The New Sudoku Players' Forum

[Marie26]

I am new to Sudoku and am enjoying doing these puzzles. I am able to do the easy ones and sometimes the medium ones. However, I am unable to complete a hard one.

What are the main differences between easy, medium and hard? Are there special techniques that are needed to complete a puzzle that is at a different level?

Thank you for any advice you can give me.

[lunababy_moonchild]

I am new to Sudoku and am enjoying doing these puzzles. I am able to do the easy ones and sometimes the medium ones. However, I am unable to complete a hard one.

What are the main differences between easy, medium and hard? Are there special techniques that are needed to complete a puzzle that is at a different level?

Thank you for any advice you can give me.

Hello Marie and welcome to the Forum.

It depends on whose puzzles you are doing. Generally speaking there are differences between the grades and it just takes practice to work your way up the grades. If you are referring to Pappacom's puzzles then yes, there are distinct differences between the grades (ie they get progressively harder) but you only need the x-wings technique to complete the Very Hards (these are only available via the Pappacom software, on this site) and the rest are completed using logic alone. As for other puzzles, it's much like driving a car, the basic principles are the same but sometimes they 'handle' differently.

However, if you are interested in the more advanced techniques please peruse the Solving Techniques section (too many of them to quote directly here) and you'll find an eye-watering array of techniques avaliable. Whether or not you choose to use them is entirely up to you - it depends on how far you want to take the solving experience, really.

You could try these (click on the blue writing) for hints/tips/solver AngusJ or Sadman or Sudoku Susser

Hope that helps.

Luna

[MCC]

... but you only need the x-wings technique to complete the Very Hards...and the rest are completed using logic alone.

Luna, are you saying that x-wings are not logic surely not

[Jeff]

x-wing is logic only if you understand how it works. Almost all do. But, I think only few know how a swordfish or a jelly fish works. This is evident by the fact that I have never seen a proof for a swordfish or jellyfish being presented. All hints available so far only describe the patterns and show how to use them.

[lunababy_moonchild]

... but you only need the x-wings technique to complete the Very Hards...and the rest are completed using logic alone.

Luna, are you saying that x-wings are not logic surely not

No, I'm not saying that but I can see how you got the impression! What I meant was : the only advanced technique needed to solve Pappacom's puzzles is the x-wings and that is only needed for the Very Hards.

Luna

[Cec]

I am new to Sudoku and am enjoying doing these puzzles. I am able to do the easy ones and sometimes the medium ones. However, I am unable to complete a hard one.

Hi Marie, At the risk of giving you too much homework the following additional link explains some more hints to help you tackle those "hard ones":

http://www.angusj.com/sudoku/hints.php

Cec

[MCC]

It was only a small point Luna but I thought it may lead to a misunderstanding with any newbie looking in.
So thank you for the clarification

Marie26, I think that sometimes it's easier to work with a puzzle you're familiar with, especially when trying to learn new techniques.
So if you want to post a puzzle that you're currently working on there are plenty of people willing to give you guidance.

[emm]

Luna, glad you’re back! Isn’t MCC such a stickler for exactitude? Such an obsession with niggly degrees of accuracy. I thought your reply was crystalline – everything that needed to be said in a (largish) nutshell.

Jeff – what do you think we are? Jellyfish maybe, but swordfish?! And why do we need a proof? I’ve never seen a proof for pairs or wings or chains or anything else for that matter. Isn’t the proof of the swordfish in the doing?

[angusj]

This is evident by the fact that I have never seen a proof for a swordfish or jellyfish being presented.

OK, I'll rise to the challenge .

Swordfish proof:

For every Sudoku, candidate C must be found exactly once in any row (or column). If a row has only 3 possible cells for C, then it must be assigned to one of these 3 cells.

Given a puzzle that has 3 rows where candidate C is restricted to exactly the same three columns (and no more than 3 columns), and since
1) candidate C must be assigned once in each of these three rows
2) no column can contain more than one of candidate C
then candidate C must be assigned exactly once in each of these columns within these three rows. Therefore, it's not possible for any other cells in these three columns to contain candidate C.

This same logic applies when a puzzle that has 3 columns where candidate C is restricted to exactly the same three rows.

[Nick67]

A nice proof!

Since the topic came up, I just can't resist
mentioning a proof that I posted in a different
forum, in this thread .

[Jeff]

Angus, Nicely done. Your proof also explains why a swordfish can have a 322, 332 or 333 formation.

BTW, swordfish can also be proven by forcing chain theory which is not as easy to grasp.

[Jeff]

– what do you think we are? Jellyfish maybe, but swordfish?! And why do we need a proof? I’ve never seen a proof for pairs or wings or chains or anything else for that matter. Isn’t the proof of the swordfish in the doing?

Hi em, If you know how to prove a technique, it means that you understand the technique. Perhaps pair is too trivial. But, all wings and chains can be proven by forcing chain and forcing net implications. Does the following look familar to you?

xy wing:

Code: Select all

 
 . . . | . . . | . . .
 . . . | . . . | . . .
 . . . | . . . | . . .
-------+-------+-------
 . . . | *-xy------yz.
 . . . | |/. . | . . .
 . . . |xz . . | . . .
-------+-------+-------
 . . . | . . . | . . .
 . . . | . . . | . . .
 . . . | . . . | . . .
 
r4c5 is either x or y. If it's x, r6c4 is z. If it's y, r4c8 is z. In either case, r4c4 cannot be z.

xyz-wing:

Code: Select all

 .  .  .  | .  .  .  |  .  .  .
 . xy----------------|-xyz .  *
 .  .  .  | .  .  .  |  . \xz .
----------+----------+----------
 .  .  .  | .  .  .  |  .  .  .
 .  .  .  | .  .  .  |  .  .  .
 .  .  .  | .  .  .  |  .  .  .
----------+----------+----------
 .  .  .  | .  .  .  |  .  .  .
 .  .  .  | .  .  .  |  .  .  .
 .  .  .  | .  .  .  |  .  .  .
 
r2c2 is either x or y. If it's x, r2c9 cannot be x. If it's y, r2c7 is xz which forms a naked pair with r3c8, therefore r2c9 cannot be x. In either case, r2c9 cannot be x.

Double implication chain:

Code: Select all

r1c1=4 => r1c6=7
r1c1<>4 => r4c1=4 => r6c3<>4 => r6c6=4 => r3c6=3 => r1c6=7
Therefore r1c6=7

Triple implication chain:

Code: Select all

r6c2=1 => r1c2<>1 => r1c2=2 => r3c3<>2
r6c2=2 => r6c2=2 => r7c2<>2 => r7c3=2 => r3c3<>2
r6c2=9 => r3c2<>9 => r3c3=9
Therefor r3c3<>2

Forcing net:

Code: Select all

r9c1=5=>r9c7=8=>r7c9=7=>r7c8=2=>r8c8=5=>r3c2=8=>r1c3=2=> r5c3 = 3
r9c1=9 => r4c1=7 => r3c1=1 => r5c1=2 => r5c3 =3
Therefore r5c3=3

[emm]

Are you trying to catch me out, Jeff? Are these really proofs? Or are you being overcompensatorily explanatory? Without being too pernickety, your proofs look pretty much like a lot of descriptions which, as you put it, just ‘describe the patterns and show how to use them’.

Possibly it’s a personal thing – proofs aren’t really what my relationship with Sudoku is all about. We have – well, not quite an open relationship since I don’t actually do any other puzzles and not exactly a serious relationship either – it's more a modern sort of arrangement based on an understanding. Perhaps for me personally, it’s the kind that doesn’t require a lot of proofs.

Anyway, enough aimless rambling. The point I was actually trying to make, albeit unsuccessfully, was that I thought it was a little condescending of you to say that other people don’t understand Swordfish because you hadn’t seen a proof.

[Guest]

Hey, I'm new here, and all this talk about x-wings and swordfish is really confusing me. What's it all about

[Myth Jellies]

Sal,

Try this link for starters:

http://www.angusj.com/sudoku/hints.php