# rabuf's solution

## to Trinary in the Common Lisp Track

Published at Jan 21 2020 · 0 comments
Instructions
Test suite
Solution

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.

## Setup

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

## Formatting

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.

### VIM

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

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.

## 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.

### trinary-test.lisp

``````(ql:quickload "lisp-unit")

(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")))

#-xlisp-test
(let ((*print-errors* t)
(*print-failures* t))
(run-tests :all :trinary-test))``````
``````(unless (find-package :iterate)
(defpackage #:trinary
(:use #:common-lisp #:iterate)
(:export #:to-decimal))

(in-package #:trinary)

(defun trinary-char->number (c)
(ecase c
(#\0 0)
(#\1 1)
(#\2 2)))

(defun to-decimal (str)
(handler-case
(iter (for c in-string (reverse str))
(for power from 0)
(sum (* (trinary-char->number c)
(expt 3 power))))
(sb-kernel:case-failure (err) 0)))``````

### rabuf's Reflection

Not the best way, perhaps, but I decided to handle junk input with `ecase` and handling the raised error.