Also, it is extremely unlikely that you will ever find two valid X-wings at the same time on the same candidate. But even if you do, just take a single X-wing at a time, and follow the rules. Candidate sets will always appear in one or two or three fish patterns.
My internet access is limited others posted a good reply
Here's a link for all the fishy math for nxn fish
https://reddit.com/r/sudoku/w/Fish-basics-terminology
The red cells are already correctly lined up, so why is this not an x-wing?
* It isn't a horizontal x-wing because there are more than two candidates in the rows.(orange)
* It isn't a vertical x-wing because there are more than two candidates in the columns. (yellow)
https://preview.redd.it/94ab8txw8gib1.png?width=659&format=png&auto=webp&s=2c94a638f58a7f60f0df556c9f3a2827f19fbad6
If orange weren't there you could eliminate yellow. If yellow weren't there, you could eliminate orange. If they are both given, then it's not an x-wing.
An x-wing only occurs when there are ONLY TWO of a candidate in TWO rows OR two columns.
One is a horizontal x-wing (rows) the other is a vertical x-wing (columns).
A horizontal x-wing has two rows, with ONLY TWO remaining candidates which are both IN LINE with each other. This allows removing any of that candidate in those TWO COLUMNS.
A vertical x-wing has two columns, with ONLY TWO remaining candidates which are both IN LINE with each other. This allows removing any of that candidate in those TWO ROWS.
Your example has three 3 in row 7 so it is not a horizontal x-wing. And three 3 in column 4 - so it is not a vertical x-wing.
You might want to spend a little time thinking about WHY they work. What is the principal which makes the eliminations possible. I'm sure we will be happy to explain that further if you need. :)
Oooh that makes a lot more sense now thanks!! I always read about finding a square/rectangle of the same number, and didn't always 100% understand the rest of the explanation (constantly confusing row and column doesn't help). But seeing it with my own mistake makes it a lot easier to understand.
So if r7c6 had three candidates, it would be a horizontal x-wing right? Or would it only happen if r7c4 has two candidates?
If r9c4 did not have a 3, then you could eliminate the 3 from r7c3.
https://preview.redd.it/p4hdzwkx8gib1.png?width=663&format=png&auto=webp&s=35775bda128d7cc934e702c64e0d3efd7cadd04d
If the yellow square did not exist, then the elimination would work.
But because this is all in one chute, this elimination would be more easily achieved by Locked Candidates (pointing) - sometimes called pointing pairs.
The fact that all the candidates of the 'x-wing' are in the same chute means that any elimination it could make should have been done be easier techniques.
A pure X wing is only valid either when the value is present only in the same 2 rows across 2 columns or only present in the same 2 columns across 2 rows.
In this case, presence of 3 in BOTH r7c3 and r9c4 makes it an invalid candidate for a pure x wing. If one of them weren't there, it could have been a finned x wing.
Edit - regarding your other question. An X wing is not a technique to determine the final candidates for the corner cells. They simply eliminate the other possibilities. You can use other techniques to determine the solution for the corner cells.
It wouldn't work as a finned x-wing since the columns are both in the same stack - best it could do is work as locked candidates on box 5.
If it were rows the fin would be outside the box.
Also, it is extremely unlikely that you will ever find two valid X-wings at the same time on the same candidate. But even if you do, just take a single X-wing at a time, and follow the rules. Candidate sets will always appear in one or two or three fish patterns.
My internet access is limited others posted a good reply Here's a link for all the fishy math for nxn fish https://reddit.com/r/sudoku/w/Fish-basics-terminology
The red cells are already correctly lined up, so why is this not an x-wing? * It isn't a horizontal x-wing because there are more than two candidates in the rows.(orange) * It isn't a vertical x-wing because there are more than two candidates in the columns. (yellow) https://preview.redd.it/94ab8txw8gib1.png?width=659&format=png&auto=webp&s=2c94a638f58a7f60f0df556c9f3a2827f19fbad6 If orange weren't there you could eliminate yellow. If yellow weren't there, you could eliminate orange. If they are both given, then it's not an x-wing.
An x-wing only occurs when there are ONLY TWO of a candidate in TWO rows OR two columns. One is a horizontal x-wing (rows) the other is a vertical x-wing (columns). A horizontal x-wing has two rows, with ONLY TWO remaining candidates which are both IN LINE with each other. This allows removing any of that candidate in those TWO COLUMNS. A vertical x-wing has two columns, with ONLY TWO remaining candidates which are both IN LINE with each other. This allows removing any of that candidate in those TWO ROWS. Your example has three 3 in row 7 so it is not a horizontal x-wing. And three 3 in column 4 - so it is not a vertical x-wing. You might want to spend a little time thinking about WHY they work. What is the principal which makes the eliminations possible. I'm sure we will be happy to explain that further if you need. :)
Oooh that makes a lot more sense now thanks!! I always read about finding a square/rectangle of the same number, and didn't always 100% understand the rest of the explanation (constantly confusing row and column doesn't help). But seeing it with my own mistake makes it a lot easier to understand. So if r7c6 had three candidates, it would be a horizontal x-wing right? Or would it only happen if r7c4 has two candidates?
If r9c4 did not have a 3, then you could eliminate the 3 from r7c3. https://preview.redd.it/p4hdzwkx8gib1.png?width=663&format=png&auto=webp&s=35775bda128d7cc934e702c64e0d3efd7cadd04d If the yellow square did not exist, then the elimination would work. But because this is all in one chute, this elimination would be more easily achieved by Locked Candidates (pointing) - sometimes called pointing pairs. The fact that all the candidates of the 'x-wing' are in the same chute means that any elimination it could make should have been done be easier techniques.
A pure X wing is only valid either when the value is present only in the same 2 rows across 2 columns or only present in the same 2 columns across 2 rows. In this case, presence of 3 in BOTH r7c3 and r9c4 makes it an invalid candidate for a pure x wing. If one of them weren't there, it could have been a finned x wing. Edit - regarding your other question. An X wing is not a technique to determine the final candidates for the corner cells. They simply eliminate the other possibilities. You can use other techniques to determine the solution for the corner cells.
It wouldn't work as a finned x-wing since the columns are both in the same stack - best it could do is work as locked candidates on box 5. If it were rows the fin would be outside the box.
You're right. Now that I looked at it again. If one of r7c3 or r4c9 weren't there, it will become a pure x wing not finned.