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# thekeele's solution

## to Strain in the Elixir Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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

• 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.

`apply` will let you pass arguments to a function, as will `fun.(args)`

## Running tests

Execute the tests with:

``````\$ elixir strain_test.exs
``````

### Pending tests

In the test suites, all but the first test have been 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:

``````# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
``````

Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the test suite.

``````# ExUnit.configure exclude: :pending, trace: true
``````

For more detailed information about the Elixir track, please see the help page.

## Source

Conversation with James Edward Gray II https://twitter.com/jeg2

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### strain_test.exs

``````if !System.get_env("EXERCISM_TEST_EXAMPLES") do
end

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

defmodule StrainTest do
use ExUnit.Case

defp is_odd?(n), do: rem(n, 2) == 1
defp is_even?(n), do: rem(n, 2) == 0
defp noop(_), do: true

# @tag :pending
test "empty keep" do
assert Strain.keep([], &noop/1) == []
end

@tag :pending
test "keep everything" do
assert Strain.keep([1, 2, 3], fn e -> e < 10 end) == [1, 2, 3]
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, 4, 5], &is_even?/1) == [2, 4]
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 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.keep(rows, fn row -> 5 in row end) == [
[5, 5, 5],
[5, 1, 2],
[1, 5, 2],
[1, 2, 5]
]
end

@tag :pending
test "empty discard" do
assert Strain.discard([], &noop/1) == []
end

@tag :pending
test "discard nothing" do
assert Strain.discard([1, 2, 3], fn e -> e > 10 end) == [1, 2, 3]
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, 4, 5], &is_even?/1) == [1, 3, 5]
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``````
``````defmodule Strain do
def keep([], _fun), do: []
def keep([car | cdr], fun) do
recur(car, fun.(car), fn -> keep(cdr, fun) end)
end

def discard([], _fun), do: []
def discard([car | cdr], fun) do
recur(car, not fun.(car), fn -> discard(cdr, fun) end)
end

defp recur(_car, false, fun), do: fun.()
defp recur(car, true, fun), do: cons(car, fun.())

defp cons(item, list), do: [item | list]
end``````