Talk:Casting out nines

I am trying to understand this task.

Would it be fair to change the final sentence to read:

The task is to write code that given a numeric base and two numbers that mark the begining and end of a range use only this test to eliminate numbers from the range which cannot be valid Kaprekar numbers -- the result is the rest of the numbers from that range.

Or have I completely misunderstood what you are trying to convey?

--Rdm 13:59, 25 June 2012 (UTC)

OK, you have not misunderstood what I am trying to convey. I wanted to accept any test based on the congruence B^n=1(mod B-1). The Kaprekar numbers are a good objective because they have the property k%(B-1) == (k*k)%(B-1) and are explained elswhere on this site. I would accept any other objective.

Ok! I think the current task description is a bit too coy about the filtering mechanism. It's almost equivalent to "filter numbers" since the description of the selection function is so general. I am currently thinking it should be:

Given two numbers which mark the beginning and end of a range of integers [LO,HI], and an integer base (BASE), return the integers which fall in that range where the remainder after dividing the number by BASE-1 is the same as the remainder after dividing the square of the number by BASE-1.

--Rdm 14:02, 26 June 2012 (UTC)

I'd like to see a different title for the task. The current description states right up front that the task is not casting out nines. —Sonia 19:46, 26 June 2012 (UTC)

OK, maybe you have misunderstood what I am trying to convey. Following Dr. Math at http://mathforum.org/library/drmath/view/55926.html describing casting out nines to the phrase "(You wouldn't normally get the same check digit for the result of the sum and the products; I just picked a weird example.)" I see that every Kaprekar is a "weird example". Using Dr. Maths "quick explanation of how to do it, without the big words" would be as slow as the String C++ Kaprekar solution. So we turn to http://mathworld.wolfram.com/CastingOutNines.html. What is said there is true for bases other than 10, therefore it is possible to develop a fast test. --Nigel Galloway 12:47, 27 June 2012 (UTC)