title: Cannot assign through subscript: 'dict' is a 'let' constant
body: <p>I have a very simple problem with Swift. 
  I created this function:</p>
  
  <pre><code>var dictAges : [String: Int] = ["John":40, "Michael":20, "Bob": -16]
  
  func correctAges(dict:[String:Int]) {
       for (name, age) in dict {
            guard age &gt;= 0 else {
            dict[name] = 0
            continue
            }
       }
  }
  correctAges(dict:dictAges)
  </code></pre>
  
  <p>But I don't understand the error:</p>
  
  <blockquote>
    <p>"cannot assign through subscript: 'dict' is a 'let' constant, dict[name] = 0"</p>
  </blockquote>
  
  <p>How can I solve it? </p>