The New Sudoku Players' Forum

[Kent]

The below example is given by Myth Jellies.

+-----------+-----------+-----------+
| . -2- 2 | . 2 2 | . 2 2 |
| . -2- 2 | . 2 2 | . 2 2 |
| . *2 #2 | . *2 . | . *2 . |
+-----------+-----------+-----------+
| . *2 . | . *2 . | . *2 . |
| . 2 2 | . 2 2 | . 2 2 |
| . *2 . | . *2 . | . *2 . |
+-----------+-----------+-----------+
| . . . | . . . | 2 . . |
| . . . | 2 . . | . . . |
| 2 . . | . . . | . . . |
+-----------+-----------+-----------+

I don't get it how can the hash become swordfish. At row 3, there's more than 3 candidates.I thought the principle is that there shouldn't be more than 3 candidates in a row.Please clarify.Thanks

[emm]

Kent - this is a finned swordfish - the hash is the fin ie the extra candidate.
The two cells marked (-) can be eliminated - they are any cells that would have been removed by the swordfish and can 'see' the fin.

Code: Select all

+-------------+-----------+-----------+
| .  -2-  2   | .   2   2 | .   2   2 |
| .  -2-  2   | .   2   2 | .   2   2 |
| .  *2  #2   | .  *2   . | .  *2   . |
+-------------+-----------+-----------+
| .  *2   .   | .  *2   . | .  *2   . |
| .   2   2   | .   2   2 | .   2   2 |
| .  *2   .   | .  *2   . | .  *2   . |
+-------------+-----------+-----------+
| .   .   .   | .   .   . | 2   .   . |
| .   .   .   | 2   .   . | .   .   . |
| 2   .   .   | .   .   . | .  .   .  |
+-------------+-----------+-----------+

PS : use the 'code' button not the 'quote' button for laying out the grid. It's a good idea to 'preview' before you 'submit' to make sure it's all lined up.

[ronk]

I don't get it how can the hash become swordfish. At row 3, there's more than 3 candidates.

That's why it's called a 'finned' swordfish.

Ron

[tso]

Say three cells in a row have been reduced to the folloing possibilites and you didn't know what a naked pair was:

Code: Select all

[12][12][123]

You could reason "If the first cell is 1 then the third cell ISN'T 1. But if the first cell is 2, then the second cell is 1 and then the third cell ISN'T 1. In either case, the third cell isn't 1." Same reasoning follows to eliminate 2 from the third cell as well.

You could write out these simple "forcing chains" to spell it out:

If r1c1=1, then r1c3<>1
If r1c1=2, then r1c2=1, then r1c3<>1
Therefore, r1c3<>3

Eventually, you wouldn't have to go through this process as you would recognize the pattern, regardless of if you ever gave it a name.

Now, lets say you had the following position and (like me) you didn't know anything about 'finned' this or 'almost' that:

Code: Select all

+-------------+-----------+-----------+
| .  -2-  2   | .   2   2 | .   2   2 |
| .  -2-  2   | .   2   2 | .   2   2 |
| .  *2  #2   | .  *2   . | .  *2   . |
+-------------+-----------+-----------+
| .  *2   .   | .  *2   . | .  *2   . |
| .   2   2   | .   2   2 | .   2   2 |
| .  *2   .   | .  *2   . | .  *2   . |
+-------------+-----------+-----------+
| .   .   .   | .   .   . | 2   .   . |
| .   .   .   | 2   .   . | .   .   . |
| 2   .   .   | .   .   . | .  .   .  |
+-------------+-----------+-----------+

You could reason, "If r3c3 were 2, then r1c2 and r2c2 must NOT be 2. But if r3c3 is NOT 2, then r346c258 would form a Swordfish in rows -- which would mean that r1c2 and r2c2 must NOT be 2. In either case, r1c2 and r2c2 must not be 2."

Again, a simple forcing chain (with one gnarley link):

If r3c3=2, then r12c2<>2
If r3c3<>2, then r346c258 form a Swordfish in rows, then r12c2<>2
Therefore, r12c2<>2

(Everything can be stated as a forcing chain -- from a simple naked pair to a deformed lunch special.)

You may or may not get to the point where you recogize this as a pattern in the future. The fact that someone gave it a name will not make it more likely that you will find it.

In the quest to label everything within an inch of its life, so specifically that our tool box overflows, we can miss the simplicity of many situations. If all roads lead to Rome, we don't need to know which road we're on -- we know we'll be having pasta for dinner.

That is, if both "A" and "NOT A" lead to "B", then "B".

[Kent]

Thanks Tso.If r3c3 is not 2 and therefore it forms a swordfish, it does eliminate the other 9 candidates(example r5c2,r5c2....) right??

[tso]

Thanks Tso.If r3c3 is not 2 and therefore it forms a swordfish, it does eliminate the other 9 candidates(example r5c2,r5c2....) right??

Yes -- but you don't KNOW if r3c3 is 2 or not! All you know is that whether it is or isn't, there will be some reason to exclude r12c2.

If you take door number one, you get an apple and a punch in the nose. If you take door two, you get an apple and a kick in the groin. Since you have to take one of the two doors, all you KNOW is that your getting an apple and some sort of pain.