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to Matching Brackets in the Elixir Track

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

Given a string containing brackets [], braces {}, parentheses (), or any combination thereof, verify that any and all pairs are matched and nested correctly.

Running tests

Execute the tests with:

$ mix test

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

Ginna Baker

Submitting Incomplete Solutions

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

matching_brackets_test.exs

defmodule MatchingBracketsTest do
  use ExUnit.Case

  # @tag :pending
  test "paired square brackets" do
    assert MatchingBrackets.check_brackets("[]")
  end

  @tag :pending
  test "empty string" do
    assert MatchingBrackets.check_brackets("")
  end

  @tag :pending
  test "unpaired brackets" do
    refute MatchingBrackets.check_brackets("[[")
  end

  @tag :pending
  test "wrong ordered brackets" do
    refute MatchingBrackets.check_brackets("}{")
  end

  @tag :pending
  test "wrong closing bracket" do
    refute MatchingBrackets.check_brackets("{]")
  end

  @tag :pending
  test "paired with whitespace" do
    assert MatchingBrackets.check_brackets("{ }")
  end

  @tag :pending
  test "simple nested brackets" do
    assert MatchingBrackets.check_brackets("{[]}")
  end

  @tag :pending
  test "several paired brackets" do
    assert MatchingBrackets.check_brackets("{}[]")
  end

  @tag :pending
  test "paired and nested brackets" do
    assert MatchingBrackets.check_brackets("([{}({}[])])")
  end

  @tag :pending
  test "unopened closing brackets" do
    refute MatchingBrackets.check_brackets("{[)][]}")
  end

  @tag :pending
  test "unpaired and nested brackets" do
    refute MatchingBrackets.check_brackets("([{])")
  end

  @tag :pending
  test "paired and wrong nested brackets" do
    refute MatchingBrackets.check_brackets("[({]})")
  end

  @tag :pending
  test "math expression" do
    assert MatchingBrackets.check_brackets("(((185 + 223.85) * 15) - 543)/2")
  end

  @tag :pending
  test "complex latex expression" do
    assert MatchingBrackets.check_brackets(
             "\\left(\\begin{array}{cc} \\frac{1}{3} & x\\\\ \\mathrm{e}^{x} &... x^2 \\end{array}\\right)"
           )
  end
end

test_helper.exs

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule BracketPush do
  @doc """
  Checks that all the brackets and braces in the string are matched correctly, and nested correctly
  """
  @spec check_brackets(String.t) :: boolean
  def check_brackets(str) do
    do_check_brackets [],
                      (str |> String.split("", trim: true)
                           |> Enum.filter(&is_bracket?/1))
  end

  defp do_check_brackets(stack, []), do: Enum.empty? stack

  defp do_check_brackets([], [next_char | rest]) do
    do_check_brackets [next_char], rest
  end

  defp do_check_brackets([prior_char | more_stack],
                         [this_char  | more_chars]) do
    cond do
      is_match?(prior_char, this_char) ->
        do_check_brackets more_stack, more_chars
      is_opener?(prior_char) && is_closer?(this_char) ->
        false
      true ->
        do_check_brackets [this_char | [prior_char | more_stack]], more_chars
    end
  end

  @openers  "([{"
  @closers  "}])"  # does not need to align with openers; we sort when needed
  @brackets "#{@openers}#{@closers}"
  @pairs    @brackets |> String.split("", trim: true)
                      |> Enum.sort
                      |> Enum.chunk(2)
                      |> Enum.map(&Enum.join/1)

  defp is_bracket?(char), do: String.contains?(@brackets, char)

  defp is_match?(prior_char, this_char) do
    Enum.member?(@pairs, "#{prior_char}#{this_char}")
  end

  defp is_opener?(char), do: String.contains?(@openers, char)

  defp is_closer?(char), do: String.contains?(@closers, char)

end

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