Published at Nov 27 2019
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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`

.

Check out Installing Common Lisp for instructions to get started or take a look at the guides available in the track's side bar.

While Common Lisp doesn't care about indentation and layout of code, nor whether you use spaces or tabs, this is an important consideration for submissions to exercism.io. Excercism.io's code widget cannot handle mixing of tab and space characters well so using only spaces is recommended to make the code more readable to the human reviewers. Please review your editors settings on how to accomplish this. Below are instructions for popular editors for Common Lisp.

Use the following commands to ensure VIM uses only spaces for indentation:

```
:set tabstop=2
:set shiftwidth=2
:set expandtab
```

(or as a oneliner `:set tabstop=2 shiftwidth=2 expandtab`

). This can
be added to your `~/.vimrc`

file to use it all the time.

Emacs is very well suited for editing Common Lisp and has many powerful add-on packages available. The only thing that one needs to do with a stock emacs to make it work well with exercism.io is to evaluate the following code:

`(setq-default indent-tabs-mode nil)`

This can be placed in your `~/.emacs`

(or `~/.emacs.d/init.el`

) in
order to have it set whenever Emacs is launched.

One suggested add-on for Emacs and Common Lisp is SLIME which offers tight integration with the REPL; making iterative coding and testing very easy.

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.

```
(ql:quickload "lisp-unit")
#-xlisp-test (load "binary")
(defpackage #:binary-test
(:use #:common-lisp #:lisp-unit))
(in-package #:binary-test)
(define-test binary-1-is-decimal-1
(assert-equal 1 (binary:to-decimal "1")))
(define-test binary-10-is-decimal-2
(assert-equal 2 (binary:to-decimal "10")))
(define-test binary-11-is-decimal-3
(assert-equal 3 (binary:to-decimal "11")))
(define-test binary-100-is-decimal-4
(assert-equal 4 (binary:to-decimal "100")))
(define-test binary-1001-is-decimal-9
(assert-equal 9 (binary:to-decimal "1001")))
(define-test binary-11010-is-decimal-26
(assert-equal 26 (binary:to-decimal "11010")))
(define-test binary-10001101000-is-decimal-1128
(assert-equal 1128 (binary:to-decimal "10001101000")))
(define-test invalid-binary-is-decimal-0
(assert-equal 0 (binary:to-decimal "carrot")))
(define-test invalid-characters-at-beginning
(assert-equal 2 (binary:to-decimal "a10")))
(define-test invalid-characters-at-end
(assert-equal 2 (binary:to-decimal "10a")))
(define-test invalid-characters-in-middle
(assert-equal 2 (binary:to-decimal "1a0")))
(define-test invalid-digits
(assert-equal 0 (binary:to-decimal "23")))
#-xlisp-test
(let ((*print-errors* t)
(*print-failures* t))
(run-tests :all))
```

```
(defpackage #:binary
(:use #:common-lisp)
(:export #:to-decimal))
(in-package #:binary)
(defun to-decimal (str)
(let ((pos 1))
(reduce
(lambda (s e)
(when (find e "01")
(incf s (* (digit-char-p e) pos))
(setf pos (* pos 2)))
s)
(reverse str)
:initial-value 0)))
```

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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