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to Binary in the Common Lisp Track

Published at Jan 05 2020 · 0 comments
Test suite

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.

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.


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


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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.


(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")))

(let ((*print-errors* t)
      (*print-failures* t))
  (run-tests :all))
(defpackage #:binary
  (:use :common-lisp)
  (:export :to-decimal))

(in-package :binary)

(defun range (end)
  (loop for n from 0 below end by 1
     collect n))

(defun to-digits (number)
  (loop for c across number
     collect (digit-char-p c)))

(defun to-decimal (input)
  (let* ((digits (remove-if-not #'numberp (to-digits input)))
         (valid-digits (remove-if-not (lambda (x)
                                        (member x '(0 1))) (copy-seq digits))))
    (if (equal digits valid-digits)
        (reduce #'+
                (map 'list #'(lambda (idx itm) (* itm (expt 2 idx)))
                     (reverse (range (length digits)))

What can you learn from this solution?

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?