title: how to set size UIView in auto layout programmatically swift?
body: <p>I have made two <code>UIView</code> with <code>Auto Layout</code> using this code :</p>
  
  <pre><code>class ViewController: UIViewController {
  
      var view_constraint_V:NSArray = [NSLayoutConstraint]();
      var originalValue:CGFloat = 0.0;
      var offsetValue:CGFloat = -100;
  
      override func viewDidLoad() {
          super.viewDidLoad()
          // Do any additional setup after loading the view, typically from a nib.
          var topView = UIView()
          topView.setTranslatesAutoresizingMaskIntoConstraints(false)
          topView.backgroundColor = UIColor.blackColor()
  
          var bottomView = UIView()
          bottomView.setTranslatesAutoresizingMaskIntoConstraints(false)
          bottomView.backgroundColor = UIColor.lightGrayColor()
  
          let button   = UIButton()
          button.backgroundColor = UIColor.darkGrayColor()
          button.setTitle("Click me!", forState: UIControlState.Normal)
          button.addTarget(self, action: "buttonAction:", forControlEvents: UIControlEvents.TouchUpInside)
          button.setTranslatesAutoresizingMaskIntoConstraints(false)
  
          self.view.addSubview(topView);
          self.view.addSubview(bottomView);
          bottomView.addSubview(button)
  
          // constraints
          let viewsDictionary = ["top":topView,"bottom":bottomView, "button":button]
  
          //position constraints
          let view_constraint_H:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("H:|-10-[top]-10-|",options: NSLayoutFormatOptions(0),metrics: nil, views: viewsDictionary)
          let view_constraint_H2:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("H:|-10-[bottom]-10-|",options: NSLayoutFormatOptions(0),metrics: nil, views: viewsDictionary)
  
          view_constraint_V = NSLayoutConstraint.constraintsWithVisualFormat("V:|-20-[top(bottom)]-[bottom]-10-|", options: NSLayoutFormatOptions.AlignAllLeading, metrics: nil, views: viewsDictionary)
  
          view.addConstraints(view_constraint_H)
          view.addConstraints(view_constraint_H2)
          view.addConstraints(view_constraint_V)
  
          originalValue = (view_constraint_V[1] as NSLayoutConstraint).constant;
        //println((view_constraint_V[1] as NSLayoutConstraint))
          (view_constraint_V[1] as NSLayoutConstraint).constant = offsetValue
  
          // Position button
          let control_constraint_H:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("H:|-30-[button]-30-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDictionary)
          let control_constraint_V:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("V:|-30-[button(50)]", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDictionary)
  
          bottomView.addConstraints(control_constraint_H)
          bottomView.addConstraints(control_constraint_V)
  
      }
      /**
          Example of dynamically changing the constraints when clicking on a button.
      */
      func buttonAction(sender:UIButton!)
      {
          if (view_constraint_V[1].constant == offsetValue) {
              (view_constraint_V[1] as NSLayoutConstraint).constant = originalValue
          } else {
              (view_constraint_V[1] as NSLayoutConstraint).constant = offsetValue
          }
      }
  }
  </code></pre>
  
  <p>Now, I want to set top view size equal to 1/3 of bottom view size programmatically but I don't know how to do this.</p>