Please note, this blog entry is from a previous course. You might want to check out the current one.
The extra credit item from last week to turn displayValue into a Double? (i.e, an Optional rather than a Double) is now required. displayValue should return nil whenever the contents of the display cannot be interpreted as a Double. Setting displayValue to nil should clear the display.
… done that, too. However, hint #1 asks to consider using optional chaining, which we did not up to now.
Assuming there is always a text string, and that we handle the internationalization problem elsewhere, we can reduce the getter of displayValue to a single line using optional chaining:
return NSNumberFormatter().numberFromString(display.text!)?.doubleValue
The complete code for the task #4 is available on GitHub.
![Share on Facebook Facebook](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/facebook.png)
![Share on Twitter twitter](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/twitter.png)
![Share on Reddit reddit](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/reddit.png)
![Pin it with Pinterest pinterest](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/pinterest.png)
![Share on Linkedin linkedin](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/linkedin.png)
![Share on tumblr tumblr](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/tumblr.png)
![Share by email mail](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/16x16/mail.png)
![Share on Facebook Facebook](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/facebook.png)
![Share on Twitter twitter](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/twitter.png)
![Share on Reddit reddit](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/reddit.png)
![Pin it with Pinterest pinterest](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/pinterest.png)
![Share on Linkedin linkedin](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/linkedin.png)
![Share on tumblr tumblr](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/tumblr.png)
![Share by email mail](https://cs193p.m2m.at/wp-content/plugins/social-media-feather/synved-social/image/social/regular/32x32/mail.png)