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How to solve big grid puzzles

Share your thoughts on how to tackle larger grid sizes.

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How to Tackle? [Larger Puzzle Grids forum]

Here are some example big grid puzzles and discussions on how to set about solving them.

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Where to solve next (re: How to Tackle?) contributed by SudokuDragon

I think I have done all the easy squares, where should I look to solve next?

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BigGridHelp (re: Where to solve next) contributed by Prudence

I took a look and the thing that I spotted was the Sudoku stripe.

It is in column l in reverse order.

When you have filled in the whole column you have lots more squares to tackle.

Square Ek now must take a 1 as no other value will go there.Square Aj must take a 7. Looking at row A Ah must be 2 and then Ap must be E. Concentrating on region Am square Cp can only be an F. Looking along row C

I can see Cc can only be E. While still in region Aa I see that because of 'F's in the other regions and Bd is only square in row B that F can go. Similarly D in row B can only go in Bb.

Looking around there are lot of 'F's allocated and in fact the two last Fs can be allocated easily - Ei and Mo.

Finally for now the region Ai is complete but for two squares and now the missing 9 can only go in Bi and 2 in Di.

With two regions completed I think you should now be able to make more progress. Finding the stripe was the clincher.

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Stuck again! (re: BigGridHelp) contributed by Georgina

That's very useful thanks.

BUT... I get stuck after only tackling just a few more squares along the way from here.

Can you help me out again?

Thanks a million

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X-Wing solution (re: Stuck again!) contributed by Alexander

Hold your breath, I think the answer to this problem is beyond the simpler good strategies. You need to deploy the dreaded X-Wing strategy to get any further with it.

First thing to do with X-Wing is to identify patterns of individual sudoku possibilities. Use the possibility bar for each symbol in turn to find the most promising number. Avoid ones with too many possibilities. In this case the possibilities for '1' are crucial.

Here is a screen shot with the '1's highlighted.

Look at Jp the 1 is excluded because column o has only two options both in region Im.

Also the same logic applied to Bm as 1 must occur in Dm or Dp (row D).

Both these simpler exclusions could be deduced also from the X-Wing in row D and row H. There are only two possibilities and these will both exclude Bm and Jp.

Now look at the rest Row B now has two options which form a box with row L and row J.

The 1 can not go in Lh so it must be in Bh. Allocating a 1 here should get you started again.

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X-Wing wrong (re: X-Wing solution) contributed by Georgina

You've slipped up on this one Alexander. There is only one X-Wing and your suggestion of box Bh as '1' is just wrong - it makes it unsolvable.

An X-Wing must have two rows (or columns) with ONLY two possibilities forming the corners of a box. This is not true for rows B and L. Yes there is an X-Wing with the 1s (Row D and Row H) that excludes a 1 in Bm and also in Jp but that not 'force' an allocation of a '1' anywhere that I can see.

That obviously leads to the choice of Be for the '1' instead of Bh as that's the only choice but I can't see a logical rationale for that without resorting to trial and error.

I think Alexander should show us how he can 'logically' select another square.

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X-Wing correction (re: X-Wing wrong) contributed by Alexander

You are right, sorry, I got it wrong - I must have not spotted all the possibilities for some reason.

And what is worse, I can not see a genuine X-Wing or Swordfish in this puzzle. So I think you have to resort to trial and error. When doing this choose a symbol that can only occur in two places in a group. Mark the square using the 'Notes' feature of Sudoku dragon. If the option turns out wrong you can use the Undo feature of Sudoku dragon to get back to where you started.

If anyone can see a proper logical strategy to make any progress from this point please let me know.

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