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# daveyarwood's solution

## to Binary in the Nim Track

Published at Jul 13 2018 · 1 comment
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 the tests

To compile and run the tests, just run the following in your exercise directory:

``````\$ nim c -r binary_test.nim
``````

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `\$EXERCISM_WORKSPACE/nim/binary` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Exercises Directory`.

For more detailed information about running tests, code style and linting, 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.nim

``````import unittest
import binary

suite "Binary":

test "binary 0 is decimal 0":
check binary("0") == 0

test "binary 1 is decimal 1":
check binary("1") == 1

test "binary 10 is decimal 2":
check binary("10") == 2

test "binary 11 is decimal 3":
check binary("11") == 3

test "binary 100 is decimal 4":
check binary("100") == 4

test "binary 1001 is decimal 9":
check binary("1001") == 9

test "binary 11010 is decimal 26":
check binary("11010") == 26

test "binary 10001101000 is decimal 1128":
check binary("10001101000") == 1128

test "binary ignores leading zeros":
check binary("000011111") == 31

test "2 is not a valid binary digit":
expect(ValueError):
discard binary("2")

test "a number containing a non-binary digit is invalid":
expect(ValueError):
discard binary("01201")

test "a number with trailing non-binary characters is invalid":
expect(ValueError):
discard binary("10nope")

test "a number with leading non-binary characters is invalid":
expect(ValueError):
discard binary("nope10")

test "a number with internal non-binary characters is invalid":
expect(ValueError):
discard binary("10nope10")

test "a number and a word whitespace spearated is invalid":
expect(ValueError):
discard binary("001 nope")``````
``````import algorithm
import math
import sequtils
import strutils

# I wish you could just cast a char to a string in Nim.
proc toString(ch: char): string =
repeat(ch, 1)

proc parseBinaryDigit(char: char): int =
if not {'0'..'1'}.contains(char):
raise newException(ValueError, "A binary digit must be 0 or 1.")

parseInt(char.toString())

proc binary*(str: string): int =
var sum = 0
var power = -1.0

for char in toSeq(str.items).reversed():
power = power + 1
var digit = parseBinaryDigit(char)
var digitValue = (digit.float * 2.pow(power)).int
sum = sum + digitValue
sum``````

## Community comments

Find this solution interesting? Ask the author a question to learn more.
Solution Author
commented over 4 years ago

I wanted to use foldr instead of reversing the seq of chars and iterating over them in a for loop. I couldn't figure out the weird expr syntax of foldr, though.

### What can you learn from this solution?

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