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collection-functions
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Collection Functions</title>
<link rel=stylesheet href="site.css">
</head>
<script>
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<body>
<div class="example" id="collection-functions">
<h2><a href="./">Go by Example</a>: Collection Functions</h2>
<table>
<tr>
<td class="docs">
<p>We often need our programs to perform operations on
collections of data, like selecting all items that
satisfy a given predicate or mapping all items to a new
collection with a custom function.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
<p>In some languages it’s idiomatic to use <a href="http://en.wikipedia.org/wiki/Generic_programming">generic</a>
data structures and algorithms. Go does not support
generics; in Go it’s common to provide collection
functions if and when they are specifically needed for
your program and data types.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
<p>Here are some example collection functions for slices
of <code>strings</code>. You can use these examples to build your
own functions. Note that in some cases it may be
clearest to just inline the collection-manipulating
code directly, instead of creating and calling a
helper function.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/uKnePZM91WV"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"fmt"</span>
<span class="s">"strings"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Index returns the first index of the target string <code>t</code>, or
-1 if no match is found.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">Index</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">t</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">int</span> <span class="p">{</span>
<span class="k">for</span> <span class="nx">i</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">vs</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">v</span> <span class="o">==</span> <span class="nx">t</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">i</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="o">-</span><span class="mi">1</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Include returns <code>true</code> if the target string t is in the
slice.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">Include</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">t</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">return</span> <span class="nf">Index</span><span class="p">(</span><span class="nx">vs</span><span class="p">,</span> <span class="nx">t</span><span class="p">)</span> <span class="o">>=</span> <span class="mi">0</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Any returns <code>true</code> if one of the strings in the slice
satisfies the predicate <code>f</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">Any</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">f</span> <span class="kd">func</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">vs</span> <span class="p">{</span>
<span class="k">if</span> <span class="nf">f</span><span class="p">(</span><span class="nx">v</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="kc">true</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="kc">false</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>All returns <code>true</code> if all of the strings in the slice
satisfy the predicate <code>f</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">All</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">f</span> <span class="kd">func</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">vs</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">!</span><span class="nf">f</span><span class="p">(</span><span class="nx">v</span><span class="p">)</span> <span class="p">{</span>
<span class="k">return</span> <span class="kc">false</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="kc">true</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Filter returns a new slice containing all strings in the
slice that satisfy the predicate <code>f</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">Filter</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">f</span> <span class="kd">func</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span><span class="p">)</span> <span class="p">[]</span><span class="kt">string</span> <span class="p">{</span>
<span class="nx">vsf</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">string</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">vs</span> <span class="p">{</span>
<span class="k">if</span> <span class="nf">f</span><span class="p">(</span><span class="nx">v</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">vsf</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">vsf</span><span class="p">,</span> <span class="nx">v</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">vsf</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Map returns a new slice containing the results of applying
the function <code>f</code> to each string in the original slice.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">Map</span><span class="p">(</span><span class="nx">vs</span> <span class="p">[]</span><span class="kt">string</span><span class="p">,</span> <span class="nx">f</span> <span class="kd">func</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="kt">string</span><span class="p">)</span> <span class="p">[]</span><span class="kt">string</span> <span class="p">{</span>
<span class="nx">vsm</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">string</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">vs</span><span class="p">))</span>
<span class="k">for</span> <span class="nx">i</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">vs</span> <span class="p">{</span>
<span class="nx">vsm</span><span class="p">[</span><span class="nx">i</span><span class="p">]</span> <span class="p">=</span> <span class="nf">f</span><span class="p">(</span><span class="nx">v</span><span class="p">)</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">vsm</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Here we try out our various collection functions.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">var</span> <span class="nx">strs</span> <span class="p">=</span> <span class="p">[]</span><span class="kt">string</span><span class="p">{</span><span class="s">"peach"</span><span class="p">,</span> <span class="s">"apple"</span><span class="p">,</span> <span class="s">"pear"</span><span class="p">,</span> <span class="s">"plum"</span><span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">Index</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="s">"pear"</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">Include</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="s">"grape"</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">Any</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="kd">func</span><span class="p">(</span><span class="nx">v</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">strings</span><span class="p">.</span><span class="nf">HasPrefix</span><span class="p">(</span><span class="nx">v</span><span class="p">,</span> <span class="s">"p"</span><span class="p">)</span>
<span class="p">}))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">All</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="kd">func</span><span class="p">(</span><span class="nx">v</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">strings</span><span class="p">.</span><span class="nf">HasPrefix</span><span class="p">(</span><span class="nx">v</span><span class="p">,</span> <span class="s">"p"</span><span class="p">)</span>
<span class="p">}))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">Filter</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="kd">func</span><span class="p">(</span><span class="nx">v</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">bool</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">strings</span><span class="p">.</span><span class="nf">Contains</span><span class="p">(</span><span class="nx">v</span><span class="p">,</span> <span class="s">"e"</span><span class="p">)</span>
<span class="p">}))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The above examples all used anonymous functions,
but you can also use named functions of the correct
type.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">Map</span><span class="p">(</span><span class="nx">strs</span><span class="p">,</span> <span class="nx">strings</span><span class="p">.</span><span class="nx">ToUpper</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code">
<pre class="chroma"><span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code">
<pre class="chroma"><span class="gp">$</span> go run collection-functions.go
<span class="go">2
</span><span class="go">false
</span><span class="go">true
</span><span class="go">false
</span><span class="go">[peach apple pear]
</span><span class="go">[PEACH APPLE PEAR PLUM]</span></pre>
</td>
</tr>
</table>
<p class="next">
Next example: <a href="string-functions">String Functions</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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