CODEWITHSUNDEEP
c#c#-8.0
Sebastian Krysmanski
Sebastian Krysmanski

Reputation: 8414

C#: Why is the upper bound of ranges exclusive?

C# 8 added the ranges syntax to C#:

var slice = myArray[0..2];

What I find not so intuitive is that the upper bound (2 in the previous example) is exclusive - not inclusive; i.e. myArray[0..2] returns a slice with 2 elements (0 and 1), not with 3 elements.

So my question is: Why did the C# language designers choose to make the upper bound exclusive?

Both the ranges documentation as well as the design champion issue simply state that the upper bound is exclusive - without giving any explanation as to why (at least none that I could find). There also was a discussion about this on GitHub but it also doesn't seem to contain any official explanation.

By searching StackOverflow I stumbled across the same question for Python. Python also treats the upper bound as exclusive. So I can imagine that the C# language designers were looking at other languages (that have ranges) and were trying to make C#'s behavior consistent with other languages. But I'm still wondering if there is any official documentation on this decision.

Upvotes: 11

Views: 2164

Answers (1)

TheGeneral
TheGeneral

Reputation: 81573

There are a lot of design decisions that just won't have satisfying answers. However, this one at least has a paper-trail you can follow in the design notes (in one or more mentions) starting at the following link.

Note : This was just an arbitrary decision, the reason they chose exclusive is because they chose exclusive. It could easily not have been the case due to the pros and cons.

C# Language Design Notes for Jan 22, 2018

...

Conclusion

Let us go with .. means exclusive. Since we've chosen to focus on the indexing/slicing scenario, this seems the right thing to do:

  • It allows a.Length as an endpoint without adding/subtracting 1.
  • It lets the end of one range be the beginning of the next without overlap
  • It avoids ugly empty ranges of the form x..x-1

And as @vc74 mentions in the comments and also stated in the document supplied, other languages like Python follow this convention, however others don't.

Upvotes: 17

Related Questions