What does case 10 ... 19 mean?

I assume it means 10 or 11 or 12,13,14,15,161,17,18 or 19 but if so how does it work? Is 10 … 19 a single object? or is case a different notation?

Hi @Hao,
This syntax case val1 ... val2 is used to write a case when a certain range of input generates the same outcome. So, instead of writing 10 cases with the same outcome, we simply create a case with values ranging from 10–19 that have the same output result.
I hope it will help. Please drop us a note if you have any other questions or concerns.