The Doppit Almanac · No. 31

Digit, a free daily sudoku

Place every number once in row, column and box

What it is

Digit is sudoku, the classic number puzzle. The grid is divided into rows, columns and boxes, and each of the three has to end up holding every number exactly once. A few numbers come printed on the board; the rest is yours to deduce. There is no arithmetic in it, only placement: if a row already shows a 4, no cell in that row can take another.

Every board has exactly one solution, and every board can be solved by looking, not guessing. Scan a box for the number with only one home, catch the cell with only one number left, and the grid tightens itself as you go.

How it plays

• Fill every cell so each row, each column and each box holds every number once.
• The printed numbers are fixed. Everything else is yours to place.
• Pick a number chip, tap a cell to place it, tap again to clear it.
• Notes pencils small candidates into a cell while you narrow it down.
• A number that breaks a row, column or box glows red.
• Stuck? Hint names the technique that moves the board forward.

The difficulties

Easy is a six-wide grid with boxes of six, a gentle half-minute solve built entirely from single-candidate moves. Medium keeps that six-wide grid but opens with about half as many numbers, so a scan has to reach further before a cell is settled. Hard is the classic nine-wide, and singles alone will not finish it: at least twice it takes a locked pair or a round of box and line pruning. Expert is nine-wide too, and none of the above finishes it either. Twice over an expert board comes down to a three-cell wing or an alternating chain, steps a hard board never carries. A fresh board is dealt every day, for nothing.

Nearby in the almanac: Rankit, Summit and Sandit