The New Sudoku Players' Forum

[bradles]

Code: Select all

 *-----------------------------------------------------------*
 | 7     ..    .     | ...   .     .     | ..    .     .     |
 | ..    .     .     | ..7.  .     ...   | 7..   7.    .     |
 | .     ..    .     | ..7.  7.    .     | .7.   .     .     |
 |-------------------+-------------------+-------------------|
 | .     .     .     | .     .     7     | .     .     .     |
 | .     .     .7-   | ..    .     ..    | .7+   .     .     |
 | .     .7+   .     | .     .     .     | .7.   .     *7*   |
 |-------------------+-------------------+-------------------|
 | .     .     .7+   | .     .     .     | .     .     .7-   |
 | ...   ...7  .     | ..7.  7.    ..    | .     .     .7.   |
 | ..    ..7   .     | ..7   .     .     | .     7.    .     |
 *-----------------------------------------------------------*

In my simple sudoku solver it suggests to exclude 7 from r6c9. While simple sudoku used colors, I am using + and - for this example.

So if:
r5c7+ => (r5c3- & r6c2+ & r7c3+ => r7c9-)

My understanding of the suggestion is that because the 7 in r6c9 can see both a - in r7c9 and a + in r6c2 then one of those must be true so r6c9 cannot contain a 7. Correct yes?

I guess my question is....how do you see this without going through and doing this for every number? Should it be somewhat obvious? Is it just experience in knowing what to look for?

I seem to be spending ages looking at a puzzle when it gets to a certain point.

Brad

[TKiel]

Brad,

Your understanding of why 7 is excluded from r6c9 is correct. (It also sees a '+' in r5c7, so even if the one in r6c2 wasn't there, it could be excluded.)

My answer to your other question is you don't know before going through every number. I usually don't use it until I've placed as many values as I can, using single, locked candidates, naked stuff (if I see it) and x-wing, swordfish, etc... The filtering function makes spotting those things easier that finding hidden stuff, IMO. As I'm going through looking for those things, I also look for conjugate pairs. If they look like they might have potential, I'll actually use the colours, rather that just in my mind. Sometimes, they lead to nothing. In several puzzles, I've seen several conjugate pairs but when I actually colored them, I realized that they were mutually exclusive x-wings and so led to no exclusion. Many times a chain can be 7-8 cells long before an exclusion can made, if at all, and other times two 2 cell chains link in such a manner that it makes an exclusion. There is no real method to finding conjugate chains that make exclusions, in my experience. It's just recognizing that a certain value has some conjugates and seeing where that takes you.

Tracy

[Myth Jellies]

I've found that, generally, the minimum requirement is the same type of grid that you might expect to find a full fledged x-wing in. You usually need at least 4 unsolved boxes in a rectanglular pattern, or 6 unsolved boxes in any pattern to be fruitful with either seafood or colors.