This is going to be another post with a title designed to confuse you if you are a sudoku solver I’m afraid. Yet again if you are learning how to play sudoku, you may think you have arrived in the wrong place and worry that we are going to deal with angling tips instead of sudoku tips.
Well, never fear, although there are several “fish” techniques we will look at, they are all just names for more things that look a bit like the earlier X-Wing and XY-Wing.
The Swordfish extends the idea of the X-Wing that uses two candidates in a row, column or box and says that if one is correct, the other must be wrong. Look at this diagram, again I have simplified the rest of the grid.
You should be able to see that there are only two 2’s in columns 2, 4 and 8. If you look closer, you will also see that those pairs are linked across the rows as well. I have highlighted them in this diagram to make it easier to see.
We know from our earlier star-wars outing that when one of the pair “is”, the other “is not” and so we can play our game of walking round the swordfish shape saying, yes, no, yes, no all the way round.
Now you can see that I have left two other cells with 2 as a candidate in them. It doesn’t matter which of the swordfish candidates we choose, this step by step walk around the figure guarantees that even in the rows it is one of our swordfish candidates that is equal to 2. That means that none of the other 2’s in the shared rows can be true and we can eliminate them. Of course, this works if we have the pairs in rows instead of columns so you will need to look sideways at your puzzle from time to time.
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