<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="">
<meta name="author" content="Loïc Hoguin based on a design from (Soft10) Pol Cámara">
<title>Nine Nines: Erlang Tic Tac Toe</title>
<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,700,400italic' rel='stylesheet' type='text/css'>
<link href="/css/99s.css?r=7" rel="stylesheet">
<link rel="shortcut icon" href="/img/ico/favicon.ico">
<link rel="apple-touch-icon-precomposed" sizes="114x114" href="/img/ico/apple-touch-icon-114.png">
<link rel="apple-touch-icon-precomposed" sizes="72x72" href="/img/ico/apple-touch-icon-72.png">
<link rel="apple-touch-icon-precomposed" href="/img/ico/apple-touch-icon-57.png">
</head>
<body class="">
<header id="page-head">
<div id="topbar" class="container">
<div class="row">
<div class="span2">
<h1 id="logo"><a href="/" title="99s">99s</a></h1>
</div>
<div class="span10">
<div id="side-header">
<nav>
<ul>
<li class="active"><a title="Hear my thoughts" href="/articles">Articles</a></li>
<li><a title="Watch my talks" href="/talks">Talks</a></li>
<li><a title="Read the docs" href="/docs">Documentation</a></li>
<li><a title="Request my services" href="/services">Consulting & Training</a></li>
</ul>
</nav>
<ul id="social">
<li>
<a href="https://github.com/ninenines" title="Check my Github repositories"><img src="/img/ico_github.png" data-hover="/img/ico_github_alt.png" alt="Github"></a>
</li>
<li>
<a title="Contact me" href="mailto:[email protected]"><img src="/img/ico_mail.png" data-hover="/img/ico_mail_alt.png"></a>
</li>
</ul>
</div>
</div>
</div>
</div>
</header>
<div id="contents">
<div class="container">
<div class="row">
<div class="span9 maincol">
<article class="blog_item">
<header>
<h1 class="lined-header"><span>Erlang Tic Tac Toe</span></h1>
<p class="date">
<span class="year">2012</span>
<span class="day-month">17 Oct</span>
</p>
</header>
<p>Everyone knows <a href="http://en.wikipedia.org/wiki/Tic-tac-toe">Tic Tac Toe</a>, right?</p>
<p>Players choose either to be the Xs or the Os, then place their symbol on a 3x3 board one after another, trying to create a line of 3 of them.</p>
<p>Writing an algorithm to check for victory sounds easy, right? It's easily tested, considering there's only 8 possible winning rows (3 horizontal, 3 vertical and 2 diagonal).</p>
<p>In Erlang though, you probably wouldn't want an algorithm. Erlang has this cool feature called pattern matching which will allow us to completely avoid writing the algorithm by instead letting us match directly on the solutions.</p>
<p>Let's first create a board. A board is a list of 3 rows each containing 3 columns. It can also be thought of as a tuple containing 9 elements. A tuple is easier to manipulate so this is what we are going to use. Each position can either contain an <code>x</code>, an <code>o</code>, or be <code>undefined</code>.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">new</font></b>() <font color="#990000">-></font>
{<font color="#000080">undefined</font>, <font color="#000080">undefined</font>, <font color="#000080">undefined</font>,
<font color="#000080">undefined</font>, <font color="#000080">undefined</font>, <font color="#000080">undefined</font>,
<font color="#000080">undefined</font>, <font color="#000080">undefined</font>, <font color="#000080">undefined</font>}<font color="#990000">.</font></tt></pre>
</div></div>
<p>Now that we have a board, if we want to play, we need a function that will allow players to, you know, actually play their moves. Rows and columns are numbered 1 to 3 so we need a little math to correctly deduce the element's position.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">play</font></b>(<font color="#009900">Who</font>, <font color="#009900">X</font>, <font color="#009900">Y</font>, <font color="#009900">Board</font>) <font color="#990000">-></font>
<b><font color="#000080">setelement</font></b>((<font color="#009900">Y</font> <font color="#990000">-</font> <font color="#993399">1</font>) <font color="#990000">*</font> <font color="#993399">3</font> <font color="#990000">+</font> <font color="#009900">X</font>, <font color="#009900">Board</font>, <font color="#009900">Who</font>)<font color="#990000">.</font></tt></pre>
</div></div>
<p>This function returns the board with the element modified. Of course, as you probably noticed, we aren't checking that the arguments are correct, or that the element was already set. This is left as an exercise to the reader.</p>
<p>After playing the move, we need to check whether someone won. That's where you'd write an algorithm, and that's where I wouldn't. Let's just pattern match all of them!</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.9
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">check</font></b>(<font color="#009900">Board</font>) <font color="#990000">-></font>
<b><font color="#0000FF">case</font></b> <font color="#009900">Board</font> <b><font color="#0000FF">of</font></b>
{<font color="#FF6600">x</font>, <font color="#FF6600">x</font>, <font color="#FF6600">x</font>,
<font color="#990000">_</font>, <font color="#990000">_</font>, <font color="#990000">_</font>,
<font color="#990000">_</font>, <font color="#990000">_</font>, <font color="#990000">_</font>} <font color="#990000">-></font> {<font color="#FF6600">victory</font>, <font color="#FF6600">x</font>};
{<font color="#FF6600">x</font>, <font color="#990000">_</font>, <font color="#990000">_</font>,
<font color="#990000">_</font>, <font color="#FF6600">x</font>, <font color="#990000">_</font>,
<font color="#990000">_</font>, <font color="#990000">_</font>, <font color="#FF6600">x</font>} <font color="#990000">-></font> {<font color="#FF6600">victory</font>, <font color="#FF6600">x</font>};
{<font color="#FF6600">x</font>, <font color="#990000">_</font>, <font color="#990000">_</font>,
<font color="#FF6600">x</font>, <font color="#990000">_</font>, <font color="#990000">_</font>,
<font color="#FF6600">x</font>, <font color="#990000">_</font>, <font color="#990000">_</font>} <font color="#990000">-></font> {<font color="#FF6600">victory</font>, <font color="#FF6600">x</font>};
<i><font color="#9A1900">%% [snip]</font></i>
<font color="#990000">_</font> <font color="#990000">-></font> <font color="#FF6600">ok</font>
<b><font color="#0000FF">end</font></b><font color="#990000">.</font></tt></pre>
</div></div>
<p>Pattern matching allows us to simply <em>draw</em> the solutions directly inside our code, and if the board matches any of them, then we have a victory or a draw, otherwise the game can continue.</p>
<p>The <code>_</code> variable is special in that it always matches, allowing us to focus strictly on the winning row. And because it's very graphical, if we were to have messed up somewhere, then we'd only need take a quick glance to be sure the winning solutions are the right ones.</p>
<p>Erlang allows us to transform algorithms into very graphical code thanks to its pattern matching feature, and let us focus on doing things instead of writing algorithms to do things.</p>
<ul><li><a href="/res/tictactoe.erl">tictactoe.erl</a>
</li>
</ul>
</article>
</div>
<div class="span3 sidecol">
<h3>More articles</h3>
<ul id="articles-nav" class="extra_margin">
<li><a href="https://ninenines.eu/articles/merry-christmas-2019/">Merry Christmas 2019: New Beginnings</a></li>
<li><a href="https://ninenines.eu/articles/github-sponsors/">GitHub Sponsors</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.7.0/">Cowboy 2.7</a></li>
<li><a href="https://ninenines.eu/articles/gun-2.0.0-pre.1/">Gun 2.0 pre-release 1</a></li>
<li><a href="https://ninenines.eu/articles/erlang-meetup-10-septembre-2019/">Erlang meetup: 10 septembre 2019</a></li>
<li><a href="https://ninenines.eu/articles/ranch-2.0.0-rc.1/">Ranch 2.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/joe_the_rubber_duck/">Joe Armstrong the rubber duck</a></li>
<li><a href="https://ninenines.eu/articles/merry-christmas-2018/">Merry Christmas 2018: A Recap</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.6.0/">Cowboy 2.6</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.7.0/">Ranch 1.7</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.5.0/">Cowboy 2.5</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.3.0/">Gun 1.3</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.2.0/">Gun 1.2</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.6.0/">Ranch 1.6</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.0.0/">Gun 1.0</a></li>
<li><a href="https://ninenines.eu/articles/asciideck/">Asciideck: Asciidoc for Erlang</a></li>
<li><a href="https://ninenines.eu/articles/gun-1.0.0-rc.1/">Gun 1.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.4.0/">Cowboy 2.4</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.3.0/">Cowboy 2.3</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.2.0/">Cowboy 2.2</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.1.0/">Cowboy 2.1</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0/">Cowboy 2.0</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-rc.2/">Cowboy 2.0 release candidate 2</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-rc.1/">Cowboy 2.0 release candidate 1</a></li>
<li><a href="https://ninenines.eu/articles/the-elephant-in-the-room/">The elephant in the room</a></li>
<li><a href="https://ninenines.eu/articles/dont-let-it-crash/">Don't let it crash</a></li>
<li><a href="https://ninenines.eu/articles/cowboy-2.0.0-pre.4/">Cowboy 2.0 pre-release 4</a></li>
<li><a href="https://ninenines.eu/articles/ranch-1.3/">Ranch 1.3</a></li>
<li><a href="https://ninenines.eu/articles/ml-archives/">Mailing list archived</a></li>
<li><a href="https://ninenines.eu/articles/website-update/">Website update</a></li>
<li><a href="https://ninenines.eu/articles/erlanger-playbook-september-2015-update/">The Erlanger Playbook September 2015 Update</a></li>
<li><a href="https://ninenines.eu/articles/erlanger-playbook/">The Erlanger Playbook</a></li>
<li><a href="https://ninenines.eu/articles/erlang-validate-utf8/">Validating UTF-8 binaries with Erlang</a></li>
<li><a href="https://ninenines.eu/articles/on-open-source/">On open source</a></li>
<li><a href="https://ninenines.eu/articles/the-story-so-far/">The story so far</a></li>
<li><a href="https://ninenines.eu/articles/cowboy2-qs/">Cowboy 2.0 and query strings</a></li>
<li><a href="https://ninenines.eu/articles/january-2014-status/">January 2014 status</a></li>
<li><a href="https://ninenines.eu/articles/farwest-funded/">Farwest got funded!</a></li>
<li><a href="https://ninenines.eu/articles/erlang.mk-and-relx/">Build Erlang releases with Erlang.mk and Relx</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.5-intermediate-module/">Xerl: intermediate module</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.4-expression-separator/">Xerl: expression separator</a></li>
<li><a href="https://ninenines.eu/articles/erlang-scalability/">Erlang Scalability</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.3-atomic-expressions/">Xerl: atomic expressions</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.2-two-modules/">Xerl: two modules</a></li>
<li><a href="https://ninenines.eu/articles/xerl-0.1-empty-modules/">Xerl: empty modules</a></li>
<li><a href="https://ninenines.eu/articles/ranch-ftp/">Build an FTP Server with Ranch in 30 Minutes</a></li>
<li><a href="https://ninenines.eu/articles/tictactoe/">Erlang Tic Tac Toe</a></li>
</ul>
<h3>Feedback</h3>
<p>Feel free to <a href="mailto:[email protected]">email us</a>
if you found any mistake or need clarification on any of the
articles.</p>
</div>
</div>
</div>
</div>
<footer>
<div class="container">
<div class="row">
<div class="span6">
<p id="scroll-top"><a href="#">↑ Scroll to top</a></p>
<nav>
<ul>
<li><a href="mailto:[email protected]" title="Contact us">Contact us</a></li><li><a href="https://github.com/ninenines/ninenines.github.io" title="Github repository">Contribute to this site</a></li>
</ul>
</nav>
</div>
<div class="span6 credits">
<p><img src="/img/footer_logo.png"></p>
<p>Copyright © Loïc Hoguin 2012-2018</p>
</div>
</div>
</div>
</footer>
<script src="/js/custom.js"></script>
</body>
</html>
