Sudoku Tricks: The X-Wing And How To Spot It

[Music] hello welcome back to cracking the cryptic and where today I'm going to take a look at a puzzle that's been requested by a viewer if you do have a puzzle you'd like to see us solve then you can email us at cracking the cryptic at gmail.com or tweet to us at cryptic cracking just take a photo of the grid and send it over and if we have time we'll certainly try and cover it on the channel big welcome to to our new subscribers we've had a quite a good few days on the channel so a lot of new new people welcome to the family we shall certainly try and keep you entertained with our puzzle solving now this puzzle as I say it's been sent in and I think what what the person's done is that they've put this into a solver and the solver has told them that it required an x-wing to solve it and they don't seem to be terribly comfortable with what the next wing is or how to spot it so that's what we're going to talk about today in the video hopefully if I can spot it so without further ado let's kick off and try and solve the puzzle now as usual I'm going to start with what I call standard notation which is a variant of pencil marking the grid that was taught to me by a guy called Thomas Snyder three-time World sodoku champion and it certainly improved my solving no end and how does that work well well the way that works is in 3x3 blocks like the one the cursors going around at the moment if I can lock a number into exactly two positions in that 3x3 block then I'll make little pencil marks to remind me of the fact so for example in this block you can see we have a 3 here and a 3 here so I would pencil mark the threes into those two positions now of course in this particular case there's a three up here so in fact we get to put in a big number but those are the sorts of pencil marks that I'm going to make you can see the three here three here so I can pencil mark up up at the top there now why do I do this well it's a good way of keeping track without getting too cluttered and one of the big problems and one of the I suppose the no nose in my opinion about efficients thank you solving is that we put in too many pencil marks in each and every block of the grid and that is not the way to improve your solving let's carry on five zero five five again we have a five there so this square here is gonna have to be a five and again I may get to make some little pencil marks at the bottom there - - and this - here so this square is going to be a - I don't know how hard this puzzle is by the way it might turn out to be monstrously difficult before we get to the X ring but I'm I'm hoping not falls falls falls into those two squares okay we start at eight eight eight and they say here so this square here is going to have to be a big eight and get to make and some art eights at the bottom there and pencil mark seven seven years seven here so there's going to be seven in one of those two squares - and this is interesting 4 & 4 here so I get to find a pair in this block here this two and four now because of the logic involved in these pencil marks once I have the two numbers 2 & 4 in just these two cells I now know this forms a 2/4 pair so I know one of these cells will be a - one of these cells will be a four so the logic of that indicates that we can't put anything else into these two squares now interesting leader doesn't really help this 5 here is therefore shifted up onto the right-hand side of this cage it does help that because that means that's going to be applied in one of those two positions and there's a flying over there so this is a five and that helps in the top block now you can see this 5 this 5 and this 5 interact on this 3x3 block to allow us to pencil marker five in there again let's keep going with pencil marks it finds into one of these two squares and we're left with a 1 and a 7 into these two positions now that's certainly something we should keep track of don't know if it's going to be enough for the X swing I don't think it will be because there we don't have enough information with the ones or the 7s if you actually look round the grid there are very few ones and sevens and they aren't really interacting strongly on these two squares so I doubt that's the X Wing we're looking for let me just stare at this for a moment okay right but we are at the point now where we can spot the X Wing in fact we don't want to look at once in sevens we want to look at eight and the reason the reason that eights are very interesting is let's take a particular start by looking at column two we look at column two and ask ourselves where we can place an 8 this 8 is a very powerful in the way that it interacts with the digits we've already placed because it rules out these two squares so we can place an 8 here we can't place an 8 here because of this site and we can place an 8 there so the eights are locked into two positions in the column now this is classic X Wing territory a bit like the ones and 7s over here but this time the eights are very powerful and let me show you why so remember with an X Wing what we're looking for is another column of the grid where the eights are restricted to exactly these two rows so Row 1 and Row a to the grid so do test yourself now pause the puzzle and see if you can identify which the interesting column is now if you take a look now at column 7 here we can ask ourselves the same question here this ain't becomes the powerful 8 because it rules out and eight from any of those position which means the eight is against locked into this position or this position and here we have this X so called X wing pattern now why is it called an x-wing I'll add some highlighting at the end of the video but the key thing to realize if you ever come across this pattern is that in the finish solution when we look at it they'll either be an A in this square and this square or they'll be an 8 in this square and this square now why is that let's ask ourselves why let's just go through it I know that some of you have been following the channel for ages will be very familiar with this but if you're new to the channel may not be as comfortable with what we're talking about the reason is that because the eights are limited to exactly two positions in these columns as soon as we decide which one is the 8 so that's for example let's just decide this is the 8 see how what would what be the consequence of that now the moment we make this square the eight you can see that because there's only two positions you can go in in column 7 it can no longer go in this position so it must go in this position like this and similarly if instead we were to say this where was the 8 that's only other possibility we immediately find the square therefore can't be an eights of this one with Billy eight so we end up with eight on the sort of edges of this X shape now you may say well how is that helpful well the reason it's helpful is we have to consider how these eights now interact on the rest of the rows and rest at row 1 and the rest of row 8 so in fact this is very useful in terms of the pencil marks let's ask ourselves the question can this square which we've got a pencil marked a tin can this square ever contain an eight let's put one in and see so the implication of this square being an eight would be looking at column 7 now this just this cannot be an 8 anymore so this will have to be an 8 up here just put it in looking now I'm sure most of you are they're already looking now at column to this square can't be an eight anymore so this is going to have to be the eight up there oops even the computer understands there's a problem here we now have a repeated eight in Row 1 because we know that from our Xtreme pattern the exist that the eights are going to have to be along the diagonals of the eight they can that they cannot obviously not be in the same row so that is the power of the x-wing once you spot one of these you can eliminate eights right along the rows so there is no way and it can be anywhere other than in either this position or this position in Row one or this position or this position in row eight definitely be not here so we can remove the eight and look out this interacts with the pencil marks we made earlier because this is not an eight here anymore this must be the eight now the moment I place this eight because of the earlier pencil marks I've just rubbed out one of my pencil marked fours so this square there must be a for the moment I do that I eliminate one of the pencil marks sevens I had in this block so again just I get three really helpful numbers as a result of the x-wing and let's see if that's enough to really break the puzzle ape and I'm hoping it will be so what can we see immediately well I suppose this 4 and this 4 interact up here to allow us to pencil mark 4 isn't it am i doing pencil not force into those two squares which means we can pencil nice and forth up there too and what next we can pencil mark 17 into one of these two squares nines now this nine here is forcing the position of the nine in this 3x3 block especially when it interacts with this one so in fact that's going to have to be a nine let's put some pencil marks in as a result of that nines there and now where can we put two 9 in this 3x3 block was only one position left and that's going to be there the same is true here we get to place this nine because this nine eliminates from this square and I think the puzzle is now solved because you can see everything starting to chain this nine now and this nine mean there's gonna have to be a nine in this square and again because of my pencil marks the moment I place this nine I can also place a three now I can continue with the nines firstly let's do the nines this nine in this line this nine interact is going to have to be a nine there again use the pencil marks that means that a mine here I think the puzzle I think we've now filled in all the nines we also got this three didn't risk let's make sure we don't ass stop using that it's not gonna be helpful I think we can just place three into one of those two squares certain beat so now we can pencil mark sevens over here we could have done that before and where can we place an eight in this block was in one position that's got to be there this is an 8 here now ball hang on and let's also remember that this 8 here was part of the x-wing so the moment I place this 8 I can slide down to the other end of the X or this stroke of the X if you like this is nothing being 8 here anymore that's gonna have to be an 8 I place that I get to put the 7 in as well so you can see I hope I hope I've explained that ok just how powerful the x-wing technique is it's now you may say yeah but you spotted it rather to be quickly well hey look it wasn't too difficult to spot if you know what you're looking for and believe me all that is is practice the more you practice these puzzles and more you're familiar with the x-wing technique the more naturally it comes to look for that sort of logic so I'm we'll just uh suppose let's just carry on finish the puzzle off today six is here ones here and that means this square here is gonna have to be an a technique so we're looking for one and six into these two squares and presumably one and six into these two squares as well to finish off these rows let's take a quick look now at column six of the grid just because we've got five numbers in it already we're looking for one three four six three four here one no that's no good to hear ah this one here and this one here that's gonna be a one that means this must be a two four pair and we have a two here so nothing magical about this now just simply tidying up the sole that gives us a two five pair in the pencil marks because of the two here and two here I need three and six complete this block which results everything at the bottom now it's a 6-1 6-1 again you can see the power of these pencil marks the ones interact on the central block now we have a one five pair in the central block and that should allow us to complete column five we slide up the block we're looking for what are we looking for two and four but in this block we've already pencil marked floors into these two squares well I know this one must be the four so the moment I place this fall I can also place the to choose those that we've depends on our tubes there and what am I looking for there three and six by the looks of it which I'm quite results in six two three six I mean at this point I think it's it's basically done isn't it you can see now this three three one results here three and one like that that gives us the one five this must be the one now five and seven to complete that column so that means that's that way around that results the five hundred to use the pencil marks to resolve that to this is now a three or six to complete the column and there's a six here I hope I'm giving you an idea of how you can the more you practice this method the more speedy you can actually get and this is the way really to become competitive in speed solving as well because these pencil marks that that they're quick to make and if you really make use of them you can burn through puzzles even if relatively sophisticated difficulty so thank you very much for watching I hope that was interesting solved today we'll be back soon with another edition of cracking the cryptic if you do enjoy the channel please do subscribe we really appreciate that give us a thumbs up our belief is that YouTube recognizes that in some way it's probably wrong but you never know and we'll be back soon as I say with another addition [Music]

Channel: Cracking The Cryptic

Views: 938,126

Rating: 4.8191075 out of 5

Keywords: sudoku, xwing, between 1 and 9, x-wing, y-wing, swordfish, advanced sudoku, sudoku tricks, sudoku tips, sudoku technique, sudoku guide, sudoku lessons, sudoku master, expert sudoku, difficult sudoku, hard sudoku, diabolical sudoku, fiendish sudoku, x wing, simon anthony, mark goodliffe

Id: az2M0V9QCXk

Length: 16min 57sec (1017 seconds)

Published: Mon Mar 18 2019