Puzzles to play on a commute with no signal
Puzzle games to play on a commute without signal: which types need no connection, how to match board size to your ride, and how to pause a level at a stop.
Even when the signal bars show nothing at all, a 4x4 board fits on a phone screen between two stops. Along a commute, trains and buses often pass through tunnels, underground stretches or rural dead zones, and a connection can drop for several minutes at a time. When a solver opens a puzzle at those moments, a page may not load, or a game may ask them to sign in first.
Paper works too: a printed grid and a pencil keep working in any tunnel. Later parts compare board sizes for short and long rides, so a solver can choose a grid that is likely to be done before the stop arrives.
Puzzle types that need no connection on a 6×6 board
Number grids that run from stored rules need no connection. The best known asks the solver to place digits 1 through 9 in a 9x9 grid made of nine 3x3 boxes, so that each digit appears once in every row, column and box. It was first published in a US magazine under the name Number Place. Solvers start by looking for a naked single, which is a cell where only one digit is still possible after row, column and box are checked. The solver writes that digit in and moves on to the next cell.
A second habit is finding a hidden single. In this step, the solver picks a digit and a unit, which means one row, column or box, and counts the empty cells where that digit can still go. When only one cell is left in a unit, the digit goes in that cell, even if the cell could also hold other digits.
Picture grids follow a different rule. Each row and column carries a clue that lists runs of filled squares in order. A clue of "3 1" means three filled squares, at least one gap, then one filled square. The solver fills or marks squares until the picture shows up. Non Ishida published these grids in Japan in 1987 and 1988.
Matching board size to the ride
A 4x4 board has sixteen cells, and a solver can scan every row, column and box in a single pass. Most people finish one in a few minutes, a good match for a short ride with a stop or two left. A 6x6 board has 2x3 boxes, so each box holds six cells and each digit from 1 to 6 appears once per row, column and box. Solvers usually need several passes over this board, returning to the same rows as new digits appear.
A 9x9 board has nine 3x3 boxes and eighty-one cells. Some large grids are truly hard: solving a general picture grid of any size has been shown to be NP-complete, a formal term meaning the hardest cases get hard quickly. Well-built grids of every size can still be solved by logic alone, with no guessing, so a stuck solver can keep looking for a naked single or hidden single instead of trying a digit to see what happens.
As a hypothetical example with round numbers, a ride of about 10 minutes suits a 4x4 board, since there is time to start and finish it. A ride of 40 minutes suits a 9x9 board, with time left to pause and check the grid again.
Pausing a level at a stop
Timed levels keep running while a solver looks out the window or waits for the doors to open, so the clock does not stop at a stop. A solver who knows a stop is coming should pause after a placement, once the digit is written in and the grid has changed. Stopping partway through a scan is harder to recover from: the solver has checked part of a unit, and that result is not written down anywhere.
On return, solvers who stopped mid-scan often recheck the same row or box, since they cannot tell which cells they already checked. Pencil marks create a similar issue once they get long. A cell with five small candidates often goes out of date after a few placements, so marks of two or three digits at most stay accurate until the solver returns.
The technique to try before leaving the board is a single pass:
- Check one row for a digit that has only one cell left.
- Check one column in the same way.
- Check one box the same way, then make any placement it shows.
After that pass, the board is in a known state, and the next stop begins with a grid where nothing is half-checked.
FAQ
Can I play puzzles on a phone with no signal?
Yes. Puzzles that run from stored rules need no network at all once the page or app has loaded. A web page that needs a fresh connection often stalls in a tunnel, so open the puzzle before the train goes underground.
How big should a puzzle be for a 10 minute bus ride?
A 4x4 board suits a short ride, since a solver can scan every row, column and box in one pass. Most people need a few minutes to finish one, so there is time for a stop or two.
What is a naked single in a number puzzle?
After the row, column and box are checked, a naked single is a cell where only one digit is still possible. The solver writes that digit in and moves to the next empty cell.
How do I pause a puzzle when my stop is coming up?
Choose a moment right after a placement, when the grid has just changed and no check is half done. Trying to remember which cells were already checked is hard when a solver stops in the middle of a scan.
Sources
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