All Sudoku Variants
9×9 · Constraint

159 Sudoku

Also known as Column Indexer Sudoku

Columns 1, 5 and 9 are index columns: in each row the digit in column 1 gives the column that holds that row's 1, and likewise column 5 indexes the 5s and column 9 the 9s.

6 1 7 5 3 9

An example 159 puzzle. Columns 1, 5 and 9 (amber) index the digits 1, 5 and 9 — here column 1 holds 6, so this row's 1 sits in column 6.

159 Sudoku — also called Column Indexer Sudoku — turns three columns into pointers. In every row, the digit in column 1 tells you which column holds that row's 1; column 5 does the same for the 5, and column 9 for the 9. The three named columns and their matching digits give the variant its name.

The rule: three columns that point

Read column 1 of any row. If it holds a 6, then this row's 1 sits in column 6. The relationship is a two-way lock: if the 1 is in column 6, then column 1 must hold a 6. Columns 5 and 9 index the 5s and 9s in exactly the same way. Below, one row is shown (its nine columns), with the index columns 1, 5 and 9 shaded.

6 1

Index column 1 (leftmost) holds 6, so this row's 1 is in column 6. Columns 5 and 9 (also shaded) index the 5s and 9s the same way.

A worked example, step by step

Step 1 — read the index, place the digit. Whenever an index column's cell is known, it hands you a placement: column 1 = 6 puts a 1 in column 6 of that row.

Step 2 — read it backwards. The lock is symmetric. If ordinary Sudoku puts this row's 5 in column 3, then column 5 of that row must be a 3.

Step 3 — chain the three digits. Because 1, 5 and 9 are each pinned by their index column, the rows fill much faster than normal — placing one indexed digit often frees the next.

Solving strategy

  • Work both directions. An index cell places its digit; a placed 1, 5 or 9 fixes its index cell. Alternate between the two.
  • Watch columns 1, 5 and 9 as a set. They still each need 1–9 down the column, on top of their indexing job.
  • Let the three digits lead. The 1s, 5s and 9s are the most constrained on the board — solve around them.
  • Feed the grid. Every indexed placement behaves like a given for its row, column and box.

Common mistakes

  • Confusing digit and position. The index cell's value is a column number, not the digit that goes there.
  • Forgetting the two-way lock. The relationship works from the placed digit back to the index cell too.
  • Indexing the wrong digit. Column 1 indexes 1s, column 5 indexes 5s, column 9 indexes 9s — never mix them.
  • Dropping ordinary Sudoku. Rows, columns and boxes still each need 1–9 once.

Can I play it here?

Not yet — 159 Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile: