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I used a step-by-step Sudoku solver for a puzzle in which I was stuck.
However I don't understand how the next number (6) was found.

Sudoku puzzle where I was stuck:

sudoku stuck

Sudoku puzzle solved by one step forward:

sudoku one step forward

The solver says the only candidate for cell (R4, C5) is a 6. Why couldn't it be in cell (R6, C5)? I need a clear explanation; I'm not a Sudoku expert.

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3 Answers 3

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R6C5 doesn't have anything ruling out it being a 6... but R4C5 could only be a 6. There are no other options for R4C5: placing a 1, 2, 3, 4, 5, 7, 8, or 9 would break the rules. You know you have to fill a box with some number, and that is the only one left.

This is one of two basic Sudoku techniques, the "naked single" -- when a cell only has one option out of the 9 allowed digits, you can fill that number in. The other basic technique is the "hidden single" -- when there's only one place a certain digit can go in a row (or a column, or a 3×3 box). The confusion here seems to be coming from you assuming this is a hidden single, but in reality it is a naked single.

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  • $\begingroup$ I feel so dumb right now, this looks obvious. Thank you very much $\endgroup$
    – Sudokunoob
    Commented Mar 20, 2021 at 15:51
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    $\begingroup$ @Sudokunoob Lots of things look obvious when you see them written out like this! Learning to find these "obvious" things is just part of the learning process. $\endgroup$
    – Deusovi ♦
    Commented Mar 20, 2021 at 15:53
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The only possible number that could go in cell (R4, C5) is 6, as shown by the the grayed out numbers: 3 and 5 are eliminated by the row, and the rest are eliminated by the column.

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Nice to see it describes this way. The column C5 will only allow one of the 3,5,6 values in the available cells. R4 has the 3 and 5, which removes the 3 and 5 from the candidates. This forces the 6 onto that position. Refered to as the naked value.

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  • $\begingroup$ Welcome to Puzzling! While this is correct, there are already two answers saying the exact same thing which were posted a while ago. Unless you have a substantial improvement on a previous answer, please don't post duplicates. $\endgroup$
    – bobble
    Commented Mar 21, 2021 at 17:22

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