Sudoku X Wing strategy

The strategy page on this site includes a section on the X-Wing strategy. There is also a step-by-step guide to X-Wing here.

I'd managed so far without using anything quite as complex looking as this. Is that experience just me or do my fellow puzzle solvers also find that they can cope without frying their brains trying to find these obscure patterns in puzzles?

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Sort of Swordfish with 5 in Bb Hf

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Yes, I agree you haven't overlooked anything too easy.

I can't see anything forced by the only choice; only square; subgroup exclusion or twin exclusion sudoku strategies.

Always useful at this stage to look through the possibilities one by one using the possibility bar.

In this case it is the '5's that are helpful. In row F there are only two options Fb or Fh. If it was Fb then that has knock on effects, in row H the 5 would have to be in Hc but in area Da the only square left would be Dc. So putting 5 in Fb would require a 5 in the same column c for Dc and Hc. This can not be right - the 5 must go in Fh - no other choice.

If you do that sudoku assignment the rest follows fairly easily I think.

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Hello,

You said, about the puzzle "Another challenge": "In this case it is the '5's that are helpful. In row F there are only two options Fb or Fh. If it was Fb then that has knock on effects, in row H the 5 would have to be in Hc but in area Da the only square left would be Dc. So putting 5 in Fb would require a 5 in the same column c for Dc and Hc. This can not be right - the 5 must go in Fh - no other choice."

When you talk about the two options for the 5s in F, I see you. When you say 5 in Fb, the 5 in the H row is Hc, I see that. But when you bring up the Da box(area), and say that "the only square left would be Dc", I don't follow you.

I assumed you are saying that the only square left for a 5 is Dc, but didn't you already a 5 in the Da box/area? Why talk about a second 5 in the Da box? The 5 has already been placed slot Fb at the beginning for the Da box/area.

Obviously if you were to place a second 5 in that box, it would have to in Dc and would be in conflict with the 5 in Hc. But that is two 5s in the same box Fb and Dc!?!

I did a check on that option, it is works, out, but I obviously don't understand the logic.

Could I get some help?

CBK

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Sorry, yes I was wrong, not sure what I was thinking.

I still think the correct strategy was to choose between Fb and Fh for where to place a 5.

If you choose Fb you can prove this is incorrect - but by a longer chain than I thought - however all the steps are straightforward 'forced' choices (only place or only square).

First look at column c. 9 must go in Dc; 4 in Gc; 6 in Ec; 7 in Cc and 5 in Hc to complete the column.

Now complete row C with 6 in Cb and 1 in Cg.

Now go to column g with 9 in Bg and 4 in Eg and 7 in Ig.

It now gets to the crunch in column b. 1 must go in Ib but this forces an impossible allocation of 3 - it must go in BOTH Bb abd Db. This means the initial Fb choice was wrong - it must be the other alternative.

There's probably a better way of proving this but I can't see one.

Apologies again.

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Yes they are useful, really.

Many of the challenging level of difficulty puzzles on this site require the use of X-Wing.

They crop a lot in the larger 16x16 sudoku grid puzzles too.

That's probably because there are so many more squares and the pattern of matching rows and columns required for the X-Wing are much more likely to occur.

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Here is a puzzle that has an X-Wing in it.

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With release 15 of Sudoku Dragon there is an animated tutorial for the X-Wing strategy as well as new ones for naked twins, naked triplets and naked chains.

To run the tutorial just download the program and select the X-Wing tutorial. There is also a step-by-step guide to X-Wing online on our site.

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