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to Hexadecimal in the Elixir Track

Published at Aug 25 2019 · 0 comments
Instructions
Test suite
Solution

Convert a hexadecimal number, represented as a string (e.g. "10af8c"), to its decimal equivalent using first principles (i.e. no, you may not use built-in or external libraries to accomplish the conversion).

On the web we use hexadecimal to represent colors, e.g. green: 008000, teal: 008080, navy: 000080).

The program should handle invalid hexadecimal strings.

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

All of Computer Science http://www.wolframalpha.com/examples/NumberBases.html

Submitting Incomplete Solutions

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

hexadecimal_test.exs

defmodule HexadecimalTest do
  use ExUnit.Case

  test "returns 1 when hex is 1" do
    assert Hexadecimal.to_decimal("1") == 1
  end

  @tag :pending
  test "returns 12 when hex is c" do
    assert Hexadecimal.to_decimal("c") == 12
  end

  @tag :pending
  test "hexadecimal is case insensitive" do
    assert Hexadecimal.to_decimal("C") == 12
  end

  @tag :pending
  test "returns 16 when hex is 10" do
    assert Hexadecimal.to_decimal("10") == 16
  end

  @tag :pending
  test "returns 175 when hex is af" do
    assert Hexadecimal.to_decimal("af") == 175
  end

  @tag :pending
  test "returns 256 when hex is 100" do
    assert Hexadecimal.to_decimal("100") == 256
  end

  @tag :pending
  test "returns 105_166 when hex is 19ace" do
    assert Hexadecimal.to_decimal("19ace") == 105_166
  end

  @tag :pending
  test "returns 0 when hex is invalid" do
    assert Hexadecimal.to_decimal("carrot") == 0
  end

  @tag :pending
  test "returns 0 when hex represents black" do
    assert Hexadecimal.to_decimal("000000") == 0
  end

  @tag :pending
  test "returns 16_777_215 when hex represents white" do
    assert Hexadecimal.to_decimal("ffffff") == 16_777_215
  end

  @tag :pending
  test "returns 16_776_960 when hex represents yellow" do
    assert Hexadecimal.to_decimal("ffff00") == 16_776_960
  end
end

test_helper.exs

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)
defmodule Hexadecimal do
  @hex_digits '0123456789ABCDEF'
  @hex_string ~r/\A[#{@hex_digits}]+\z/
  @base 16
  @invalid_value 0

  @doc """
    Accept a string representing a hexadecimal value and returns the
    corresponding decimal value.
    It returns the integer 0 if the hexadecimal is invalid.
    Otherwise returns an integer representing the decimal value.

    ## Examples

      iex> Hexadecimal.to_decimal("invalid")
      0

      iex> Hexadecimal.to_decimal("af")
      175

  """
  @spec to_decimal(binary) :: integer
  def to_decimal(hex) do
    hex = String.upcase(hex)

    if String.match?(hex, @hex_string) do
      hex
      |> String.to_charlist()
      |> Enum.reverse()
      |> Enum.with_index()
      |> Enum.reduce(0, &add_hex_value/2)
    else
      @invalid_value
    end
  end

  defp add_hex_value({hex_char, index}, acc) do
    acc +
      Enum.find_index(@hex_digits, &(&1 == hex_char)) *
        :math.pow(@base, index)
  end
end

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