The New Sudoku Players' Forum

[Borneq]

I see example on http://www.thonky.com/sudoku/hidden-pairs-triples-quads/ (maybe only this example is bad?) There are cell 4 (counting from 0) and have possibilities:3,5,7,9. Is pair (5,9) and other possibilities was cross out.
But I analyze:
1,3->0,1,5,7
1,5->5
1,7->0,1
1,8->0,1,5,8
1,9->5
3,5->4,5
3,7->0,1,4
3,8->0,1,5
3,9->4,5
5,7->4
5,8->5
5,9->4,5
7,8->0,1
7,9->4
8.9->5
This mean: pair (1,3) is at positions 0,1,5,7, pair (1,5) at position 5. Let choose this pair which have exactly two positions:
1,7->0,1
3,5->4,5
3,9->4,5
5,9->4,5
7,8->0,1
Is five pairs with this feature and choosing (5,9) is arbitrary (?). Three pairs belongs to cell 4: (3,5),(3,9) and (5,9)
Cell 4 has possibilities: 3,5,7,9
(3,5) -> no 7,9 in this cell
(3,9) -> no 5,7 in this cell
(5,9) -> no 3,7 in this cell
This means that there was no possibility of any digit. Hidden Pairs strategy is misleading?

[JasonLion]

The example was:

1378 1378 2 4 3579 13589 6 13 18

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You are misunderstanding just slightly. You aren't looking for a pair that only occurs in two places, you are looking for individual digits that only occur in two places, and you also have two digits that only occur in the same two places.

1 occurs in 0,1,5,7,8 - can't be used with this technique
2 is already placed
3 occurs in 0,1,4,5,7 - can't be used with this technique
4 is already placed
5 occurs in 4,5 - a possibility
6 is already placed
7 occurs in 0,1,4 - can't be used with this technique
8 occurs in 0,1,5,8 - can't be used with this technique
9 occurs in 4,5 - a possibility

Since 5 and 9 each occur in only two places, and those two places are the same for both 5 and 9, the hidden pair technique applies. All other possibilities in those two cells can be eliminated.

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By the by, we normally use 1 origin column positions, instead of 0 origin as you used. I used 0 origin here to match your approach, though that would not normally be done.

[Borneq]

Good, this also significantly simplifies algorithm I want implement. Similar is in Naked Pair: 7 6 2348 9 1 348 23 5 23? not pairs but digit?
For example: if 4 would be in last cell and it was 7 6 2348 9 1 348 23 5 234 , not pair should be find?

[JasonLion]

Naked pairs use a slightly different logic: you are looking for two cells which only contain pencil marks for the same two digits (and do not contain any other pencil marks).

In your example, adding a 4 to the final cell eliminates the naked pair. The final cell no longer contains exactly two pencil marks, so it can not be part of a naked pair.

[StrmCkr]

hidden uses pencil marks

naked uses pencil marks that are missing.

logic is identical.
{hopefully that makes sense}

[pjb]

Hidden sets usually come with a complementary naked set. These I find easier to detect. For example in above example there is the naked quad 1378 1378 13 18.
Phil