title: @dimen/activity error, trying to figure out why
body: <p>I am taking an android beginners course through udacity. I just copy and pasted this code for an app but it already has errors without any edits. It was supposed to work right away. The primary problem seems to be with these lines in the xml . . .</p>
  
  <pre><code>android:paddingBottom="@dimen/activity_vertical_margin"
  android:paddingLeft="@dimen/activity_horizontal_margin"
  android:paddingRight="@dimen/activity_horizontal_margin"
  android:paddingTop="@dimen/activity_vertical_margin"
  tools:context=".MainActivity"&gt;
  </code></pre>
  
  <p>I am unfamiliar with <code>@dimen/activity</code> so any help would be appreciated.</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:tools="http://schemas.android.com/tools"
      android:layout_width="match_parent"
      android:layout_height="match_parent"
      android:orientation="vertical"
      android:paddingBottom="@dimen/activity_vertical_margin"
      android:paddingLeft="@dimen/activity_horizontal_margin"
      android:paddingRight="@dimen/activity_horizontal_margin"
      android:paddingTop="@dimen/activity_vertical_margin"
      tools:context=".MainActivity"&gt;
  
      &lt;TextView
          android:id="@+id/menu_item_1"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="Mango sorbet"
          android:textAppearance="?android:textAppearanceMedium" /&gt;
  
      &lt;TextView
          android:id="@+id/menu_item_2"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_marginTop="8dp"
          android:text="Blueberry pie"
          android:textAppearance="?android:textAppearanceMedium"
          android:textSize="18sp" /&gt;
  
      &lt;TextView
          android:id="@+id/menu_item_3"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_marginTop="8dp"
          android:text="Chocolate lava cake"
          android:textAppearance="?android:textAppearanceMedium"
          android:textSize="18sp" /&gt;
  
      &lt;Button
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:layout_marginTop="8dp"
          android:onClick="printToLogs"
          android:text="Print menu to logs" /&gt;
  &lt;/LinearLayout&gt;
  </code></pre>