Pattern Recognition for Beginners: Completed Segments and Using Phantom or Ghost Numbers

welcome to this tutorial for beginners in this video i'll build on previous lessons to help you solve puzzles more efficiently this lesson is on recognizing the completed segment pattern it's easy to recognize and a powerful tool to help place numbers let's start by first defining what a segment is and then i'll show you how to use it strategically first what is a segment a segment is when three cells line up together across a row or down a column the cells must line up in the same three by three block let's take a look at this three by three block these three cells all line up horizontally within the block so these three cells make up a segment and these three cells all line up vertically within the block so this is also a segment the pattern we're looking for in this lesson is a completed segment this means that all three cells in the segment have numbers in them here's an example of a completed segment and here's another example you see how easy it is to recognize this pattern look for it when you're solving a puzzle it can help you find numbers you might otherwise not see sometimes the puzzle starts off with a completed segment already in the puzzle and sometimes it appears later as you fill in the numbers let's have a look at how this pattern can help and then i'll give you some tips on what to look for in this example there's only one four in the middle rank that is the middle group of horizontal blocks but there is a completed segment in block five that's going to enable me to place the 4 here in row d column 2. do you see why think about it let's have a closer look at block 5 where can the 4 go in block 5 well since there's a completed segment in row d and there's already a 4 in row e then the 4 has to go in one of these two cells in row f now do you see why the 4 must go here row f is eliminated since the four is penciled into two locations where it must go in block five if we're using snyder notation then we know the four must be in one of those two cells and nowhere else in block five these cells contain what is sometimes referred to as ghost force since we know for sure a four is in one of those two cells we don't know which so they're ghost numbers sometimes called phantom numbers and row e is eliminated because there is already a 4 in row e block 6 so the 4 in block 4 must go in row d and the only open cell is this one row d column two using the completed segment in block five we were able to place the number easily in block four i'll give you some more powerful ways you can use the completed segment pattern in a minute but first i need to point out two constraints on using this pattern one the number you're trying to place in this case the number four cannot appear in the same block as the completed segment so we can't have a four anywhere in block five and two the number you're trying to place cannot appear in the same row or column as the completed segment so if the four that is in block six was in row d the same row as the completed segment then this strategy won't work the number has to be in a different row or column than the completed segment in this example the 4 in block 6 is in row e so that works i think we can all agree that it's easy to spot the completed segment pattern but what do you do when you see it take a look at this example you can see right away the completed segment in block 7 right great so now what whenever you see a completed segment look outside that block either up the same stack or across the same rank look for a number that is not in the same block as the completed segment and not in the same row or column of the completed segment do you see a number that fits that description in this example think do you see it pause this video and look the completed segment is here in block 7 and the only numbers in that stack that are not in block 7 are the numbers 1 and 6 but the 6 is in the same column as the completed segment so the only number that conforms to the rules is the number 1. it's not in block seven and it's not in the same column as the completed segment great now that we've identified the number one do you see where it can be placed in block four it would go right here in row e column three do you see that let's look at block seven because it has a completed segment in column three and there's already a one in column two then the one has to go in column one block seven either here or here these are viewed as ghost or phantom numbers since we know the one is somewhere here we just can't see it yet now when we look at block four the one can't go in column one because of the ghost numbers in column one and there's already a one in column two block one so the only place the one can go is here in column three row e do you see that here's another example pause the video and think about what i'm about to say can you guess well first there is a completed segment here in block 5 row e now can we do anything with the number six and block four yes we can do you see it it would go right here in block six row e column eight do you see that let's take a look at block five where can a six go it can't go in row d since there is already a six in row d and it can't go in row e because of the completed segment so the six must go in one of the three open cells in row f those numbers are ghosts but still it means that a six cannot go in row f in block six so now do you see where the six goes it can only go here so using the completed segment pattern along with the ghost numbers in block five we can place the six in block six there is a variation on completed segments where the segment isn't totally completed but can be considered completed for example take a look at this puzzle grid it's the same as before except now we don't have a completed segment in block 5 but we have a 6 in column 6. that's six in column six effectively blocks this cell from being a six and we can think of this segment as a virtual completed segment since it acts the same way for the number six even though we don't have a completed segment here it acts like one and these two cells again contain ghost sixes so now we can place the six in block six here now let's take a look at another way we can use the completed segment when we have candidates penciled in first let's look at a puzzle without candidates penciled in we see here we have a five in block six and it can only go in row f in block five because of the completed segment here does that help us to place a 5 somewhere in block 4 well in this case no since the 5 can either go here or here but let's say we had previously penciled in a matching pair using snyder notation like this now we can deduce where the five goes in block four do you see where well since we have ghost fives here in block five then this cell can no longer be a five it must therefore be a two then this cell would be the five that's it for this lesson i will continue to build on this lesson by lesson working from the beginner level where we are now up through intermediate and on to advanced until i've covered most of the strategies used by sudoku solvers if you enjoy these lessons please subscribe to my channel and hit the bell notification so you know when i upload new lessons thanks for watching and i hope you learned something you

Channel: Learn Something

Views: 36,424

Keywords: #Sudoku, how to use a completed segment, what is a segment in sudoku, what are ghost numbers in sudoku, what are phantom numbers in sudoku, sudoku guy, cracking the cryptic, sudoku solving techniques for beginners, Sudoku help, sudoku help for beginners

Id: ZxvS7BOIOsw

Length: 9min 5sec (545 seconds)

Published: Thu Jul 08 2021