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to Accumulate in the Elixir Track

Published at Jun 26 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Implement the accumulate operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

  • 1, 2, 3, 4, 5

And the operation:

  • square a number (x => x * x)

Your code should be able to produce the collection of squares:

  • 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.

Restrictions

Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

Running tests

Execute the tests with:

$ elixir accumulate_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

If you're stuck on something, it may help to look at some of the available resources out there where answers might be found.

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.

accumulate_test.exs

if !System.get_env("EXERCISM_TEST_EXAMPLES") do
  Code.load_file("accumulate.exs", __DIR__)
end

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

defmodule AccumulateTest do
  use ExUnit.Case

  test "accumulate empty list" do
    assert Accumulate.accumulate([], fn n -> n * n end) == []
  end

  @tag :pending
  test "accumulate square numbers" do
    assert Accumulate.accumulate([1, 2, 3], fn n -> n * n end) == [1, 4, 9]
  end

  @tag :pending
  test "accumulate upcased strings" do
    fun = fn w -> String.upcase(w) end
    assert Accumulate.accumulate(["hello", "world"], fun) == ["HELLO", "WORLD"]
  end

  @tag :pending
  test "accumulate reversed strings" do
    fun = fn w -> String.reverse(w) end
    words = ~w(the quick brown fox etc)
    expected = ["eht", "kciuq", "nworb", "xof", "cte"]
    assert Accumulate.accumulate(words, fun) == expected
  end

  @tag :pending
  test "nested accumulate" do
    chars = ~w(a b c)
    nums = ~w(1 2 3)
    fun = fn c -> Accumulate.accumulate(nums, &(c <> &1)) end
    expected = [["a1", "a2", "a3"], ["b1", "b2", "b3"], ["c1", "c2", "c3"]]
    assert Accumulate.accumulate(chars, fun) == expected
  end
end
defmodule Accumulate do
  @doc """
    Given a list and a function, apply the function to each list item and
    replace it with the function's return value.

    Returns a list.

    ## Examples

      iex> Accumulate.accumulate([], fn(x) -> x * 2 end)
      []

      iex> Accumulate.accumulate([1, 2, 3], fn(x) -> x * 2 end)
      [2, 4, 6]

  """

  @spec accumulate(list, (any -> any)) :: list
  def accumulate([head | tail], fun) do
    [fun.(head) | accumulate(tail, fun)]
  end

  def accumulate([], _fun), do: []
end

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