title: how to get index list in arrayList filter
body: <p>I want to get index list as follows.</p>
  
  <pre><code>val a = booleanArrayOf(true,false,true,false)
  </code></pre>
  
  <p>above code, True number is two. -> indexList = {0, 2}
  how to get indexList in Kotlin.</p>