title: Julia: What is undef in Array in Julia
body: <ol>
  <li>This is a constructor for arrays:</li>
  </ol>
  
  <p><code>Array{T}(undef, dims)</code></p>
  
  <p>I am new to Julia, and don't have a good background in programming. In this syntax, why is <code>undef</code> used for creating the array?</p>
  
  <ol start="2">
  <li>What is a constructor in Julia, in what situation do we use a constructor?</li>
  </ol>
  
  <p>If we don't type constructor, Julia will automatically create a constructor. Then, Why we use constructor?</p>