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to Binary 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.

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:

$ elixir binary_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

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

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

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

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
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
    string
    |> String.graphemes
    |> do_binary(0)
  end

  defp do_binary(["0"|more], acc), do: do_binary(more, acc * 2)
  defp do_binary(["1"|more], acc), do: do_binary(more, acc * 2 + 1)
  defp do_binary([],         acc), do: acc
  defp do_binary(_,          _  ), do: 0

end

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