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215 lines
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HTML
215 lines
9.7 KiB
HTML
<!DOCTYPE html>
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<html>
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<head>
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<meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
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<title>Maps – Wren</title>
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<link rel="stylesheet" type="text/css" href="style.css" />
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<link href='//fonts.googleapis.com/css?family=Source+Sans+Pro:400,700,400italic,700italic|Source+Code+Pro:400|Lato:400|Sanchez:400italic,400' rel='stylesheet' type='text/css'>
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the viewport. -->
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<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1"/>
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</head>
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<body id="top">
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<header>
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<div class="page">
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<div class="main-column">
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<h1><a href="./">wren</a></h1>
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<h2>a classy little scripting language</h2>
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</div>
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</div>
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</header>
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<div class="page">
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<nav class="big">
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<ul>
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<li><a href="getting-started.html">Getting Started</a></li>
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<li><a href="contributing.html">Contributing</a></li>
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</ul>
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<section>
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<h2>language guide</h2>
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<ul>
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<li><a href="syntax.html">Syntax</a></li>
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<li><a href="values.html">Values</a></li>
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<li><a href="lists.html">Lists</a></li>
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<li><a href="maps.html">Maps</a></li>
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<li><a href="method-calls.html">Method Calls</a></li>
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<li><a href="control-flow.html">Control Flow</a></li>
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<li><a href="variables.html">Variables</a></li>
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<li><a href="functions.html">Functions</a></li>
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<li><a href="classes.html">Classes</a></li>
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<li><a href="concurrency.html">Concurrency</a></li>
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<li><a href="error-handling.html">Error Handling</a></li>
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<li><a href="modularity.html">Modularity</a></li>
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</ul>
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</section>
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<section>
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<h2>reference</h2>
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<ul>
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<li><a href="modules">Modules</a></li>
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<li><a href="embedding">Embedding</a></li>
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<li><a href="performance.html">Performance</a></li>
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<li><a href="qa.html">Q & A</a></li>
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</ul>
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</section>
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</nav>
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<nav class="small">
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<table>
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<tr>
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<td><a href="getting-started.html">Getting Started</a></td>
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<td><a href="contributing.html">Contributing</a></td>
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</tr>
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<tr>
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<td colspan="2"><h2>language guide</h2></td>
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<td><h2>reference</h2></td>
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</tr>
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<tr>
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<td>
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<ul>
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<li><a href="syntax.html">Syntax</a></li>
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<li><a href="values.html">Values</a></li>
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<li><a href="lists.html">Lists</a></li>
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<li><a href="maps.html">Maps</a></li>
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<li><a href="method-calls.html">Method Calls</a></li>
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<li><a href="control-flow.html">Control Flow</a></li>
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</ul>
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</td>
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<td>
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<ul>
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<li><a href="variables.html">Variables</a></li>
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<li><a href="functions.html">Functions</a></li>
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<li><a href="classes.html">Classes</a></li>
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<li><a href="concurrency.html">Concurrency</a></li>
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<li><a href="error-handling.html">Error Handling</a></li>
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<li><a href="modularity.html">Modularity</a></li>
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</ul>
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</td>
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<td>
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<ul>
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<li><a href="modules">Modules</a></li>
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<li><a href="embedding">Embedding</a></li>
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<li><a href="performance.html">Performance</a></li>
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<li><a href="qa.html">Q & A</a></li>
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</ul>
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</td>
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</tr>
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</table>
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</nav>
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<main>
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<h1>Maps</h1>
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<p>A map is an <em>associative</em> collection. It holds a set of entries, each of which
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maps a <em>key</em> to a <em>value</em>. The same data structure has a variety of names in
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other languages: hash table, dictionary, association, table, etc. </p>
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<p>You can create a map by placing a series of comma-separated entries inside
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curly braces. Each entry is a key and a value separated by a colon: </p>
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<pre class="codehilite"><code class="language-wren">{
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"George": "Harrison",
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"John": "Lennon",
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"Paul": "McCartney",
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"Ringo": "Starr"
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}</code></pre>
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<p>This creates a map that associates the first name of each Beatle with his last
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name. Syntactically, in a map literal, keys can be any literal, a variable
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name, or a parenthesized expression. Values can be any expression. Here, we’re
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using string literals for both keys and values. </p>
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<p><em>Semantically</em>, values can be any object, and multiple keys may map to the same
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value. Keys have a few limitations. They must be one of the immutable built-in
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<a href="values.html">value types</a> in Wren. That means a number, string, range, bool, or <code>null</code>.
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You can also use a <a href="classes.html">class object</a> as a key. </p>
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<p>The reason for this limitation—and the reason maps are called “<em>hash</em>
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tables” in other languages—is that each key is used to generate a numeric
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<em>hash code</em>. This lets a map locate the value associated with a key in constant
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time, even in very large maps. Since Wren only knows how to hash certain
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built-in types, only those can be used as keys. </p>
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<h2>Adding entries <a href="#adding-entries" name="adding-entries" class="header-anchor">#</a></h2>
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<p>You add new key-value pairs to the map using the <a href="method-calls.html#subscripts">subscript operator</a>: </p>
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<pre class="codehilite"><code class="language-wren">var capitals = {}
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capitals["Georgia"] = "Atlanta"
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capitals["Idaho"] = "Boise"
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capitals["Maine"] = "Augusta"</code></pre>
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<p>If the key isn’t already present, this adds it and associates it with the given
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value. If the key is already there, this just replaces its value. </p>
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<h2>Looking up values <a href="#looking-up-values" name="looking-up-values" class="header-anchor">#</a></h2>
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<p>To find the value associated with some key, again you use your friend the
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subscript operator: </p>
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<pre class="codehilite"><code class="language-wren">System.print(capitals["Idaho"]) //> Boise</code></pre>
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<p>If the key is present, this returns its value. Otherwise, it returns <code>null</code>. Of
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course, <code>null</code> itself can also be used as a value, so seeing <code>null</code> here
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doesn’t necessarily mean the key wasn’t found. </p>
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<p>To tell definitively if a key exists, you can call <code>containsKey()</code>: </p>
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<pre class="codehilite"><code class="language-wren">var belief = {"nihilism": null}
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System.print(belief["nihilism"]) //> null (though key exists)
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System.print(belief["solipsism"]) //> null
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System.print(belief.containsKey("nihilism")) //> true
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System.print(belief.containsKey("solipsism")) //> false</code></pre>
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<p>You can see how many entries a map contains using <code>count</code>: </p>
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<pre class="codehilite"><code class="language-wren">System.print(capitals.count) //> 3</code></pre>
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<h2>Removing entries <a href="#removing-entries" name="removing-entries" class="header-anchor">#</a></h2>
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<p>To remove an entry from a map, call <code>remove()</code> and pass in the key for the
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entry you want to delete: </p>
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<pre class="codehilite"><code class="language-wren">capitals.remove("Maine")
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System.print(capitals.containsKey("Maine")) //> false</code></pre>
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<p>If the key was found, this returns the value that was associated with it: </p>
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<pre class="codehilite"><code class="language-wren">System.print(capitals.remove("Georgia")) //> Atlanta</code></pre>
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<p>If the key wasn’t in the map to begin with, <code>remove()</code> just returns <code>null</code>. </p>
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<p>If you want to remove <em>everything</em> from the map, like with <a href="lists.html">lists</a>, you call
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<code>clear()</code>: </p>
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<pre class="codehilite"><code class="language-wren">capitals.clear()
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System.print(capitals.count) //> 0</code></pre>
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<h2>Iterating over the contents <a href="#iterating-over-the-contents" name="iterating-over-the-contents" class="header-anchor">#</a></h2>
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<p>The subscript operator works well for finding values when you know the key
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you’re looking for, but sometimes you want to see everything that’s in the map.
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For that, map exposes two methods: <code>keys</code> and <code>values</code>. </p>
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<p>The first returns a <a href="modules/core/sequence.html">Sequence</a> that <a href="control-flow.html#the-iterator-protocol">iterates</a> over all of the keys in the
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map, and the second returns one that iterates over the values. </p>
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<p>If you want to see all of the key-value pairs in a map, the easiest way is to
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iterate over the keys and use each to look up its value: </p>
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<pre class="codehilite"><code class="language-wren">var birds = {
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"Arizona": "Cactus wren",
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"Hawaii": "Nēnē",
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"Ohio": "Northern Cardinal"
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}
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for (state in birds.keys) {
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System.print("The state bird of " + state + " is " + birds[state])
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}</code></pre>
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<p>This program prints the three states and their birds. However, the <em>order</em>
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that they are printed isn’t defined. Wren makes no promises about what order
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keys and values are iterated in when you use these methods. All it promises is
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that every entry will appear exactly once. </p>
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<p><a class="right" href="method-calls.html">Method Calls →</a>
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<a href="lists.html">← Lists</a> </p>
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</main>
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</div>
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<footer>
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<div class="page">
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<div class="main-column">
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<p>Wren lives
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<a href="https://github.com/munificent/wren">on GitHub</a>
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— Made with ❤ by
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<a href="http://journal.stuffwithstuff.com/">Bob Nystrom</a> and
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<a href="https://github.com/munificent/wren/blob/master/AUTHORS">friends</a>.
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</p>
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<div class="main-column">
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</div>
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</footer>
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</body>
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</html>
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