I agree that it is easier to understand than find, but today I discovered what it takes. Every chain or structure can join to another chain or structure. In many cases, like the fin, it just adds to the complexity, but if we just focus on the cells that the structures see, then it makes sense.

@Sam De Roeck I find XY chains pretty easy! I love chains. Swordfishes, Jellyfishes and finned varieties of them will stump me!!

I have been using this idea without even knowing it has a name

I suck with this kind of logic, but I thiiiiink that arrangement would eliminate the 7 from R4C3 (which in turn would eliminate it from R6C6, but through pointing rather than the wing). Not sure if that's what you meant anyway, but just in case, I don't think the fin on R6C2 would eliminate the 7 from R6C6 as part of the actual swordfish logic in itself, because either R6C2 is the 7 and R6C6 is eliminated, or it's not the 7 and you have the swordfish, but R6C6 is now a part of that swordfish and _can't_ be eliminated.

The 4 in R8C4 would be still available. You must not put both a 4 in R2C4 and R5C6. With this pattern you can eliminate all the other 4s in C4, C6 and C7, which aren't in the example. So if you can put a 4 in only those 6 positions you will have a useless swordfish, like the useless X-Wings later in the video. That's what I understood.

A swordfish does two things: first it allows you to eliminate possibilities in the opposite direction it was created (if it's a swordfish in columns, you eliminate in the rows, or if it's in rows, you eliminate in columns), but because of sudoku it also means that if you solve any of the positions for the number in the swordfish, you can immediately eliminate at least 2 other candidates -- along the row and column -- which will result in either an x-wing or a solvable angle configuration like the r8c6/r5c6/r5c7 arrangement you're talking about. In that configuration, r5c6 CANNOT contain the 4, because as you noted it eliminates all valid possibilities. This is why sparser swordfish are more powerful.

Ding, ding, ding, you are absolutely correct sir. Well done.

The key is that eliminating the 7 from r4c4 is actually not straightforward at all. The beauty of the Finned Swordfish technique is that we're able to say "whether or not there is a 7 is r4c4, I don't care - because I know that, either way, I can logically eliminate the possibility of a 7 in r6c6".

Thanks for reply

That’s not a swordfish because yep the 3 candidate in r4c8 is not confined within c2, c3, and c4 like the other cells. If 3 wasn’t a candidate in that cell, then there would be one

@@slashclaw14 Ah, alright! Perfect! Thank you! I've been trying to wrap my head around this. I just discovered swordfish today from his recent video.

You're unclear on why 6,6 cannot contain a 7? Obviously if 4,4 *is* 7, 6,6 is ruled out by sharing the same box. If 4,4 is *not* 7, then there is a valid swordfish in rows 2/4/7. It's an incomplete 222 fish instead of the full 333 type which would also include 2,6; 4,8; and 7,3, but the logic still holds: no other 7 candidates allowed in columns 3, 6, or 8 apart from those rows. This applies to 6,6 once again, so either way, 6,6 cannot be 7, and you can eliminate it as a candidate in that cell. (apologies if you were asking a different question and I'm simply restating the points from the video) Beyond that, you simply have to move on with whatever advantage that gains you. I am realizing there are many 'trivial' examples of these patterns where it's pointless to find them because they don't eliminate anything. For instance, there is a second finned swordfish on 7s in rows 2,6, and 7, using the same columns, with 6,2 as the 'fin'. It isn't at all useful, because there are no other 7 candidates to eliminate in box 4 where the fin is found. (The finned fish only operates on cells both the fin and the possible fish can 'see'--ie, in the same box as the fin. It doesn't affect, say, 1,2.)

Just look up 'Hodoku'

thx!

well next to be safe I see and realize your using but when the site says 'not secure' I get iffy... are you comfortable with the suggestion with a cautionary yes?

This is where I downloaded Hodoku from... I'm not IT-competent enough to guarantee the software but it hasn't broken my computer yet :) hodoku.sourceforge.net/en/index.php

@@The_Cali_Dude_88 Here's the link to the SSL-encrypted site: sourceforge.net/projects/hodoku/

I think he is saying that if there was no 7 in R4C4 then the it would be a ‘normal’ swordfish and all the green 7’s in C3, C6 & C8 would be eliminated.

thank you for your reply@@Has39.bfpo43 , now I'm gonna try and make sense of it and watch this video again, haha, I'm just so slow at this

@Amr Okasha Unfortunately that is not a swordfish. All the possible entries in the 3 rows have to lineup in EXACTLY 3 columns. Your example has 4 columns occupied.