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Solving Sudoku puzzle containing C shapes

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C Shape [Game solving Strategies forum] contributed by Alexander

It's useful to look out for 'C' or similar shaped solved squares in a region. What I mean by this is an arrangement of one completed row/column of three with one other row/column with two squares solved. That can make the missing Sudoku square in that row/column easy to solve.

In this case the 'C' is in region Da. The contents of column c have been solved and there are two solved in column a. Now look at column b the symbols 7 and 5 must go somewhere in region Da, the 7 is already allocated but the 5 must go somewhere and because of the C shape it can only go in Ea.

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C Shaped? (re: C Shape) contributed by Lynne

Not sure I understand what you are on about here, Alexander.

The shape is only very rarely a 'C' it can be any shape, I think you mean that there is one row of three and then row of two solved. As there is only one square left in the second row it's fairly obvious that this square should be worth checking to see if it can be solved.

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Other Shapes (re: C Shaped?) contributed by Alexander

Yes, that's more or less what I meant, poorly described, and I don't think 'C' describes the pattern very well anyway.

It can be a completed sub-row (three squares horizontal) or sub-column (three squares vertical) in the same sudoku region. Then you need in the same region, another sub-row or sub-column with only two squares allocated and the rest clear. I think that makes lots of other shapes other than 'C'.

My point was that the unallocated square in the sub-row or sub-column is often a good one to investigate. Look at the regions in the same row (or column) and see if any numbers are forced to go into that square.

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