thekeele's solution to Binary on the Elixir track

Instructions

Test suite

Solution

Note:

This exercise has changed since this solution was written.

Convert a binary number, represented as a string (e.g. '101010'), to its decimal equivalent using first principles.

Implement binary to decimal conversion. Given a binary input string, your program should produce a decimal output. The program should handle invalid inputs.

Note

Implement the conversion yourself. Do not use something else to perform the conversion for you.

About Binary (Base-2)

Decimal is a base-10 system.

A number 23 in base 10 notation can be understood as a linear combination of powers of 10:

The rightmost digit gets multiplied by 10^0 = 1
The next number gets multiplied by 10^1 = 10
...
The nth number gets multiplied by 10^(n-1).
All these values are summed.

So: 23 => 2*10^1 + 3*10^0 => 2*10 + 3*1 = 23 base 10

Binary is similar, but uses powers of 2 rather than powers of 10.

So: 101 => 1*2^2 + 0*2^1 + 1*2^0 => 1*4 + 0*2 + 1*1 => 4 + 1 => 5 base 10.

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/input/?i=binary&a=*C.binary-_*MathWorld-

Submitting Incomplete Solutions

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

binary_test.exs

defmodule BinaryTest do
  use ExUnit.Case

  # @tag :pending
  test "binary 1 is decimal 1" do
    assert Binary.to_decimal("1") == 1
  end

  @tag :pending
  test "binary 10 is decimal 2" do
    assert Binary.to_decimal("10") == 2
  end

  @tag :pending
  test "binary 11 is decimal 3" do
    assert Binary.to_decimal("11") == 3
  end

  @tag :pending
  test "binary 100 is decimal 4" do
    assert Binary.to_decimal("100") == 4
  end

  @tag :pending
  test "binary 1001 is decimal 9" do
    assert Binary.to_decimal("1001") == 9
  end

  @tag :pending
  test "binary 11010 is decimal 26" do
    assert Binary.to_decimal("11010") == 26
  end

  @tag :pending
  test "binary 10001101000 is decimal 1128" do
    assert Binary.to_decimal("10001101000") == 1128
  end

  @tag :pending
  test "invalid binary is decimal 0" do
    assert Binary.to_decimal("carrot") == 0
  end

  @tag :pending
  test "invalid binary is decimal 0 II" do
    assert Binary.to_decimal("convert01") == 0
  end

  @tag :pending
  test "invalid binary is decimal 0 III" do
    assert Binary.to_decimal("10convert") == 0
  end

  @tag :pending
  test "invalid binary is decimal 0 IV" do
    assert Binary.to_decimal("1carrot0") == 0
  end

  @tag :pending
  test "22 is not a binary number" do
    assert Binary.to_decimal("22") == 0
  end
end

test_helper.exs

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

defmodule Binary do
  @spec to_decimal(String.t()) :: non_neg_integer
  def to_decimal(string) do
    binary? = Regex.match?(~r/^[10]*$/, string)
    if binary?, do: string |> String.reverse() |> binary_powers(), else: 0
  end

  defp binary_powers(binary, position \\ 0)
  defp binary_powers("", _),
    do: 0
  defp binary_powers("0" <> binary, position),
    do: binary_powers(binary, position + 1)
  defp binary_powers("1" <> binary, position),
    do: (1 * 2 |> :math.pow(position) |> round()) + binary_powers(binary, position + 1)
end

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

What compromises have been made?
Are there new concepts here that you could read more about to improve your understanding?

thekeele's solution

to Binary in the Elixir Track