By PM:
Fredrik wrote:Thanks,
Your web site DID help.
My blockage was in believing that the chain (e.g. of 8's) was to "solve its own true/false" rather than that the members of the chain would serve to knock off the number 8's NOT part of the chain.
I have crawled through sveral examples and DO get them right, eventually. However, I have difficulties picking the proper chain members, so as to zero in on those I want to eliminate.
Most of the time, I find that the chain "members" I am using to eliminate non-chain numbers are of the same color (to be expected, if I linked them properly). I.e., if I am intersecting a target using two numbers I have assigned to be TRUE, what should be my conclusion as to the T/F of the target, and how would that go if I had assigned them a FALSE status.
Thanks for your help,
Fred
Fred,
(I hope you don't mind me posting your PM, and replying in the public forum, but the information could also be useful to other readers of this thread.)
If I understand you correctly, and you are eliminating non-chain candidate because they are at the intersection of two same-colour chain members, then you're doing it wrong!
Any non-chain candidate to be eliminated must be at the intersection of
different colour chain members. Then we know that whichever colour members in the chain are the true ones, one of them will exclude the candidate at the intersection. This is an "exclusion".
The other useful information that can sometimes be found through colouring is a contradiction. If, when colouring the chain, you come across a unit (row, column or block) that contains two nodes of the same colour, then you know that colour must be false. You can then eliminate the candidate from all cells of that colour. This is because as the unit cannot contain two cells with the same value, both nodes cannot be true, so they must both be false. The other colour must be the true one.
S