# Sudoku Move Marked Wrong, Seems Legal: Software Bug? [duplicate]

The included screenshot shows an attempted Sudoku move. The computer is marking it incorrect. There is no other 7 in the row, column, or 3x3 cell. What gives?

• This question might help, it seems to be asking the exact same thing.
– Bass
Nov 13, 2019 at 11:01

This program likely has the solution built into it, so it knows that this 7 is wrong.

Each Sudoku puzzle has a single solution that can be figured out with pure logic.¹ Even if you place something without it directly contradicting anything else, it may be the case that you would find out later that that was wrong. You should only place numbers that you've deduced to go in that location -- that you're 100% sure of. Anything else could be a mistake, and would lead to the puzzle being unsolvable.

¹ (Or at least, it should, but apps are often poorly designed.)

• With this program you can solve every sudoku puzzle by brainlessly click every possibility on every cell... Nov 12, 2019 at 23:56

The problem is that the unique solution to the sudoku does not have a 7 there. At the moment it is not immediately clear what number should be in that corner, and there is no direct reason for it to be a 7 rather than a 2 or a 6.

In fact, with some thought you can show it will actually have to be a 6:

Column E only has one spot for its 6, namely cell E5.
Row 6 now has only one spot for its 6, namely cell C6.
Row 1 now has only one spot for its 6, namely cell A1.

The $$6$$'s at r4c9 and r8c2 mean that the $$6$$ in block left-middle is on the right, so the $$6$$ in row 1 is also in column 1 (due to r3c6 or r7c4, r4c9 and r9c8).