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Published at Dec 31 2020
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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`

.

Execute the tests with:

```
$ mix test
```

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.

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.

```
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
```

```
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
codepoints = string |> String.reverse() |> String.codepoints()
valid? = codepoints |> Enum.all?(fn n -> n == "1" or n == "0" end)
case valid? do
false ->
0
true ->
codepoints
|> Enum.reduce({0, 0}, fn x, {acc, index} ->
d = x |> Integer.parse() |> elem(0)
{acc + d * :math.pow(2, index), index + 1}
end)
|> elem(0)
end
end
end
```

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?
- Are there new concepts here that you could read more about to improve your understanding?

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