Published at Nov 18 2018
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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.

- 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
*n*th 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`

.

Follow the setup instructions for Crystal here:

http://exercism.io/languages/crystal

More help installing can be found here:

http://crystal-lang.org/docs/installation/index.html

Execute the tests with:

```
$ crystal spec
```

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing `pending`

to `it`

.

All of Computer Science http://www.wolframalpha.com/input/?i=binary&a=*C.binary-_*MathWorld-

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

```
require "spec"
require "../src/*"
describe "Binary" do
it "binary 0 is decimal 0" do
Binary.to_decimal("0").should eq(0)
end
pending "binary 1 is decimal 1" do
Binary.to_decimal("1").should eq(1)
end
pending "binary 10 is decimal 2" do
Binary.to_decimal("10").should eq(2)
end
pending "binary 11 is decimal 3" do
Binary.to_decimal("11").should eq(3)
end
pending "binary 100 is decimal 4" do
Binary.to_decimal("100").should eq(4)
end
pending "binary 1001 is decimal 9" do
Binary.to_decimal("1001").should eq(9)
end
pending "binary 11010 is decimal 26" do
Binary.to_decimal("11010").should eq(26)
end
pending "binary 10001101000 is decimal 1128" do
Binary.to_decimal("10001101000").should eq(1128)
end
pending "binary ignores leading zeros" do
Binary.to_decimal("000011111").should eq(31)
end
pending "2 is not a valid binary digit" do
expect_raises(ArgumentError) do
Binary.to_decimal("2")
end
end
pending "a number containing a non-binary digit is invalid" do
expect_raises(ArgumentError) do
Binary.to_decimal("01201")
end
end
pending "a number with trailing non-binary characters is invalid" do
expect_raises(ArgumentError) do
Binary.to_decimal("10nope")
end
end
pending "a number with leading non-binary characters is invalid" do
expect_raises(ArgumentError) do
Binary.to_decimal("nope10")
end
end
pending "a number with internal non-binary characters is invalid" do
expect_raises(ArgumentError) do
Binary.to_decimal("10nope10")
end
end
pending "a number and a word whitespace separated is invalid" do
expect_raises(ArgumentError) do
Binary.to_decimal("001 nope")
end
end
end
```

```
require "big"
module Binary
extend self
def to_decimal(si : String) : BigInt
si.each_char.reduce(BigInt.new(0)) { |b, c|
case c
when '1'
(b << 1) + 1
when '0'
b << 1
else
raise ArgumentError.new
end
}
end
end
```

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?

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