Published at Sep 19 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`

.

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.

```
(ns binary-test
(:require [clojure.test :refer [deftest is]]
binary))
(deftest binary-1-is-decimal-1
(is (= 1 (binary/to-decimal "1"))))
(deftest binary-10-is-decimal-2
(is (= 2 (binary/to-decimal "10"))))
(deftest binary-11-is-decimal-3
(is (= 3 (binary/to-decimal "11"))))
(deftest binary-100-is-decimal-4
(is (= 4 (binary/to-decimal "100"))))
(deftest binary-1001-is-decimal-9
(is (= 9 (binary/to-decimal "1001"))))
(deftest binary-11010-is-decimal-26
(is (= 26 (binary/to-decimal "11010"))))
(deftest binary-10001101000-is-decimal-1128
(is (= 1128 (binary/to-decimal "10001101000"))))
(deftest invalid-binary-is-decimal-0
(is (= 0 (binary/to-decimal "carrot"))))
```

```
(ns binary)
;; I wrote this function to split a number in its digits but then I
;; saw that the tests all use strings. So this is not used.
;; (defn digits [n]
;; (loop [n n digits '()]
;; (if (zero? n)
;; digits
;; (recur (quot n 10) (conj digits (mod n 10))))))
(defn pow [x n]
(int (Math/pow x n)))
(defn parse-digits [n]
(try
(doall (map #(Long/parseLong (str %)) n))
(catch NumberFormatException _)))
(defn to-decimal [n]
(let [digits (parse-digits n)]
(reduce + (map-indexed #(* %2 (pow 2 %1)) (reverse digits)))))
```

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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