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to Trinary in the Common Lisp Track

Published at Mar 01 2020 · 0 comments
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Convert a trinary number, represented as a string (e.g. '102012'), to its decimal equivalent using first principles.

The program should consider strings specifying an invalid trinary as the value 0.

Trinary numbers contain three symbols: 0, 1, and 2.

The last place in a trinary number is the 1's place. The second to last is the 3's place, the third to last is the 9's place, etc.

# "102012"
    1       0       2       0       1       2    # the number
1*3^5 + 0*3^4 + 2*3^3 + 0*3^2 + 1*3^1 + 2*3^0    # the value
  243 +     0 +    54 +     0 +     3 +     2 =  302

If your language provides a method in the standard library to perform the conversion, pretend it doesn't exist and implement it yourself.


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

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(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:

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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-

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 "trinary")

(defpackage #:trinary-test
  (:use #:common-lisp #:lisp-unit))

(in-package #:trinary-test)

(define-test trinary-1-is-decimal-1
  (assert-equal 1 (trinary:to-decimal "1")))

(define-test trinary-2-is-decimal-2
  (assert-equal 2 (trinary:to-decimal "2")))

(define-test trinary-10-is-decimal-3
  (assert-equal 3 (trinary:to-decimal "10")))

(define-test trinary-11-is-decimal-4
  (assert-equal 4 (trinary:to-decimal "11")))

(define-test trinary-100-is-decimal-9
  (assert-equal 9 (trinary:to-decimal "100")))

(define-test trinary-112-is-decimal-14
  (assert-equal 14 (trinary:to-decimal "112")))

(define-test trinary-222-is-decimal-26

  (assert-equal 26 (trinary:to-decimal "222")))

(define-test trinary-1122000120-is-decimal-32091
  (assert-equal 32091 (trinary:to-decimal "1122000120")))

(define-test invalid-input-is-decimal-0
  (assert-equal 0 (trinary:to-decimal "carrot")))

(define-test invalid-input-with-digits-is-decimal-0
  (assert-equal 0 (trinary:to-decimal "0a1b2c")))

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

(in-package #:trinary)

(defun to-decimal (str)
     :for sym :across (reverse str)
     :for order :from 0
     :while (char<= #\0 sym #\2)
     :summing (* (expt 3 order) (- (char-code sym) 48)) :into ret
     :finally (return (or ret 0))))

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