A map of arrays question

<p>So, I am confused. I am coming from a Python background and trying to learn Go. So, in Python I can do:</p> <pre><code>age_dict = {} age_dict[40] = [] age_dict[40].append(MyData(someone) age_dict[40].append(MyData(someonelse)) age_dict[2] = [MyData(kid)] </code></pre> <p>This would produce:</p> <pre><code>{2: [&lt;MyData object at 0x7fea79de96d0&gt;], 40: [&lt;MyData object at 0x7fea79de9ed0&gt;, &lt;MyData object at 0x7fea79de9e10&gt;]} </code></pre> <p>This would give me a dictionary (read: map) which has an integer as a key and has a list (read:array) of values. I would like to do something similar in Go, but I am just not getting it. </p> <pre><code>type MyData struct { name string age int8 } age_map = map[int][]MyData age_map[40].append(age_map[40], MyData{&#34;someone&#34;, 40}) age_map[40].append(age_map[40], MyData{&#34;someone&#34;, 40}) </code></pre> <p>So, what am I doing wrong? I am just not understanding how to do this - if it&#39;s possible at all. My real issue is understanding the whole:</p> <pre><code> map[int][]MyCustomType </code></pre> <p>Syntax. Can someone point me in the correct direction?</p> <hr/>**评论：**<br/><br/>anaerobic_lifeform: <pre><p>The new slice is given by the return value of &#34;append&#34;.</p> <p><a href="https://blog.golang.org/slices" rel="nofollow">https://blog.golang.org/slices</a></p></pre>Kraigius: <pre><p>To expand and to help avoid a common pitfall when starting go, a slice is basically a view on an array. 98% of the time when appending, you will want to assign the result back to the very same slice, this isn&#39;t a new variable. The view changed but it&#39;s using the same array in the back.</p> <p>When you append you add something at <code>s[len(s)]</code> of the view. If you start to assign the result of an append to a different slice you will end up overwriting values.</p> <p>Demonstration: <a href="https://play.golang.org/p/xlQZs7uyNf" rel="nofollow">https://play.golang.org/p/xlQZs7uyNf</a></p> <p>This might seems like I&#39;m throwing random knowledge that isn&#39;t related to OP problem at hand but it&#39;s such a common pitfall when starting using slice and it isn&#39;t well communicated that I just want to share it.</p></pre>TheMerovius: <pre><p>Does <a href="https://play.golang.org/p/ii7aB7uWMA" rel="nofollow">this</a> help a bit?</p></pre>TheMerovius: <pre><p>to explain:</p> <ul> <li><code>map[int][]MyData</code> is a type, read it as: &#34;<code>map</code> from <code>int</code> to slice of (<code>[]</code>) <code>MyData</code>&#34;</li> <li><code>make(T)</code> will create a new value of type <code>T</code> - in this case, above map. So it&#39;s equivalent to the <code>{}</code> from the first line of your python code, just statically typed</li> <li><code>age_map := …</code> declares a new variable with name <code>age_map</code> and assigns <code>…</code> to it. See <a href="https://golang.org/ref/spec#Short_variable_declarations" rel="nofollow">the spec</a> for details.</li> <li>In Go, <code>append</code> is not a method on slices (which is the correct equivalent of a python list; in Go, an array has a fixed length and is something different). Instead, there is a builtin function <code>append</code>, which takes a slice and some elements and returns the slice with the elements appended. So, it doesn&#39;t work in-place and you need to assign the return value to use it.</li> </ul></pre>petezhut: <pre><p>This did it. I was confused by the append syntax. Many, many thanks.</p></pre>