what's the meaning of short circuit evaluation?

I'm going to guess. In some languages, they only evaluate expressions if required. so if the condition is (x && y), if x is false, it doesn't evaluate y. If the conditional is (x || y) then if x is true, then evaluating y is bypassed. It can lead to conditionals like if(myobject==null || myobject.method()). THe second half is only tested if the first part is false. In this case, you're relying on it not testing when true, since it would result in an exception (calling a method on a null). So the short of it is, it only evaluates what it needs to, and there are some situations where this is useful knowledge. It's discussed at https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/BasicOperators.html . Just search for "circuit"