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davearonson's solution

to Wordy 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.

Parse and evaluate simple math word problems returning the answer as an integer.

Iteration 1 — Addition

Add two numbers together.

What is 5 plus 13?

Evaluates to 18.

Handle large numbers and negative numbers.

Iteration 2 — Subtraction, Multiplication and Division

Now, perform the other three operations.

What is 7 minus 5?

2

What is 6 multiplied by 4?

24

What is 25 divided by 5?

5

Iteration 3 — Multiple Operations

Handle a set of operations, in sequence.

Since these are verbal word problems, evaluate the expression from left-to-right, ignoring the typical order of operations.

What is 5 plus 13 plus 6?

24

What is 3 plus 2 multiplied by 3?

15 (i.e. not 9)

Bonus — Exponentials

If you'd like, handle exponentials.

What is 2 raised to the 5th power?

32

Running tests

Execute the tests with:

$ elixir wordy_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

Inspired by one of the generated questions in the Extreme Startup game. https://github.com/rchatley/extreme_startup

Submitting Incomplete Solutions

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

wordy_test.exs

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

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

defmodule WordyTest do
  use ExUnit.Case

  test "addition" do
    assert Wordy.answer("What is 1 plus 1?") == 2
  end

  @tag :pending
  test "more addition" do
    assert Wordy.answer("What is 53 plus 2?") == 55
  end

  @tag :pending
  test "addition with negative numbers" do
    assert Wordy.answer("What is -1 plus -10?") == -11
  end

  @tag :pending
  test "large addition" do
    assert Wordy.answer("What is 123 plus 45678?") == 45801
  end

  @tag :pending
  test "subtraction" do
    assert Wordy.answer("What is 4 minus -12?") == 16
  end

  @tag :pending
  test "multiplication" do
    assert Wordy.answer("What is -3 multiplied by 25?") == -75
  end

  @tag :pending
  test "division" do
    assert Wordy.answer("What is 33 divided by -3?") == -11
  end

  @tag :pending
  test "multiple additions" do
    assert Wordy.answer("What is 1 plus 1 plus 1?") == 3
  end

  @tag :pending
  test "addition and subtraction" do
    assert Wordy.answer("What is 1 plus 5 minus -2?") == 8
  end

  @tag :pending
  test "multiple subtraction" do
    assert Wordy.answer("What is 20 minus 4 minus 13?") == 3
  end

  @tag :pending
  test "subtraction then addition" do
    assert Wordy.answer("What is 17 minus 6 plus 3?") == 14
  end

  @tag :pending
  test "multiple multiplication" do
    assert Wordy.answer("What is 2 multiplied by -2 multiplied by 3?") == -12
  end

  @tag :pending
  test "addition and multiplication" do
    assert Wordy.answer("What is -3 plus 7 multiplied by -2?") == -8
  end

  @tag :pending
  test "multiple division" do
    assert Wordy.answer("What is -12 divided by 2 divided by -3?") == 2
  end

  @tag :pending
  test "unknown operation" do
    assert_raise ArgumentError, fn ->
      Wordy.answer("What is 52 cubed?")
    end
  end

  @tag :pending
  test "Non math question" do
    assert_raise ArgumentError, fn ->
      Wordy.answer("Who is the President of the United States?")
    end
  end
end
defmodule Wordy do

  @doc """
  Calculate the math problem in the sentence.
  """
  @spec answer(String.t) :: integer
  def answer(question) do
    # scan for hits composed of:
    # any string of non-digits (and stash it), a space,
    # and an optional minus sign followed by digits (and stash it and them).
    # at this point we're not bothering to validate the non-number part.
    pieces = Regex.scan(~r/([^\d]*) (-?\d+)/, question)
    # if we got no hits, or there is more (or less!) left over than a "?",
    # then the question is malformed. it may be malformed in other ways,
    # that will raise no-matching-clause errors below.
    if pieces == nil || remove_pieces(pieces, question) != "?" do
      raise ArgumentError
    end
    fixed = pieces |> Enum.map(&fixup/1)
    do_answer(fixed, nil)
  end

  defp remove_pieces([[whole_hit|_]|more], question) do
    remove_pieces(more, question |> String.replace(whole_hit, ""))
  end

  defp remove_pieces([], question), do: question

  # match on nil means it must be the very first thing.
  # anything else as first thing will also produce error
  # when we try to do math with it in the other defps.
  defp do_answer([["What is"|[number]]|more], nil) do
    do_answer(more, number)
  end

  defp do_answer([["plus"|[number]]|more], acc) do
    do_answer(more, acc + number)
  end

  defp do_answer([["minus"|[number]]|more], acc) do
    do_answer(more, acc - number)
  end

  defp do_answer([["multiplied by"|[number]]|more], acc) do
    do_answer(more, acc * number)
  end

  defp do_answer([["divided by"|[number]]|more], acc) do
    do_answer(more, acc / number)
  end

  defp do_answer([], acc), do: acc

  defp do_answer(_, _), do: raise ArgumentError

  defp fixup([_|[words|[number]]]) do
    [String.trim(words), String.to_integer(number)]
  end
end

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