Sudoku 101: The Swordfish

Simon takes a look at a difficult puzzle and discusses how it could be approached using the "swordfish" technique.

[@matthewlui1004]

The reason for looking at columns 3,5,7 is not explained. So I'll explain: You have to consider all possible locations of 8 You have to find 3 columns/rows that contain 3 or less 8s and one of those 8s must overlap each other. *The meaning of overlapping is defined by drawing a grid using the 8s you found in the 3 columns/rows, then select any 2 rows/columns (If you found 3 rows, select anu 2 columns; If you found 3 colums, select any 2 rows). In the 2 rows/columns, you would find each row/column have at least an 8 in the same column/row (If you have selected 2 columns, two 8s are on the same row; If you have selected 2 rows, two 8s are on the same column).* That pattern is the swordfish pattern and you can eliminate any other 8 that is on any line of the grid which extends to the edge of the puzzle, except the 8s part of the swordfish pattern. Same logic applies to x-wing, therefore if you know the priciples of an x-wing (especially when you are the one that fills everything in) you would know how the swordfish pattern works. *X-wing is finding any 2 columns/rows that only have two of a certain number that form a rectangular/square shape (aka only two of a certain number found in a column/row and they share the same row/column. Then you find a pair of that on the same columns but on different rows/on the same rows but on different columns). You can then draw a # pattern and eliminate that certain number that is on that line but not on the cell where the 2 lines intersect (aka the numbers part of the x-wing).* There can be different reasons for him to not explain it (speculation). 1. It is hard to explain 2. He has faster methods 3. It takes too long to explain 4. He didn't feel like explaining Whatever the reason, most of these KZbinrs never explain how to spot such patterns and it is quite the challenge to learn this. I learnt this from reading a text tutorial. And it cost me my childhood because I believe demonstrating live is more powerful than plain text and graphs, plus, text tutorials are very confusing. As I said, text explanations are very confusing. So if you still don't understand, you can ask. But if it requires a graphical solution, forgive me for I have no patience in drawing a sudoku puzzle in a KZbin comment. Find another day when you had a good sleep and learn it through text tutorials.

[@sandyhafenbrack141]

Thank you for explaining because right after I realized that I was not going to get an explanation the entire tutorial was meaningless.

[@deonster1]

Dude if you do a swordfish tutorial I will watch it.

[@lsmart]

Wouldn't an easier explanation be that since you want columns with 8's restricted to 3 positions, you search for those that already have numerous filled cells. The only columns with 4 or more cells filled are 3, 5, 7 and 8, but column 8 is eliminated since it has an 8 in it.

[@matthewlui1004]

Tbh, if you know the x-wing, the swordfish shouldn't be hard to grasp. I'm just disappointed that the video only says the position of the number is 'special' for no apparent reason. Also, it is really hard to explain in a KZbin comment. If you know how x-wing works: www.thonky.com/sudoku/sword-fish This website shouldn't be too hard to understand yet I feel it is also a bit vague. Highly recommend learning how to find x-wings first.

[@grolich]

That was actually quite well explained for a KZbin comment ;) Thank you.

[@shellmichael9665]

It’s crazy to go back and watch videos like this one now, given how advanced the channel has become.

[@CarlintVeld]

Would be cool if you would do a fresh recording of a swordfish tutorial with Sven's latest sudoku software and your upskilled vlog competency 😅 Maybe you would like to showcase a few puzzles that require swordfish. How do you identify that a swordfish is the only way out and how would you be able to rule out digits that don't yield a swordfish.

[@6stones]

Great channel, just what I was looking for right now.

[@mensamoo]

swordfish starts at 6 minutes in

[@thsone]

Ty!

[@growingtruedisciples]

Thank you!

[@dismantledbrain5910]

Thanks

[@paulreader1777]

Thanks for the technique which I shall have to study again to fully understand Although a lot slower I found this puzzle almost soluble with pairs alone. The exceptions being the 3 in row 8 and the 9 in row 7 that could be inferred from pairs around them and those singles that resolved as final entries in some blocks, rows and columns.

[@clickayna6157]

Most Complete Sudoku Solving Techniques you Will Find! from here bit.ly/2ui8Q2f From Beginner, Intermediate, Advanced to Very Advanced Levels of Solving Strategies.

[@paulreader1777]

Thanks - I don't need a paid course of videos for my level of interest.

[@LinXiaoChaun]

I am impressed. Your method of finding the swordfish is subtle indeed.

[@LimitedRegicide]

A very diplomatic way of saying "unexplained", huh

[@saganandroid4175]

9:42 you never explain why columns 3, 5 and 7 should be examined.

[@ThePinkimojca]

Yes. I don't understand that ether.

[@CharlieR1]

Sagan Android those just happened to be where the swordfish existed. There isn’t any way to specially find out which rows/columns are involved, you have to just spot it

[@StuartCox]

Excellent. Thank you.

[@swainscheps]

Good gravy. After watching this I am now more confused about what a swordfish is than when I started.

[@CFood0]

this is nice, but i still have trouble identifying swordfishes sometimes i think i see one and operate off of that logic, but i end up being wrong and the puzzle gets broken sometimes i'm right tho, i really don't understand what i'm looking for

[@goldenera7090]

nice one. - thanks Simon

[@atis1976]

I tried to solve this alone. After a lots a time I found an almost X in C5 and C7. The 8 can be there three different ways. C5-R2 and C7-R4 or C7-R2 and C5-R8 or C5-R8 and C7-R4. It looked really useless at first sight. After a while I finally see that, how this interact with C3. Does not matter which pair is real, there is only one possible place for 8 in C3. This means there are seven cells with three 8s in C3, C5, C7 and R2, R4, R8. No more place for 8 in those 6 vertical lines, so my struggle was over with that happy moment. After the 8 disappeared from C4-R8, I suspected a hidden triple in the near, so the quad gave me the pairs. This was an extreme challenge for me, but I learned a lots.

[@rabidsamfan]

Thank you. I don’t think it would have ever occurred to me to look for places to use a swordfish like that. (I generally dodge doing them at all.) It suggested some strategies that don’t occur to me very often, and will probably be quite useful! Interestingly, your swordfish had a lot in common with my bifurcation, since they were both dependent on the 8s to take the next step. (I also never thought of finding the 7s marks in MC as using an X-wing, so that was neat.) It’s nice to know that you’d go the easier way when speed solving. I would like to quibble with your assessment of bifurcation being less stringent logic, or guesswork though. Because there’s a difference between trial-and-error and either/or logic, even if you can only make two trials. And it sounded like what you were describing was trial-and-error. (I may have misunderstood, though.) Longwinded explanation to follow. In trial-and-error, you can only reach two ends -- either a string of deductions dead ends (a contradiction) or it goes all the way to the end of the puzzle (a conclusion.) That’s because the logic involved in each string says: “If a is true (and b is false) then c is true if c is true than d is true, etc...” and you go until you’ve either done the whole puzzle (conclusion) or you find out that the string is trying to make b true or something similar (contradiction) and go back to the beginning to try again. But if you bifurcate, running two threads at once, you can see where the two possibilities both match. Taking the logic to its simplest form it looks like this: Given four statements:: If A is true B is false. If B is true A is false. If A is true C is false. If B is true C is false. Then logically you know that C is always false. This is what an X wing does. A swordfish takes an extra step, sometimes, but it is still working on the principle of either/or. It’s just that these are such common patterns that people who are good at visual patterns can spot them fairly easily and skip over the logical argument to “either A or B is true, therefore C is false”. But I’m bad at visual patterns, so trying to find them is an exercise in counting how often a digit appears in each row or column and then doing comparisons. But looking for “C is always false” isn’t a limitation that applies to bifurcating when done with logic instead of patterns. You can also find a “C” that is true for both A and B. For example in this puzzle, I began with the 3/9 split in TLtl the way you would have. I put a circle around the 3 and a triangle around the 9. Then I followed both paths, enclosing digits (even if I had to write them in) with a circle for the logical path deriving from “TLtl3 is true” and a triangle for “TLtl9 is true”. It was a little easier for me to spot some deductions because I write marks for cells that only have two candidates dividing them with a /, but that’s a side issue. Anyway, one path went: TLtl3 > TLtr9 > MLtr8 > TLmr4... And the other went TLtl9 > TLtr3 > BLbl3 > BCmc3 > TCmc8 > TLmr4... Which of course meant I could place TLmr4. That wasn’t especially useful, but continuing on in the same way eventually eliminated TLtr3. In the meantime I had completely solved the Top Right square and established several other numbers in other places. So I guess the next question is, how do you keep track of your train of thought when you’re speed solving with bifurcation? (I am really enjoying this series!)

[@jimkyle8008]

I failed to fully understand why you were trying to find out where the number 8 could go in columns 3, 5, and 7. I sort of get columns 3 and 5, because you are trying to figure out whether these contain 3 and 8 at row 7, but where does column 7 come into it?

[@Aerialdan1]

Agreed. No explanation. The video was useless as a result.

[@khurgar8120]

Its quite easy to understand actually. In order for the swordfish to work you need to have more correlating numbers than 2 rows/columns. If you look at column 7 you will see that 8 can only be in 2 of those 9 squares, so he use them in correlation with the rest of the swordfish to figure out if 8 can be in column 4 row 8. Now spotting the swordfish to begin with is a whole other story.

[@leftysheppey]

A nice reminder of where you've come from. This is the software I first started watching you from. Amusing to see you saying that you'd be inclined to bifurcate. I know you wouldn't be caught dead bifuricating now lol

[@johnnye87]

Yes that struck me as well! I wonder if that's a change of mind or just the natural difference between what someone's willing to demonstrate as a streamer and what they'd be willing to do to win a competition against the clock 😅

[@jimkyle8008]

Unfortunately I found your explanation impenetrable, and didn't follow the video to the end, because, unlike the X-wing, where one can see the X pattern, I could not visualize the "swordfish" and could not see why you picked the particular columns you did initially.

[@dukefrywokker6470]

The explanation is given at the end of the video.

[@RylanStorm]

It's really not

[@duffelesser6210]

thx from germany!

[@madanaramakrishna9979]

101 suduko solution we want

[@lisanelson2993]

Diffficult to follow 💚

[@sahelanthropus7415]

Ok.... but why swordfish????

[@multipleSpiders]

why's it called that

[@tedsilver5219]

7:06

[@bethearl8686]

couldn't figure out your point. Sorry

[@Mark-ul2lf]

so no logic provided to spot the swordfish, nor any explanation of what a swordfish (iotehr the saying its a variation of the x wing, but that's like saying a bicycle is a variation of a skelter)

[@spacedragon2753]

the swordfish has the same concept as an x-wing. (I'M ASSUMING U ALREADY KNOW AND UNDERSTAND THE X-WING). so basically instead of beeing 2 columns or 2 rows with 2 possible solutions u have 3 columns or 3 rows with 3 possible solutions for a certain number. considering any possible solution for a certain number to fit into thoose 3 cells of 3 columns or rows, there cannot be the same number in any other columns or rows that intersect the 3 cells

[@jacquesdelgado6424]

Thanks but too fast for me.

[@Tb40556]

The most crucial part of the video is why he picked columns 3, 5, and 7. It’s also the only part he left out, rendering the video useless

[@tarubewildin6931]

Trevor Bailey ur right he did leave it out, and it is hard to explain, but it has to do with the three columns that can only contain 8s intersecting with the exact three rows that can only contain 8s. So it looks like there can be more 8s in rows 2,4,7 and columns 3,5,7 but those are the intersections

[@applejackjam2514]

Really not understanding the logic of Looking at the columns and all the other steps really

[@craigmowrey3627]

Simon, I don't know if you still read the comments on these older videos, but I'm hoping... I have watched a few (dozen!) of your tutorials, and normally I REALLY like the way you present new material. This is one of your worst. Two particular things stand out: 1. You run your cursor all over the puzzle while you are talking - very distracting. 11:45-48 is a good example. 12:57-13:03 is another. 2. Your illustrated explanation suffers from yoru inability to highlight a cell or cells while talking about it/them. I believe both of these things could be easily corrected if you were to delete this video and remake it using your new puzzle app (the one you hyperlink in your later videos, for viewers to try their own hand). That would allow you to color the cells in question, it would eliminate the distracting "Creator" popups, and it would let you leave a cell or group of cells highlighted while you explain the nuance. One final suggestion - add a link below the video to one or more of your earlier tutorials on x-wings. Understanding x-wings is crucial to understanding swordfish.

[@Vantier]

I fail to see how this swordfish isn't just a form of bifurcation

[@bitti1975]

To explain it you need "a form of bifurcation", once you know the pattern it isn't necessary anymore.

[@IngoziDanger]

Classic example of understanding but struggling to explain it

[@jeffreystenquist]

While I appreciate the knowledge, the explanation was painfully slow.

[@B3Band]

That's a summary of basically every video on this channel.

[@martinpolach6171]

Just watched the first 4.30 minutes and I am deeply shocked, disappointed and let down.....that you would even Consider”guessing” It’s like you were spewing. four letter curse words ... the shame ...my illusions are shattered....there is no Santa Claus !

[@Frits34000]

This has to be the worst user interface for a sudoku I have ever seen xd

[@TheGuitarded1]

Terrible explanation; so much useless info. Get to the point and stick to it!