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# laszpio's solution

## to Binary in the Elixir Track

Published at May 01 2020 · 0 comments
Instructions
Test suite
Solution

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.

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 0 is decimal 0" do
assert Binary.to_decimal("0") == 0
end

@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 "binary ignores leading zeros" do
assert Binary.to_decimal("000011111") == 31
end

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

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

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

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

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

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

defp to_decimal([], acc), do: acc

defp to_decimal([?0 | t], acc), do: to_decimal(t, acc * 2)

defp to_decimal([?1 | t], acc), do: to_decimal(t, 1 + acc * 2)

defp to_decimal(_, _), do: 0
end``````