Watch Where You Get Those Puzzles! (Part 2 of 2)

In the last newsletter, I gave you two puzzles that I consider to be “bad.”


One of the e-mails I sent out had the subject line: “I can’t solve these Sudoku puzzles. Can you?”

Well hundreds of readers took it on as a personal challenge to solve those puzzles. And I got over a hundred replies with solutions. So does that mean that Your Sudoku Professor is losing his edge? Am I useless has-been? (I certainly hope you don’t think so.)

Brian from Tauranga, New Zealand was kind enough to run the puzzles through a computer program. He came up with the following analysis:

“Both are rated Diabolical by my computer. The first one has 32 solutions. The second has 172 solutions.”

I was shocked! There should only be one solution! I had no idea that the puzzles were that bad. Thank you, Brian!

You have proved my point!

The point is that these bad puzzles lead you to a point where you have to guess. There is no other way to proceed except to guess.

And at times one guess is as good as another.

That’s why there are so many possible solutions.

Sudoku Puzzles are supposed to be solved using logic.

If you’re guessing, you’re not using logic. If you get to a point where you can flip a coin to choose your next step (and no matter whether you get head or tails, you’ll be right), then that’s not logic.

So if you keep getting to the point where you have to guess, the problem is one of two things:

  1. You don’t know what logic to use, or
  2. It’s a bad puzzle

All my lessons show you how to use logic only and not guess, so that takes care of problem #1.

My point with this article and the prior one is to help you with problem #2.

How to Spot Bad Puzzles

There’s no sure-fire way how to spot bad puzzles. But there are two common characteristics:

  1. They are not symmetrical
  2. They come from web sites, computer programs or gaming machines

Or in a nut-shell: Symmetry and Source.


Good puzzles have symmetry. That is, the pattern of the given numbers is similar top-to-bottom, left-to-right or even on a diagonal.

In other words, it almost like if you were to fold the piece of paper horizontally, vertically or diagonally, the given numbers (and hence the empty spaces) line up.

Take this one for example:

NL04_Puzzle03Puzzle Copyright © 2006, Nikoli Co., Ltd., reprinted with permission

Notice how box 9 looks like an upside down version of box 1? Box 2 and box 8 are mirror images of each other. Same with boxes 4 & 6.

That’s symmetry. All good puzzles have it, because symmetry is typically the result of careful puzzle creation. In other words, someone is paying attention.

Every bad puzzle I’ve ever encountered was asymmetrical. Notice the two bad puzzles in the last newsletter have little or no symmetry at all.

Now that’s not to say that every asymmetrical puzzle is bad. That’s not the case. But if it’s not symmetrical then it’s my first clue that I could have a problem.


There are a ton of sources for puzzles out there. The internet is full of sites that offer free Sudoku puzzles.

But you need to realize that the goal of many of these web sites is NOT to offer you quality Sudoku puzzles. Their goal is to get you to click on the ads, so they make money.

Now don’t get me wrong. I’m OK with people making money on advertising. We wouldn’t have newspapers or television without it.

But what if your newspaper or TV station only offered false, misleading or inaccurate information?

And then they sensationalized everything, just to get you to read or watch? (Oh, wait! I guess they already do that…)

Well, at least with the newspapers and TV stations, you typically know what you’re getting. You can tell what biases they have, and you can roll your eyes when they make such a big deal out of small things.

But with these Sudoku puzzle web sites, you’re expecting, at a minimum, a puzzle that’s solvable. Seems reasonable. Right?

To be fair, many web sites do offer pretty good puzzles. But which ones are they? They may be good now, but are they going to go bad later?

Here’s the deal: Most of those web sites use computers to generate the puzzles, but they often don’t make the effort to check for solvability. Or if they do check for solvability, they use a computer program to do it, and not a human being.

But what about all those computer programs and apps you can download that can generate an infinite number of puzzles?

Well, same as the websites. The program is generating the puzzle, so who’s there checking to see that a human can solve it? Just you. Only that you didn’t know you were the tester, so after hours of frustration, you finally give up. Like one of my friends told me:

“It’s like banging your head against the wall, only with fewer opportunities for reward.”

What to do?

I continue to recommend Sudoku puzzle books.

They’re relatively cheap, portable, a lot more flexible than an computer program (at least for pencil marks), and best of all, I’ve never encountered a book where the puzzles lacked symmetry or the puzzles were unsolvable.

You see, the book publishers have a different goal than the website operators.

A book publisher wants to sell you more books. So if they give you a bad product, you’ll find another publisher. But if you like what they offer, you’ll buy more.

Also book publishers are going through a great deal of expense to get this book to you. By comparison, a website operator has virtually no expense. So who are you going to trust more? Someone who’s got a lot to lose or someone who’s got nothing to lose?

And these books are cheap (even at full price). Most books have 300 or more puzzles in them and cost less than $8 (US). If I can do about 2 puzzles a day (which is a stretch), most books will last me almost 6 months!

The great thing is that you can find many good books in the discount section of your book store or you can order them from used book sellers off Amazon.

A few months ago, I got a book for $2 (including shipping) off Amazon and the only thing wrong with it was someone had started to solve the first puzzle. No big deal. I still had 319 puzzles left to solve. A couple of years ago I got a book of 588 Sudoku puzzles for $5 at my local Barnes and Noble.

By the way, this time of year is a great time to find lots of great bargains on books at your local book store.

Now if, despite my love of books, you’ve just got to have a Sudoku program or app, then I suggest this process.

  1. Find a puzzle maker you trust by looking for their puzzles in print. So, for example, look in your daily newspaper and see who produces that puzzle. Many times they’ll have a web address right there or you can Google the company name.
  2. Go to their web site and see if they have a program or app you can download.

I wouldn’t expect it to be free. But whatever the price (I’m sure it’s not too high) it may be worth it to avoid the frustration of unsolvable puzzles.

My Recommendations

For books, there are two primary sources I recommend.

And I can recommend these without reservation because I’ve completed many hundreds of their puzzles, and I’ve never had a problem.

  • Any book by Nikoli Publishing. Nikoli is the Japanese company that popularized the game in Japan before it spread to the rest of the world.

Some of their titles include Original Sudoku, More Original Sudoku, X-Treme Sudoku, etc.

Even though these puzzle are never extremely hard, they are the most fun to solve because the have the most unique patterns of starting numbers.

  • Any of the Martial Arts Sudoku Book series by Frank Longo.

He’s got White Belt Sudoku, Green Belt Sudoku, Brown Belt, and Black Belt.

He also has multiple editions of those books. So, for example, there’s Third Degree Black Belt Sudoku. (I don’t know that those puzzles are harder than “Second Degree.” I think it’s just the third black belt book.) The great thing is that you can buy a book of puzzles that exactly at the right level for you.

Now there are plenty of other really good books out there. But the two above are my favorites.

If you’re looking for websites that you can download and print puzzles from, here are my recommendations.

I’ve tried puzzles at all these websites. (I’ve tried more of KrazyDad’s). I’ve never had a problem, but don’t get angry with me if you ever do get an unsolvable one. So they seem good, but I’m not making any guarantees. – just click on Sudoku Collections, select your difficulty level, then pick a collection. These are free, but a donation is suggested. – You can solve on-line or you can print one puzzle at a time. – You can solve on-line or you can print one puzzle at a time.

Bottom line, beware of where you get your puzzles.

Be vigilant. If you’ve got a problem trying to solve a puzzle that’s rated well within your skill level, then consider the source. If you can’t trust the source, then throw that puzzle out and find another one.

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The Professor