On Tue, Oct 05, 2010 at 01:52:39PM -0700, Chris Rebert wrote: > On Tue, Oct 5, 2010 at 1:31 PM, Wolfgang Rohdewald > <wolfg...@rohdewald.de> wrote: > > On Dienstag 05 Oktober 2010, MRAB wrote: > >> > About notation, even if loved right-hand-half-open > >> > intervals, I would wonder about [a,b] noting it. I guess > >> > 99.9% of programmers and novices (even purely amateur) have > >> > learnt about intervals at school in math courses. Both > >> > notations I know of use [a,b] for closed intervals, while > >> > half-open ones are noted either [a,b[ or [a,b). Thus, for > >> > me, the present C/python/etc notation is at best > >> > misleading. So, what about a hypothetical language using > >> > directly math unambiguous notation, thus also letting > >> > programmers chose their preferred semantics (without > >> > fooling others)? End of war? > >> > >> Dijkstra came to his conclusion after seeing the results of > >> students using the programming language Mesa, which does > >> support all 4 forms of interval. > > > > what was his conclusion? > > That right-hand-half-open intervals (i.e. a <= i < b, equivalently [a, > b) ), which are what Python uses, are to be preferred. > (See aforelinked PDF: http://www.cs.utexas.edu/users/EWD/ewd08xx/EWD831.PDF)
The problem is that the slice notation is sometimes handy in situations where an open interval doesn't allow easily to mark what you want. For instance I have at one time implemted a Tree. This is a dict like structure but it allows to visit the keys in order. Because there is an order, slice notation can make sense. e.g. if T is a tree with names as keys, T['bea':'mike'] is a subtree where we have for each key that 'bea' <= key < 'mike'. But what if I wanted a subtree where 'mike' was still included, but nothing further? Or what if the keys were floats or tuples and I wanted an inclusive upper boundary? And what if you needed the reverse sequence. If you start with inclusive limit, the reverse of a <= item <= b is b >= item >= a. If the second limit is to be exclusive the reverse of a <= item < b becomes (b - 1) >= item > (a - 1). So what do you do in python, if you are given a list, a lower inclusive and an upper exclusive limit and you have to return the reverse slice. Better don't use L[b-1:a-1:-1], because that may result in a nasty surprise, a surprise that could have been avoided by allowing the second limit to be inclusive. So I don't think the results in MESA are that relevant when you are working in an environment that is not limited to integers in upgoing sequences. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
