On Wed, Jul 13, 2011 at 04:04:15PM -0400, Stephen Bloch wrote: > > On Jul 13, 2011, at 1:08 PM, H Bakkum wrote: > > > "Develop the function check-range1?, which consumes a list of > > temperature measurements (represented as numbers) and checks whether > > all measurements are between 5*C and 95*C." > > ... > > Everything works as it should, except when testing the program against > > an empty list...it returns true, naturally. However, logically I feel > > that an empty list should produce false, as it doesn't contain any > > temperatures between 5*C and 95*C. > > The question wasn't "are there any temperatures between 5 and 95?", it was > "are there NO temperatures OUTSIDE the range of 5 to 95?" Which is clearly > true of the empty list: it has no temperatures at all, hence certainly none > outside that range. > > This is one of my favorite problems. A closely related problem is > > "Develop the function multiply-all, which consumes a list of numbers and > returns the result of multiplying them all together." > > As with check-range1, the interesting question is "what's the right answer to > the empty case?". >
This was fought out at great length by philosophers and logicians in the 19th century. The final consensus, generalized by mathematicians, is that when you foo together a bunch of things, and the 'foo' operation is associative and has an identity, the reult on an empth bunch is the identity of the operator. -- hendrik _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
