. . 9 | . 2 8 | 7 . .
8 . 6 | . . 4 | . . 5
. . 3 | . . . | . . 4
------+-------+------
6 . . | . . . | . . .
? 2 . | 7 1 3 | 4 5 .
. . . | . . . | . . 2
------+-------+------
3 . . | . . . | 5 . .
9 . . | 4 . . | 8 . 7
. . 1 | 2 5 . | 3 . .
In a comment to a previous post Flavio Poletti proposed a very interesting test case for a function which solves the Sudoku game - semantically invalid input, i.e. an input that passes intermediate validation checks (no duplicates in any row/col/9-square) but that cannot possibly have a solution.
Until then I thought that Sudoku was a completely deterministic game and if input followed all validation checks then we always have a solution. Apparently I was wrong! Reading more on the topic I discovered these Sudoku test cases from Sudopedia. Their Invalid Test Cases section lists several examples of semantically invalid input in Sudoku:
- Unsolvable Square;
- Unsolvable Box;
- Unsolvable Column;
- Unsolvable Row;
- Not Unique with examples having 2, 3, 4, 10 and 125 solutions
The example above cannot be solved because the left-most square of the middle row (r5c1) has no possible candidates.
Following the rule non-repeating numbers from 1 to 9 in each row for row 5 we're left with numbers: 6, 8 and 9. For (r5c1) 6 is a no-go because it is already present in the same square. Then 9 is a no-go because it is present in column 1. Which leaves us with 8, which is also present in column 1! Pretty awesome, isn't it?
Also check the Valid Test Cases section which includes other interesting examples and definitely not ones which I have considered previously when testing Sudoku.
On a more practical note I have been trying to remember a case from my QA practice where we had input data that matched all conditions but is semantically invalid. I can't remember of such a case. If you do have examples about semantically invalid data in real software please let me know in the comments below!
Thanks for reading and happy testing!
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