Sudoku Rules vs Answer Checks: A Worked Example

Native Sudoku has a Check button, but its feedback answers a different question from checking for duplicates. A digit can avoid every visible duplicate and still differ from the puzzle’s stored answer. This example shows the difference, without making a claim about health or cognitive benefits.

The starting puzzle and its source solution

A dot is an empty cell. Coordinates start at row 1, column 1. This fixture comes from the site’s Sudoku generator and is reproduced here for explanation; the player does not have an import function.

Starting puzzle:

. . 2 | . . 1 | 3 . .
. 4 3 | . . 7 | 8 . 1
. 1 . | . 2 . | 7 . 9
------+-------+------
. 5 9 | . 3 . | . . .
1 7 . | 9 . . | 5 3 8
. . . | . . . | 9 4 2
------+-------+------
6 . . | . . 9 | . 7 3
. . 5 | 1 . . | 4 . 6
. . . | 2 . 6 | 1 . 5

Complete source solution:

7 9 2 | 8 5 1 | 3 6 4
5 4 3 | 6 9 7 | 8 2 1
8 1 6 | 3 2 4 | 7 5 9
------+-------+------
2 5 9 | 4 3 8 | 6 1 7
1 7 4 | 9 6 2 | 5 3 8
3 6 8 | 7 1 5 | 9 4 2
------+-------+------
6 8 1 | 5 4 9 | 2 7 3
9 2 5 | 1 7 3 | 4 8 6
4 3 7 | 2 8 6 | 1 9 5

Each row, column and 3x3 box of the source solution contains digits 1 through 9 exactly once. The original givens match it. This is a concrete answer to compare against, not a claim that any arbitrary partly filled grid is solvable.

Try 5 in row 1, column 1

The given digits in that row are 2, 1 and 3. The column contains 1. The top-left box contains 2, 4, 3 and 1. None contains 5, so writing 5 creates no immediate duplicate.

However, the source solution has 7 there. Entering 5 and pressing Check marks row 1, column 1 as incorrect. The board accepts the entry before checking: input itself does not enforce the three Sudoku constraints.

Question	Result for this 5
Does it duplicate a visible peer?	No
Does it match the stored answer?	No: the source digit is 7
Does entering it fill the board?	No
What does Check return?	One incorrect entry; the puzzle is not complete

No immediate duplicate is a useful first filter, but it is not a proof of the answer. To establish a digit by reasoning, use information from other cells too. Solving basics covers missing numbers and candidate elimination.

What the different feedback states mean

State	Result	Limit of that result
Original givens and blanks	No incorrect entries found	The board is still unfinished
A wrong normal entry, such as this 5	Incorrect entry marked	It differs from the stored solution
Only a wrong pencil note added	No incorrect entries found	Notes are not checked
Every cell filled with the source answer	Puzzle complete	All entries match and no blanks remain

An entry or note edit clears visible error marks; that does not establish that other wrong entries were corrected. Selecting a cell also redraws the board, but retains existing error marks. Check again when you want another answer comparison.

Check does not calculate candidates, explain a deduction, fill an answer or validate notes. For the input steps, use touch, keyboard and notes. The useful distinction is between a candidate you have not ruled out, a chosen entry, and feedback against the generated answer.