Scala：seq.size vs 模式匹配

Question

I have to make a decision based on size of a Seq.

So, either I can do:

if(mySeq.size() > 0) // do your stuff

Or, I can do:

mySeq match {
  case x :: _ => // do your stuff
}

Which one should I prefer?

Answer 1

If your checking are exactly the size > 0 or not, I prefer to

if (mySeq.nonEmpty) { .. }

This solution without performance losing and works for all version of scala. In my opinion nonEmpty is more intuitive than size checking.

Answer 2

Seq is an abstraction of the underlying collection type. This presents a few problems.

case x :: _ => won't work unless the underlying collection is a List , but you can change it to case x +: _ => for more universal coverage.

mySeq.size might be reasonably efficient if the underlying collection is something like a Vector but it's a linear operation on a List .

My recommendation would be to use mySeq.sizeIs > 0 (Scala 2.13). sizeIs is designed to terminate as soon as the comparison can be determined, so even on a List it should return true after traversing only one element.