1, 5, 2],
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[1, 2, 5]
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]
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end
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end
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describe "discard" do
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# @tag :pending
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test "on empty list returns empty list" do
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assert Strain.discard([], fn _ -> true end) == []
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end
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# @tag :pending
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test "discard everything" do
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assert Strain.discard([1, 3, 5], fn _ -> true end) == []
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end
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# @tag :pending
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test "discard nothing" do
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assert Strain.discard([1, 3, 5], fn _ -> false end) == [1, 3, 5]
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end
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# @tag :pending
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test "discard first and last" do
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assert Strain.discard([1, 2, 3], &is_odd?/1) == [2]
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end
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# @tag :pending
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test "discard neither first nor last" do
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assert Strain.discard([1, 2, 3], &is_even?/1) == [1, 3]
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end
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# @tag :pending
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test "discard strings" do
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words = ~w(apple zebra banana zombies cherimoya zelot)
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assert Strain.discard(words, &String.starts_with?(&1, "z")) == ~w(apple banana cherimoya)
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end
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# @tag :pending
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test "discard arrays" do
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rows = [
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[1, 2, 3],
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[5, 5, 5],
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[5, 1, 2],
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[2, 1, 2],
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[1, 5, 2],
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[2, 2, 1],
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[1, 2, 5]
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]
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assert Strain.discard(rows, fn row -> 5 in row end) == [[1, 2, 3], [2, 1, 2], [2, 2, 1]]
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end
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end
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end
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ExUnit.start()
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ExUnit.configure(exclude: :pending, trace: true)
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