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:
And the predicate:
Then your keep operation should produce:
While your discard operation should produce:
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.
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)
Execute the tests with:
$ elixir strain_test.exs
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.
Conversation with James Edward Gray II https://twitter.com/jeg2
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
if !System.get_env("EXERCISM_TEST_EXAMPLES") do
Code.load_file("strain.exs", __DIR__)
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
@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(list, fun) do
filter(list, fn (el) -> fun.(el) end)
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
filter(list, fn (x) -> !fun.(x) end)
end
defp filter([h | t], fun) do
if fun.(h) do
[h | filter(t, fun)]
else
filter(t, fun)
end
end
defp filter([], fun) do
[]
end
end
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