Column Indexer Sudoku is a 9x9 Sudoku variant in which columns 1, 5 and 9 double as an index. In every row, the digit sitting in column 1 says which column of that row holds the 1, the digit in column 5 says which column holds the 5, and the digit in column 9 says which column holds the 9. Those three columns are still ordinary Sudoku columns at the same time. The variant is sometimes called 159 Sudoku, and it is the only one in the app with no 6x6 version.
What is Column Indexer Sudoku?
Every other Sudoku variant adds something to the board: a cage, a dot, a thermometer, a shaded window, a number in the margin. Column Indexer adds nothing at all. The grid looks exactly like a Classic puzzle. What changes is how you are allowed to read three of the columns that are already there.
In a Classic Sudoku, a digit is only ever a value. In Column Indexer, the digits in columns 1, 5 and 9 are values and addresses. A 7 in column 1 of row 3 is genuinely a 7 — it competes with every other 7 in its row, column, and box exactly as it always would — but it simultaneously announces that the 1 of row 3 is over in column 7. One cell, two jobs, and both must be satisfied by the final solution.
That self-reference is what gives the variant its character. Deductions in Column Indexer tend to arrive in pairs, jumping across the row from an index column to the cell it names and back again. It is the most abstract variant in the collection and, for solvers who enjoy that turn of mind, the most satisfying.
Column Indexer Sudoku Rules
- Standard Sudoku rules: Each row, each column, and each 3x3 box must contain the digits 1 through 9 exactly once.
- Column 1 indexes the 1s: In each row, the digit in column 1 is the column number of that row's 1.
- Column 5 indexes the 5s: In each row, the digit in column 5 is the column number of that row's 5.
- Column 9 indexes the 9s: In each row, the digit in column 9 is the column number of that row's 9.
- The index columns are still columns: Columns 1, 5 and 9 must each contain the digits 1 through 9 exactly once, like every other column on the board.
How the Indexing Rule Works
Take row 4 and suppose the cell at row 4, column 1 holds a 6. Column 1 indexes the 1s, so this says: the 1 of row 4 lives in column 6. You can immediately write a 1 into row 4, column 6 — one clue, two cells resolved.
Now suppose instead that row 4, column 1 holds a 5. That says the 1 of row 4 is in column 5. But column 5 is itself an index column: whatever sits there names the position of row 4's 5. And what sits there is now a 1 — so the 5 of row 4 is in column 1, which is where we started. The two statements agree, and they resolve each other.
The three swap pairs
That last example generalises into the three most useful deductions in the variant. Within any single row:
- A 5 in column 1 forces a 1 in column 5 — and the reverse.
- A 9 in column 1 forces a 1 in column 9 — and the reverse.
- A 9 in column 5 forces a 5 in column 9 — and the reverse.
Each swap resolves two cells from one placement, and each one is cheap to check. Scanning the three index columns for these six digits is the fastest opening move in any Column Indexer puzzle.
The fixed points
A 1 in column 1 says the row's 1 is in column 1 — which is the cell itself. It is self-consistent and tells you nothing further, but it does tell you that this row's 1 is settled. The same holds for a 5 in column 5 and a 9 in column 9. Because columns 1, 5 and 9 are ordinary Sudoku columns, each of these can happen in at most one row on the whole board.
Solving Strategies
1. Open with the six swap digits
Before anything else, look for 1s, 5s and 9s already given in columns 1, 5 and 9, and for givens of those digits anywhere in a row that has an index cell filled. Each hit fires a swap and resolves a pair of cells. This is where the puzzle opens, and on easier difficulties it can carry you a third of the way through.
2. Work each row as a small system
The indexing rule never crosses a row boundary: a digit in column 1 of row 3 says nothing about any row but row 3. That makes each row a self-contained puzzle of its own, three index cells pointing at three targets. When you are stuck globally, pick the row with the most index cells filled and exhaust it.
3. Follow the pointer in both directions
An index cell tells you where a digit is. Equally, knowing where a digit is tells you what the index cell must be. If you have already placed the 9 of row 7 in column 3, then row 7, column 9 holds a 3 — no further reasoning needed. Solvers new to the variant reliably use the first direction and forget the second, which is where half the available deductions live.
4. Eliminate impossible pointers
An index cell cannot point at a column where the indexed digit is already impossible. If the 1 of row 2 has been ruled out of columns 3, 4 and 8 by ordinary box logic, then row 2, column 1 cannot be 3, 4 or 8. This turns every elimination you make about the 1s, 5s and 9s into an elimination in the index columns, and vice versa. It is the workhorse technique of the variant.
5. Keep the standard toolkit
Everything in our solving techniques library applies unchanged. Column Indexer puzzles are still Sudoku underneath, and the indexing rule mostly serves to convert progress on one digit into progress on another.
Why There Is No Mini Column Indexer
Every other variant added in version 1.2.4 ships as a free Mini 6x6 alongside its premium 9x9. Column Indexer does not, and the reason is structural rather than a matter of effort.
The rule names three digits and three columns simultaneously, and it depends on those columns being meaningfully placed: 1, 5 and 9 are the left edge, the exact centre, and the right edge of a nine-wide grid, and 1, 5 and 9 are the smallest, middle, and largest digit. The elegance of the variant is that those two facts coincide.
A 6x6 grid has six columns, so it has no centre column at all — the middle falls between columns 3 and 4 — and no set of three digits from 1 to 6 reproduces the smallest-middle-largest spacing that makes 1-5-9 work. Any 6x6 version would have to pick an arbitrary pair or triple of columns, and the rule would stop being the clean, self-mirroring statement it is on a 9x9 board. Rather than ship a compromise, Column Indexer is 9x9 only, and it is part of the premium collection.
If you want a gentle entry point into variant Sudoku, the four other 1.2.4 additions all have free Mini versions: Mini Skyscraper, Mini Anti-Knight, Mini Windoku, and Mini Odd/Even.
Tips for Beginners
- Say the sentence. "The digit in column 1 of this row tells me where the 1 is." Reading it as a sentence rather than a rule makes the double meaning stick.
- Learn the three swaps. 5↔1 across columns 1 and 5, 9↔1 across columns 1 and 9, 9↔5 across columns 5 and 9. They are the cheapest deductions on the board.
- Point both ways. Placing a 1, 5 or 9 anywhere immediately fills an index cell. Do not only read the index columns — write to them.
- Rows are independent. The indexing rule never crosses rows, so a stuck row can simply be left for later without losing anything.
- Watch the fixed points. A 1 in column 1, a 5 in column 5, or a 9 in column 9 can each occur in at most one row on the whole grid.
- Try Skyscraper or Windoku first. If Column Indexer feels abstract, the other 1.2.4 variants have free Mini versions and build the variant-solving habit on more concrete rules.
Column Indexer is the variant that changes the least about a Sudoku board and the most about how you look at it. Nothing is drawn, nothing is shaded, nothing sits in the margin — three columns simply stop being only values and start being addresses too. Solvers who like the moment a puzzle turns and looks back at itself will find it the most rewarding of the twenty-seven.