h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The "solution" of the puzzle at the top doesn't seem to be a solution at all. It is just a re-statement.

In solving the Times Su Doku puzzles, I have never had to resort to "what if" and I believe it should always be possible to solve them without that.

Hmm. Blame the editors for that one. I originally put the solution to the puzzle in a separate entry. You can find the original at A3819729

As to 'what if', that language does imply guessing, which is not really what I had intended. The method I was trying to get across is the one used where you have one of two possible values for a cell, and work forwards from that, discovering that, say, placing one of those numbers yields a situation where another cell has no possible solution, therefore that was the wrong choice. (Aaargh, I hope you know what I mean, I'm having difficulty making sense today...)

Jim

The editors tell me that they have fixed this now. However something hasn't updated yet. Probably lost in the cache somewhere.. Hopefully it will soon...

Well you made sense to me. (post 2)

I'd like to quickly point out a slight jump in deduction... In the third segment, where you say: "As there is already a 7 in the middle column (right at the top of the puzzle), the 7 must go in the vacant square to the left."
This only seems obvious after the reader realises that the /other/ vacant square, the one to the right, must be used by the 2, because of the placement of the 2s in the squares above it, and so the 7 can't go there.

Great entry, though!

Oh wait I just saw the 7s in the squares on either side.

I suppose the question of what is "what if" and what is pure logic is a difficult one. I don't believe I have ever had to "look forward" in solving puzzles (you know, if I put the 7 here, that means the 6 must be here and so on).

But, there are certain rules that I absolutely know to be true and you could say they are equivalent to looking ahead. For example:

- if in any set (row, column, mini-grid) you have two cells that both contain the same two numbers, then those two cells are the only two cells where those numbers can occur in that set.

- if in any set you have three cells that can contain XY, YZ and XYZ then those are the only three cells that can contain X, Y or Z in that set.