Smash hit

My program uses a multi-part algorithm to determine a puzzle’s difficulty. There is no one mathematical precept which determines it.

Can we please have a few puzzles where we will need to choose between two candidate numerals for a particular cell, and settle on one only by showing that the other choice leads to a contradiction? Tom Hennessy, West Sussex

I think you have set out the definition of trial and error. So No, there won’t be any Su Doku puzzles from Pappocom where you need to use trial and error. On the other hand, you might choose to guess – that’s up to you. Some people enjoy solving puzzles with guesses. Myself, I find it more satisfying to solve a puzzle using logic alone. All The Times puzzles from Pappocom are solvable using only logic.

How often is there more than one solution to Su Doku? Elizabeth Frost, Saudi Arabia

Never. See the answers to questions by Andrew Marjoribanks and Katherine Edgcombe. Incidentally, I get a kick out of the fact that Su Doku is being enjoyed in Saudi Arabia.

These puzzles are great, but easy. Friday's takes a couple of minutes and a bit of thought, but Monday's can be done in one’s head. I have resorted to making it harder by allowing myself to use only one or two of the several logical rules available. Please give us a bit more of a challenge. How about making the published puzzle “hard” and providing extra numbers via optional hints on the website? Stephen Davis, Alton

The idea about optional website hints for Hard puzzles is interesting. Let me think about it for a while.

Do you object to computer programmes being created to solve the Su Doku puzzle? Name and address witheld

No, I don’t object – I just don’t understand it. As a programmer myself, I can understand the appeal in writing the solver program. But I can’t understand the appeal in actually using it to solve a puzzle. Unless, perhaps, one needs the solution to enter a contest and doesn’t have the ability to get it any other way. A computer solver can solve a Hard puzzle in less than a second. If some people imply that the puzzle is therefore worthless, I don’t agree. It just highlights the computer’s ability at some things and our own limitations in others.

Either I am getting too clever or your puzzles are getting easier. Have you such a thing as a level high and if so will we get this for Christmas? What is the minimum number of clues where logic will still apply? DJ Bracher, West Sussex

If you are finding them easier, it’s because you have sharpened your abilities. Good for you. That has to be a good thing, doesn’t it, for everyday living. Yes, there are harder puzzles available, and they may get an airing over the Christmas period.

I am finding these puzzles are addictive. Are you planning to make a book full of them? Jeff M Dacombe

There have been so many enquiries about a book. Discussions are under way and it looks likely that a book will be available in the United Kingdom soon.

What plans do you have to develop Su Doku, for example, 4x4 and 5x5 grids? Neil Barrett, Nottingham

The Su Doku program lets you choose the size you want. Classic Su Doku is 9x9, but the Su Doku program can deal with grid-sizes from 4x4 to 16x16.

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