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# lujanfernaud's solution

## to Binary in the Ruby Track

Published at Jul 13 2018 · 3 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.

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`.

For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

``````gem install minitest
``````

If you would like color output, you can `require 'minitest/pride'` in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

``````ruby binary_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride binary_test.rb
``````

## 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.rb

``````require 'minitest/autorun'
require_relative 'binary'

# Common test data version: 1.0.0 969717d
class BinaryTest < Minitest::Test
def test_binary_0_is_decimal_0
# skip
assert_equal 0, Binary.to_decimal('0')
end

def test_binary_1_is_decimal_1
skip
assert_equal 1, Binary.to_decimal('1')
end

def test_binary_10_is_decimal_2
skip
assert_equal 2, Binary.to_decimal('10')
end

def test_binary_11_is_decimal_3
skip
assert_equal 3, Binary.to_decimal('11')
end

def test_binary_100_is_decimal_4
skip
assert_equal 4, Binary.to_decimal('100')
end

def test_binary_1001_is_decimal_9
skip
assert_equal 9, Binary.to_decimal('1001')
end

def test_binary_11010_is_decimal_26
skip
assert_equal 26, Binary.to_decimal('11010')
end

def test_binary_10001101000_is_decimal_1128
skip
assert_equal 1128, Binary.to_decimal('10001101000')
end

skip
assert_equal 31, Binary.to_decimal('000011111')
end

def test_2_is_not_a_valid_binary_digit
skip
assert_raises(ArgumentError) { Binary.to_decimal('2') }
end

def test_a_number_containing_a_non_binary_digit_is_invalid
skip
assert_raises(ArgumentError) { Binary.to_decimal('01201') }
end

def test_a_number_with_trailing_non_binary_characters_is_invalid
skip
assert_raises(ArgumentError) { Binary.to_decimal('10nope') }
end

skip
assert_raises(ArgumentError) { Binary.to_decimal('nope10') }
end

def test_a_number_with_internal_non_binary_characters_is_invalid
skip
assert_raises(ArgumentError) { Binary.to_decimal('10nope10') }
end

def test_a_number_and_a_word_whitespace_spearated_is_invalid
skip
assert_raises(ArgumentError) { Binary.to_decimal('001 nope') }
end

# Problems in exercism evolve over time, as we find better ways to ask
# questions.
# The version number refers to the version of the problem you solved,
#
# Define a constant named VERSION inside of the top level BookKeeping
# module, which may be placed near the end of your file.
#
# In your file, it will look like this:
#
# module BookKeeping
#   VERSION = 1 # Where the version number matches the one in the test.
# end
#
# http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html

def test_bookkeeping
skip
assert_equal 3, BookKeeping::VERSION
end
end``````
``````# Convert a binary number to its decimal equivalent.
class Binary
BINARY = /\A[0-1]+\z/

class << self
def to_decimal(binary)
raise ArgumentError, "#{binary} is not a correct binary number" unless
binary.match(BINARY)

binary = parse_and_reverse(binary)
powers = powers_for_each(binary)

convert(binary, powers)
end

def parse_and_reverse(binary)
binary.chars.map(&:to_i).reverse
end

def powers_for_each(binary)
binary.map.with_index { |_number, index| 2**index }
end

def convert(binary, powers)
binary.map.with_index do |number, index|
case number
when 0 then 0
when 1 then powers[index]
end
end.inject(:+)
end
end
end

module BookKeeping
VERSION = 3
end``````

Not sure if I prefer this one over the previous one. Maybe it's easier to read this way.

Don't forget to check out other implementations by clicking "See other implementations of" near the top of the page. You can see mine by clicking on the </> below this message as well.

Part of the value of Exercism is reading others code (and reviewing).

Thanks @kotp. I'll check it out.

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?