Expert level in app

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Expert level in app

Postby peddidle » Thu Apr 09, 2020 5:48 am

Hello, I’m quite new to Sudoku and very new to the expert level. I’ve been stuck at this point in the puzzle for a couple hours now. I’m looking for help as to what technique involving which cells I seem to be missing in order to progress. Thank you!

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Re: Expert level in app

Postby Wepwawet » Sun Apr 12, 2020 3:32 am

You have 2 Skyscrapers that will solve the puzzle. As you are new to what you call the Expert Level, and you haven't seen the Skyscrapers, then, I am assuming that you are not familiar with them. Skyscraper are short AIC chains.

They can be represented by this notation:
(a) Skyscraper: 1 in r1c6,r5c4 (connected by r15c2) > r4c6<>1
(b) Skyscraper: 2 in r6c8,r8c9 (connected by r68c2) > r4c9,r7c8<>2

In the example (a) the notation above is informing you that r4c6 cannot be a 1.
Now, either r1c6, is a 1 or it is not.
If r1c6 is a 1, then r4c6 cannot be a 1, conversely, if r1c6 is not a 1, then r1c2 is a 1, therefore, r5c2 cannot be a 1, so r5c4 must be a 1, which again means r4c6 cannot be a 1.
So you can see, whether r1c6 is a 1 (or not), r4c6 can never be 1.

In example (b) the notation above, is informing you that both r4c9 and r7c8 cannot be a 2.
Now, lets look at r8c9, it is either a 2 or it is not a 2.
If r8c9 is a 2, then r4c9 and r7c8 cannot be a 2, conversely, if r8c9 is not a 2, then r8c2 is a 2, therefore, r6c2 cannot be a 2, so r6c8 is a 2, which means, again, that r4c9 and r7c8 cannot be a 2.
So you can see, whatever, r8c9 is (a 2 or not a 2), r4c9 and r7c8 can never be a 2.

Hope that explanation was useful.

Edited: To explain Skyscrapers notation
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Re: Expert level in app

Postby Leren » Sun Apr 12, 2020 4:11 am

A link to the Hodoku explanation of Skyscrapers is here.

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Re: Expert level in app

Postby dxSudoku » Tue Apr 14, 2020 3:06 am

If you can download and use Hodoku I highly recommend it. It has "Learning" mode where every time you create a new puzzle the very next step will be a Skyscraper. This way you can practice over and over again until you become an expert with this one technique.

If you interested I did a training video on Skyscrapers and how to find them in puzzle:

I did a bunch of videos on downloading and using Hodoku if you are interested:

Just cut-n-paste the link into your browser to see the YouTube playlist.
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