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to Binary in the Elixir Track

Published at May 01 2020 · 0 comments
Test suite

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.


  • 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!"

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.


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.


defmodule BinaryTest do
  use ExUnit.Case

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

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

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

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

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

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

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

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

  @tag :pending
  test "binary ignores leading zeros" do
    assert Binary.to_decimal("000011111") == 31

  @tag :pending
  test "2 is not a valid binary digit" do
    assert Binary.to_decimal("2") == 0

  @tag :pending
  test "a number containing a non-binary digit is invalid" do
    assert Binary.to_decimal("01201") == 0

  @tag :pending
  test "a number with trailing non-binary characters is invalid" do
    assert Binary.to_decimal("10nope") == 0

  @tag :pending
  test "a number with leading non-binary characters is invalid" do
    assert Binary.to_decimal("nope10") == 0

  @tag :pending
  test "a number with internal non-binary characters is invalid" do
    assert Binary.to_decimal("10nope10") == 0

  @tag :pending
  test "a number and a word whitespace separated is invalid" do
    assert Binary.to_decimal("001 nope") == 0

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

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

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

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

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


ExUnit.configure(exclude: :pending, trace: true)
defmodule Binary do
  @doc """
  Convert a string containing a binary number to an integer.

  On errors returns 0.
  @spec to_decimal(String.t()) :: non_neg_integer
  def to_decimal(string) do
    |> to_charlist()
    |> Enum.map(&(&1 - 48))
    |> Enum.reverse()
    |> do_to_decimal()

  defp do_to_decimal(binary, counter \\ 1, acc \\ 0)
  defp do_to_decimal([], _counter, acc), do: acc
  defp do_to_decimal([0 | rest], counter, acc), do: do_to_decimal(rest, counter * 2, acc)

  defp do_to_decimal([digit | rest], counter, acc) when digit in [0, 1],
    do: do_to_decimal(rest, counter * 2, acc + counter)

  defp do_to_decimal(_binary, _counter, _acc), do: 0

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