How to Solve Digital Sudoku

The rules of Digital Sudoku

Fill the grid so every row, every column, and every bold box holds each digit exactly once. The easy board is 6x6 with boxes of two rows and three columns and uses the digits 1 to 6. The medium and hard boards are 9x9 with 3x3 boxes and use the digits 1 to 9.

Digits are drawn the way a digital clock draws them, from seven segments named top, upper right, lower right, bottom, lower left, upper left, and middle. A 1 uses only upper right and lower right. A 7 adds the top. An 8 uses all seven.

Lit segments are clues. Whatever digit you place in a cell must use every lit segment in that cell. Faint segments are unknown, not off. A faint segment may or may not be part of the answer, so never rule a digit out because it uses a faint segment.

Reading a cell

Each lit segment cuts the list of digits a cell can hold. A cell with one lit segment can hold any digit that uses that segment. On a 9x9 board, top allows 2, 3, 5, 6, 7, 8, or 9. Upper right allows 1, 2, 3, 4, 7, 8, or 9. Lower right allows every digit except 2. Bottom allows 2, 3, 5, 6, 8, or 9. Lower left allows only 2, 6, or 8. Upper left allows 4, 5, 6, 8, or 9. Middle allows every digit except 1 and 7.

More lit segments means fewer candidates, because the digit must use all of them. Upper left and lower left together allow only 6 or 8. Upper left and upper right together allow 4, 8, or 9. A cell with nothing lit tells you nothing on its own, and Sudoku logic has to fill it.

How the 6x6 board changes candidates

The easy board only uses 1 to 6, so 7, 8, and 9 drop out of every list. That makes single segments much stronger. Lower left now means 2 or 6. Upper left means 4, 5, or 6. Upper right means 1, 2, 3, or 4.

Some pairs of segments settle a cell outright on 6x6. Upper left with lower left can only be 6, because the other digit that fits, 8, is not in play. Upper left with upper right can only be 4. Watch for these on easy boards, they are the quickest placements on the grid.

Useful facts about digital digits

8 uses every segment, so it fits any clue at all. That is why 8 is the hardest digit to pin down from segments on a 9x9 board, and why you will usually place it last, by Sudoku logic. A cell never starts with all seven segments lit, since that would simply be a given 8.

1 and 4 are the only digits without a top. 2 is the only digit without lower right. Lower left is the rarest segment, used only by 2, 6, and 8. 7 uses top, upper right, and lower right, and 3, 9, and 8 all contain those same three, so a cell lit like a 7 can still be 3, 7, 8, or 9.

Shapes nest inside one another. A lit 1 shape fits 1, 3, 4, 7, 8, and 9. A lit 9 shape fits only 9 and 8. Thinking in terms of which digits contain a pattern is faster than checking digits one by one.

Combining segments with Sudoku logic

Start by writing each cell's segment candidates as pencil notes, beginning with the cells that have the most lit segments. Then solve the way you would any Sudoku. Remove a placed digit from its row, column, and box. Look for a cell with one candidate left, and for a digit that has only one possible cell in a row, column, or box.

Segments keep paying off as the grid fills. A cell with only lower left lit can hold 2, 6, or 8, so once its row already has a 6 and an 8, it must be the 2. On medium and hard boards you will also need pairs, pointing digits in a box, and other standard Sudoku techniques, now working from candidate lists that the segments made for you.

Work through a small example

Take the top row of a 6x6 board. From left to right, its cells have these lit segments. The first has upper right. The second has lower left. The third has top and lower right. The fourth has upper left and upper right. The fifth has upper left and bottom. The sixth has lower left and upper left.

Read each cell. The first can be 1, 2, 3, or 4. The second can be 2 or 6. The third can be 3, 5, or 6. The fourth can only be 4. The fifth can be 5 or 6. The sixth can only be 6. Place the 4 and the 6 straight away.

Now the row does the rest. With 6 placed, the fifth cell must be 5 and the second cell must be 2. The third cell loses 5 and 6 and becomes 3. The first cell loses 2, 3, and 4 and becomes 1. The row reads 1, 2, 3, 4, 5, 6, solved from segments and the row rule alone.

Move to the second row, inside the same left box. The box already holds 1, 2, and 3, so its three cells in the second row must be 4, 5, and 6 in some order. The first of them has upper right lit, which allows 1 to 4, so it is 4. The third has lower left lit, which allows 2 or 6, so it is 6. The middle cell has nothing lit at all, and the box makes it 5. Keep alternating between segments and Sudoku logic and the whole board falls into place.

Notes, checking, and finishing

Tap the pencil button, or press P, to switch to notes. Notes fade the faint segments behind them so your candidates stay readable, and lit clue segments stay visible underneath. Erase clears the selected cell.

You can enter any digit in any cell, even one that does not fit its lit segments. The pad never blocks a digit. It is just a wrong answer. In Check mode nothing is flagged as you play, and you press Check to test the whole grid. In instant mode a wrong digit is flagged at once, and three mistakes ends the puzzle. Digital Sudoku is currently marked beta, so presentation may still improve while the rules stay fixed.