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{
"authors": [
"petehuang"
],
"contributors": [
"angelikatyborska",
"Cohen-Carlisle",
"devonestes",
"DoggettCK",
"fxn",
"kytrinyx",
"neenjaw",
"sotojuan"
],
"files": {
"solution": [
"lib/strain.ex"
],
"test": [
"test/strain_test.exs"
],
"example": [
".meta/example.ex"
]
},
"blurb": "Implement the `keep` and `discard` operation on collections.",
"source": "Conversation with James Edward Gray II",
"source_url": "http://graysoftinc.com/"
}

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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"
[
inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
]

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# The directory Mix will write compiled artifacts to.
/_build/
# If you run "mix test --cover", coverage assets end up here.
/cover/
# The directory Mix downloads your dependencies sources to.
/deps/
# Where third-party dependencies like ExDoc output generated docs.
/doc/
# Ignore .fetch files in case you like to edit your project deps locally.
/.fetch
# If the VM crashes, it generates a dump, let's ignore it too.
erl_crash.dump
# Also ignore archive artifacts (built via "mix archive.build").
*.ez
# Ignore package tarball (built via "mix hex.build").
strain-*.tar

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elixir/strain/HELP.md Normal file
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# Help
## Running the tests
From the terminal, change to the base directory of the exercise then execute the tests with:
```bash
$ mix test
```
This will execute the test file found in the `test` subfolder -- a file ending in `_test.exs`
Documentation:
* [`mix test` - Elixir's test execution tool](https://hexdocs.pm/mix/Mix.Tasks.Test.html)
* [`ExUnit` - Elixir's unit test library](https://hexdocs.pm/ex_unit/ExUnit.html)
## Pending tests
In test suites of practice exercises, all but the first test have been tagged to be skipped.
Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.
For example:
```elixir
# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
```
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:
```bash
$ mix test --include pending
```
Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the file `test/test_helper.exs`.
```elixir
# ExUnit.configure(exclude: :pending, trace: true)
```
## Useful `mix test` options
* `test/<FILE>.exs:LINENUM` - runs only a single test, the test from `<FILE>.exs` whose definition is on line `LINENUM`
* `--failed` - runs only tests that failed the last time they ran
* `--max-failures` - the suite stops evaluating tests when this number of test failures
is reached
* `--seed 0` - disables randomization so the tests in a single file will always be ran
in the same order they were defined in
## Submitting your solution
You can submit your solution using the `exercism submit lib/strain.ex` command.
This command will upload your solution to the Exercism website and print the solution page's URL.
It's possible to submit an incomplete solution which allows you to:
- See how others have completed the exercise
- Request help from a mentor
## Need to get help?
If you'd like help solving the exercise, check the following pages:
- The [Elixir track's documentation](https://exercism.org/docs/tracks/elixir)
- The [Elixir track's programming category on the forum](https://forum.exercism.org/c/programming/elixir)
- [Exercism's programming category on the forum](https://forum.exercism.org/c/programming/5)
- The [Frequently Asked Questions](https://exercism.org/docs/using/faqs)
Should those resources not suffice, you could submit your (incomplete) solution to request mentoring.
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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elixir/strain/HINTS.md Normal file
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# Hints
## General
- `apply` will let you pass arguments to a function, as will `fun.(args)`.

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# Strain
Welcome to Strain on Exercism's Elixir Track.
If you need help running the tests or submitting your code, check out `HELP.md`.
If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
## Instructions
Implement the `keep` and `discard` operation on collections.
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.
For example, given the collection of numbers:
- 1, 2, 3, 4, 5
And the predicate:
- is the number even?
Then your keep operation should produce:
- 2, 4
While your discard operation should produce:
- 1, 3, 5
Note that the union of keep and discard is all the elements.
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.
## Restrictions
Keep your hands off that filter/reject/whatchamacallit functionality provided by your standard library!
Solve this one yourself using other basic tools instead.
## Source
### Created by
- @petehuang
### Contributed to by
- @angelikatyborska
- @Cohen-Carlisle
- @devonestes
- @DoggettCK
- @fxn
- @kytrinyx
- @neenjaw
- @sotojuan
### Based on
Conversation with James Edward Gray II - http://graysoftinc.com/

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defmodule Strain do
@doc """
Given a `list` of items and a function `fun`, return the list of items where
`fun` returns true.
Do not use `Enum.filter`.
"""
@spec keep(list :: list(any), fun :: (any -> boolean)) :: list(any)
def keep([], _fun), do: []
def keep([head | tail], fun) do
if fun.(head), do: [head | keep(tail, fun)], else: keep(tail, fun)
end
@doc """
Given a `list` of items and a function `fun`, return the list of items where
`fun` returns false.
Do not use `Enum.reject`.
"""
@spec discard(list :: list(any), fun :: (any -> boolean)) :: list(any)
def discard(list, fun), do: keep(list, &!fun.(&1))
end

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defmodule Strain.MixProject do
use Mix.Project
def project do
[
app: :strain,
version: "0.1.0",
# elixir: "~> 1.8",
start_permanent: Mix.env() == :prod,
deps: deps()
]
end
# Run "mix help compile.app" to learn about applications.
def application do
[
extra_applications: [:logger]
]
end
# Run "mix help deps" to learn about dependencies.
defp deps do
[
# {:dep_from_hexpm, "~> 0.3.0"},
# {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
]
end
end

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defmodule StrainTest do
use ExUnit.Case
defp is_odd?(n), do: rem(n, 2) == 1
defp is_even?(n), do: rem(n, 2) == 0
describe "keep" do
# @tag :pending
test "on empty list returns empty list" do
assert Strain.keep([], fn _ -> true end) == []
end
# @tag :pending
test "keep everything" do
assert Strain.keep([1, 3, 5], fn _ -> true end) == [1, 3, 5]
end
# @tag :pending
test "keep nothing" do
assert Strain.keep([1, 3, 5], fn _ -> false end) == []
end
# @tag :pending
test "keep first and last" do
assert Strain.keep([1, 2, 3], &is_odd?/1) == [1, 3]
end
# @tag :pending
test "keep neither first nor last" do
assert Strain.keep([1, 2, 3], &is_even?/1) == [2]
end
# @tag :pending
test "keep strings" do
words = ~w(apple zebra banana zombies cherimoya zelot)
assert Strain.keep(words, &String.starts_with?(&1, "z")) == ~w(zebra zombies zelot)
end
# @tag :pending
test "keep lists" do
rows = [
[1, 2, 3],
[5, 5, 5],
[5, 1, 2],
[2, 1, 2],
[1, 5, 2],
[2, 2, 1],
[1, 2, 5]
]
assert Strain.keep(rows, fn row -> 5 in row end) == [
[5, 5, 5],
[5, 1, 2],
[1, 5, 2],
[1, 2, 5]
]
end
end
describe "discard" do
# @tag :pending
test "on empty list returns empty list" do
assert Strain.discard([], fn _ -> true end) == []
end
# @tag :pending
test "discard everything" do
assert Strain.discard([1, 3, 5], fn _ -> true end) == []
end
# @tag :pending
test "discard nothing" do
assert Strain.discard([1, 3, 5], fn _ -> false end) == [1, 3, 5]
end
# @tag :pending
test "discard first and last" do
assert Strain.discard([1, 2, 3], &is_odd?/1) == [2]
end
# @tag :pending
test "discard neither first nor last" do
assert Strain.discard([1, 2, 3], &is_even?/1) == [1, 3]
end
# @tag :pending
test "discard strings" do
words = ~w(apple zebra banana zombies cherimoya zelot)
assert Strain.discard(words, &String.starts_with?(&1, "z")) == ~w(apple banana cherimoya)
end
# @tag :pending
test "discard arrays" do
rows = [
[1, 2, 3],
[5, 5, 5],
[5, 1, 2],
[2, 1, 2],
[1, 5, 2],
[2, 2, 1],
[1, 2, 5]
]
assert Strain.discard(rows, fn row -> 5 in row end) == [[1, 2, 3], [2, 1, 2], [2, 2, 1]]
end
end
end

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ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)