Su Doku

[ThePenguin]

Without getting into an argument whether or not Su Doku puzzles suck, are there any strategies to defeating the harder ones besides the obvious look down the lines and in the larger squares options?

[yellowjack]

Apply logic, not everything is that cut and dry. Some of the normal difficulty sudoku puzzles (and all the more difficult ones) require a few instances where you have to use the process of elimination for some squares.

[mute]

Soduko

Check out "Terms related to solving".

[ThePenguin]

I understand the logic and process of elimination points. That all seems fundamental. However, I find that I get into spots where I have seemingly eliminated all spaces that have certain answers, and must start guessing between one or another number and see if the guess works from there. Is this right? Or do all SuDoku puzzles follow a process of elimination in which the next number can be found with certainty?

[youtalkfunny]

All Soduko/Sudoku/Whatever puzzles can be solved without guessing.

Or so they tell me. [img]/images/graemlins/frown.gif[/img]

[El Diablo]

TP,

Sometimes you have to do multiple square combos at once because you can't with certainty identify an individual square.

[posnera]

I find the most useful thing when stuck is to look for multiple squares with multiple number possibilities. You can then eliminate those numbers from the rest of the row/box.

I know that isn't in English, so here's an example:

You have 4 squares in a box. The possible numbers for each are:

1,2
1,2,3
2,3,4
1,4

No other square in this box can use the numbers 1-4.

[DougShrapnel]

So lets say the you have these for squares in a row or collumn or a box.

1,2
1,2,3
2,3,4
1,4

the possible placements are
1, 2, 3, 4
1, 3, 2, 4
2, 1, 3, 4
2, 3, 4, 1

If we have no other easy eliminations and we are forced to guess it is best to guess 4 for the 4th square since it is the correct answer 75% of the time. Just looking at that box, collumn, or row.

Looking further. If these boxes were in a row, look at the collumn corresponding to box 4. If 4 is also the most likely solution for that box in a collumn as well as the row, Guessing 4 is a well educated guess. If however 4 is the correct solution only 25% of the time we haven't made a well educated guess and should find another square that we can educate a guess better on.

The trick to guessing is find a way to mark the part of the solution up until the guess. I copy the whole thing. Someone I know color codes it.

[posnera]

The point is that you never have to guess. In our example, you may be able to finish the other 5 boxes using 5-9 and be able to move to another area. The odds of being right don't matter.

[januarymute]

IMO the most interesting thing about Sudoku (seen this spelled Soduku elsewhere btw) is the construction of the original puzzle, with various number of givens (pre-specified squares) and so on. Wikipedia happens to touch on this:

"A puzzle is a partially completed grid. The initially defined values are known as givens or clues. A proper puzzles has a single (unique) solution. A proper puzzle that can be solved without trial and error (guessing) is known as a satisfactory puzzle. An irreducible puzzle (a.k.a. minimum puzzle) is a proper puzzle from which no givens can removed leaving it a proper puzzle (with a single solution). It is possible to construct minimum puzzles with different number of givens. The minimum number of givens refers to the minimum over all proper puzzels and identifies a subset of minimum puzzles. See Mathematics of Sudoku-Minimum number of givens for values and details."

I suspect that the most difficult Sudoku puzzles are also very tricky to construct.